Jupiter outstanding as the Geminids meteors fly
December brings our longest nights of the year and what may be 2014’s richest meteor shower. Indeed, there is an argument for ranking December nights as the most spectacular of the year if only because Orion, and the sparkling constellations that attend him, stand at their highest near the meridian at midnight. Of the bright planets, Jupiter outshines every star and is well placed from mid-evening onwards, but the others are lurking shyly near the Sun and require a little more effort.
Jupiter is unmistakable from the moment it rises in the east-north-east some 35 minutes after our star map times. Improving in brightness from magnitude -2.3 to -2.5 this month, it climbs to pass high in the south and onwards into the south-west before dawn. We find it in Leo, to the right of the Sickle and less than 8° above-right of Regulus. It is here that it reaches a stationary point on the 9th before beginning a westerly motion which carries it back into Cancer just a day before its opposition in early February.
With its large disk and changing cloud-patterns, Jupiter is always an rewarding telescopic sight while the motions from side to side of its four main moons may be followed using nothing more than decent binoculars. When Jupiter lies near the Moon on the night of the 11th-12th, it is 717 million km distant and its globe appears 41 arcsec in diameter.
Orion stands clear of the horizon in the east-south-east at the map times. Its main stars, the blue-white supergiant Rigel at Orion’s knee and the contrasting red supergiant Betelgeuse at his shoulder, are among the ten brightest. the trio of stars between them form Orion’s Belt while hanging below the Belt is Orion’s Sword and the fuzzy glow of the Orion Nebula where new stars and planets are forming, albeit slowly, before our eyes.
A line upwards along the Belt extends to Aldebaran (close to the Moon on the 5th-6th) and onwards to the Pleiades or Seven Sisters star cluster. Carry the line downwards towards Sirius which rises one hour after our map times and is our brightest star after the Sun.
North and east (above-left) of Orion lies Gemini with its twins Castor and Pollux, while close to Castor (see chart) is the radiant point for the annual Geminids meteor shower. Bright medium-slow meteors streak in all parts of the sky between the 8th and 17th but all radiate away from this point as they follow parallel paths into the upper atmosphere. The radiant climbs from the north-north-east horizon at nightfall to pass high in the south at about 02:00. Meteor rates are expected to be highest during the 24 hours around 07:00 on the morning of the 14th when more than 80 Geminids per hour might be counted under ideal conditions. The Moon is much less obtrusive than during the Geminids last year.
The Square of Pegasus crosses the high meridian in the early evening and shifts to the south-west by our map times as Andromeda stretches up from its upper-left corner. High in the south are the two smaller constellations of Triangulum the Triangle and Aries the Ram. Aries’ main star, Hamal, is identical in brightness to Polaris, the Pole Star, but lies perhaps five times closer to us at 66 light years, It also appears to have a planet that is larger than Jupiter and takes 381 days to orbit at a distance slightly greater than that between the Earth and the Sun.
Aries also gives its name to the celestial counterpart of the Greenwich meridian. Longitudes in the sky are measured eastwards from the so-called First Point of Aries where the Sun crosses the sky’s equator at the spring or vernal equinox. When the Greek astronomer Hipparchus assigned the name more than two thousand years ago this point was located in Aries. However, the Earth wobbles on its axis over a period of 26,000 years with the result that the First Point of Aries has slipped more than 30° westwards against the stars and now lies to the south of the Square of Pegasus in the dim constellation of Pisces.
The Sun is furthest south in the sky at 23:03 GMT on the 21st, the moment of our winter solstice. Sunrise/sunset times for Edinburgh change from 08:19/15:44 on the 1st, to 08:43/15:40 on the 21st and 08:44/15:48 on the 31st. Nautical twilight persists for around 94 minutes at dawn and dusk. The Moon is full on the 6th, at last quarter on the 14th, new on the 22nd and at first quarter on the 28th.
Mars, the best of the planets after Jupiter, is the brightest object low in the south-south-west at nightfall and climbs a little higher from night to night as it slides northwards in relation to the Sun. It does, though, dim from magnitude 1.0 to 1.1 as it tracks eastwards through Capricornus. It sets at about 19:15 and stands left of the young earthlit Moon on Christmas Eve.
By mid-month, and provided we have a clear south-western horizon, we may be able to spot the brilliant (magnitude -3.9) evening star Venus just after sunset. At Hogmanay, Venus stands 6° high at sunset and sets itself 76 minutes later. Mercury slips around the Sun’s far side on the 8th and is destined to join Venus as an evening star in the New Year.
Saturn is emerging as a pre-dawn object low in the south-east where it shines at magnitude 0.5 as it tracks from Libra into Scorpius. Catch it 7° below-left of the waning Moon on the 19th.
This is a slightly-revised version of Alan’s article published in The Scotsman on November 28th 2014, with thanks to the newspaper for permission to republish here.
Europe’s Philae probe to attempt first touchdown on comet
In an exciting month in astronomy and space exploration, November should bring the first soft landing on a comet when the European Space Agency’s Philae craft detaches from the Rosetta probe and drops gently onto the icy nucleus of Comet Churyumov-Gerasimenko.
Our sky at nightfall s similar to that of a month ago although, with our return to GMT, darkness arrives more than two hours earlier in the evening. Mars continues as the only bright planet at these times, visible low in Edinburgh’s south-south-western sky and fading only a little from magnitude 0.9 to 1.0 as it tracks eastwards above the Teapot of Sagittarius.
However, even though Mars is drawing closer to the Sun, its altitude at the end of nautical twilight improves from 5° to 9° during November as the Sun plunges more than 7° southwards in the sky and Mars edges almost 3° northwards. This also means that Mars-set in the south-west occurs at about 19:05 throughout the period. It stands below the young crescent Moon on the 26th.
Sunrise/sunset times for Edinburgh change from 07:19/16:33 GMT on the 1st to 08:17/15:45 on the 30th as the duration of nautical twilight at dawn and dusk extends from 83 to 93 minutes. The Moon is full on the 6th, at last quarter on the 14th, new on the 22nd and at first quarter on the 29th.
Comet Churyumov-Gerasimenko lies 6° south-east of Mars on the 12th but is a very dim telescopic object some 450 million km from the Sun. On that day Philae is due to unlatch from Rosetta and take about seven hours to fall 22.5km, coming to rest on tripod legs at about 16:00 GMT atop the head of the comet’s strange “rubber-duck” shape. To stop itself bouncing off into space in defiance of the comet’s feeble gravitational pull, it should then fire a tethered harpoon to anchor itself to the surface.
The comet’s 6-year orbit is carrying it closer to the Sun, eventually to reach perihelion at a distance of 186 million km next August. Meantime, its activity is picking up and Rosetta is imaging jets of dust and gas emerging, mainly from the duck’s neck region at present. With Philae in position to also monitor conditions at the surface, and even below the crust using sonar, seismographs and permittivity probes, our knowledge of what makes comets tick should soon be transformed.
The Summer Triangle of Vega, Deneb and Altair, lies in the west at our map times as Orion rises in the east below Taurus and the Pleiades. The Square of Pegasus stands high on the meridian with the three main stars of Andromeda, Alpheratz, Mirach and Almach, leading off from its top-left corner. The Andromeda Galaxy, M31, could hardly be better placed, being visible to the naked eye in a decent sky and not difficult at all through binoculars. It stands 2.5 million light years (ly) away and appears as an oval smudge some 8° above Mirach.
A line through the Square’s two right-hand stars points the way to Fomalhaut, bright but very low in the south. I mentioned last time that it may have at least a couple of planets. In fact, the first so-called extrasolar planet circling a solar-type star was discovered in 1995 and is about half the size of Jupiter yet orbits in only 4.2 days at a distance only one seventh of that of Mercury from the Sun. The star concerned is 51 Pegasi, magnitude 5.5 and 50 ly distant, which is unmistakable through binoculars just 1.5° or 3 Moon-widths to the right of the Scheat-Markab line.
Of the 1,800-plus extrasolar planets now known, no less than four orbit Upsilon Andromedae, a fourth magnitude star at 44 ly that stands between Mirach and Almach (see chart).
Jupiter, is creeping eastwards to the right of the famous Sickle of Leo. Rising in the east-north-east at about 23:20 on the 1st and as early as 21:40 on the 30th, it is prominent until dawn as it climbs through our south-eastern sky to pass about 50° high on our meridian before dawn. The Jovian disk is 38 arcseconds across when Jupiter lies near the Moon on the night of 13/14th.
The annual Leonids meteor shower lasts from the 15th to the 20th, building to a sharp peak on the morning of the 18th. Its super-swift meteors flash in all parts of the sky, though their paths radiate from a point in the Sickle. There is little moonlight interference this year, but meteor rates may be well down on what they were a few years ago when the shower’s parent comet was in the vicinity.
Venus sets too soon after the Sun to be seen, and with Saturn reaching conjunction on the Sun’s far side on the 18th, our only other observable bright planet is Mercury, fortunately putting on its best morning show of 2014.
On the 1st Mercury rises two hours before the Sun and shines at magnitude -0.5 as it climbs to an altitude of 10° in the east-south-east forty minutes before sunrise. Although it soon brightens to magnitude 0.8, it also slips back towards the Sun, so that by the 14th it rises 89 minutes before the Sun and is 6° high forty minutes before sunrise. Given a clear horizon, though, binoculars should show it easily and it should be a naked-eye object until it is swamped by the brightening twilight. Look for Virgo’s leading star, Spica, climbing from below Mercury to pass 5° to its right on the 7th.
This is a slightly-revised version of Alan’s article published in The Scotsman on October 31st 2014, with thanks to the newspaper for permission to republish here.
Ice giants lurk in our southern sky
Our October nights are some of the finest for stargazing in the entire year. The temperatures have yet to plumb the bone-chilling depths of winter, but the constellations visible between dusk and dawn include all the highlights of our summer and winter skies. It is just a shame that most of the bright planets are poorly placed at present.
The nights begin with the Summer Triangle high in the south. Formed by the prominent stars Vega in Lyra, Altair in Aquila and Deneb in Cygnus, its stands just to the west of the meridian at nightfall, but tumbles into the west by the star map times. In a dark sky, the diffuse band of the Milky Way flows through it is it arches overhead through Cepheus and Cassiopeia.
Look up in the south at our map times for the large, and largely empty, Square of Pegasus, and very low in the south, less than 5° high for Edinburgh, to find Fomalhaut in Pisces Austrinus, the Southern Fish. A young star only 25 light years away, it is surrounded by disks of dust and probably orbited by two or more planets.
Only two planets are visible at our map times as they lurk to the south of the Square. Uranus and Neptune are plotted on our chart in Pisces and Aquarius respectively, but they are binocular-brightness at magnitude 5.7 and 7.8 and demand more detailed charts, perhaps from the Internet, to identify them. They show tiny bluish disks through a telescope, with Uranus only 3.7 arcseconds wide when it comes to opposition at a distance of 2,845 million km on the 7th, while Neptune is currently 2.3 arcseconds and 1,500 million km further away. Both have ring systems, invisible under normal circumstances, and a plethora of moons.
For decades, these distant worlds have been classed among the gas giants to distinguish them from the smaller rocky planets closer to the Sun. Both are of similar size, some four times wider than Earth, with Uranus being 51,118 km in equatorial diameter and Neptune only 1,600 km smaller. Unlike Jupiter and Saturn, though, they contain a much smaller proportion of raw hydrogen and helium and instead are predominantly composed of the ices of water, methane and ammonia. Indeed, they are more often now classed as ice giants.
Taurus, climbing in the east, is the forerunner of the spectacular constellations of winter centred around Orion. The latter rises below Taurus over the following two hours and is unmistakable in the south before dawn as Sirius, the brightest star, twinkles furiously in the south-south-east.
In northern Orion, 10° to the north-east of Betelgeuse at Orion’s shoulder, lies the radiant point for the Orionids meteor shower which is active in the mornings from the 16th to the 30th. Fast meteors diverge from the point, particularly around the 22nd when numbers may approach 25 per hour under dark moonless skies. The meteoroids were released by Comet Halley.
The Sun sinks another 11° southwards during October as sunrise/sunset times for Edinburgh change from 07:15/18:48 BST (06:15/17:48 GMT) on the 1st to 07:17/16:35 GMT on the 31st. British Summer Time ends at 02:00 BST on the 26th when clocks are set back one hour to 01:00 GMT. Nautical twilight at dawn and dusk persists for a little over 80 minutes.
The Moon is at first quarter on the 1st and full on the 8th when observers around the Pacific, including N America, see a total lunar eclipse. Last quarter occurs on the 15th with new moon on the 23rd which brings a partial solar eclipse visible over most of N America and the north-eastern Pacific. First quarter comes round again on the 31st.
The solitary conspicuous planet is Jupiter but we must wait until the morning hours to see it. The largest of the gas giants shines at magnitude -1.9 as it rises in the east-north-east at about 02:00 BST at present and before 23:30 at the month’s end, climbing high into the south-east and even the south before dawn later in the period. Mid-October sees it slip from Cancer into Leo and by the 31st it has drawn to within 10° of Leo’s main star Regulus. The Moon stands 6° below Jupiter on the 18th when the planet is 35 arcseconds wide and 841 million km away.
Venus may be brilliant at magnitude -3.9 but it rises in the east only 40 minutes before sunrise on the 1st and is soon lost from view as it tracks towards superior conjunction on the Sun’s far side on the 25th. Mercury, though, slips through inferior on the Sun’s near side on the 16th and becomes a morning star during the final week of the month. By the 31st, it rises almost two hours before sunrise and shines at magnitude -0.4 low in the east-south-east.
Saturn and Mars are challenging evening planets just above the south-west horizon as darkness falls. Saturn, magnitude 0.6 in Libra, is lost from view later in the month as it is swallowed by the twilight, though experienced telescope users may be able to observe it being occulted by the young Moon in the late afternoon of the 25th. It is 11° high in Edinburgh’s south-west when it disappears behind the Moon’s eastern edge at 16:55 BST, though since they are 21° to the right of the Sun, caution is advised.
Mars, now well to the left of Saturn, dims from magnitude 0.8 to 0.9 as it tracks eastwards from 4° above Antares in Scorpius. Catch it 6° below the young Moon on the 28th.
This is a slightly-revised version of Alan’s article published in The Scotsman on September 30th 2014, with thanks to the newspaper for permission to republish here.
Mars greets a rival and two new orbiters
The Summer Triangle, formed by the bright stars Vega, Deneb and Altair, still has pride of place high in our southern sky at nightfall. Mars and Saturn are visible on our September evenings, too, but we must look low in the south-west to catch them. Both are well past their best and less interesting telescopic targets than Jupiter which is now resplendent in the east before dawn.
Having swept 3°, or six Moon-widths, to the south of Saturn on 27 August, Mars has a trio of further notable encounters later in September. Two new spacecraft, NASA’s MAVEN and India’s MOM or Mangalyaan, are on course to enter orbit around Mars on the 21st and 24th respectively while the planet is due to pass 3° north of the enormous red supergiant star Antares in Scorpius on the 27th. The name Antares comes from the Ancient Greek for “rival to Mars” and, while they may indeed be similar in brightness by the month’s end, it will be fascinating to see how their colours compare.
Meanwhile, Mars, or rather the spacecraft in orbit around it, are due for a more challenging encounter when the icy nucleus of comet C/2013 A1 Siding Spring sweeps within some 130,000 km of the planet on 19 October. The operators of NASA’s Mars Reconnaissance Orbiter and Mars Odyssey, and of Europe’s Mars Express, are arranging to shield their craft from the worst of the dust storm that is likely to be accompanying the comet, and similar precautions may be needed for MAVEN and MOM.
In other space news, Europe’s Rosetta craft is now studying five potential landing sites for its Philae lander on the nucleus of Comet Churyumov-Gerasimenko. The landing is not due until November, but it is planned to choose a primary and a backup site this month as Rosetta closes to with 30 km of the nucleus.
By our star map times, the Summer Triangle lies just west of our meridian as it gives way to the stars of autumn led by the topsy-turvy winged horse Pegasus whose nose is marked by the star Enif. Use binoculars to look 4° north-west of Enif for the star cluster M15 which appears as a fuzzy blob less than half as wide as the Moon. In fact, it is one of the finest globular clusters in the sky and contains more than 100,000 stars at a distance in excess of 30,000 light years.
The Sun tracks 11.5° southwards in the sky during September and crosses the equator at 03:29 BST on the 23rd, the time of this year’s autumnal equinox. Sunrise/sunset times for Edinburgh change from 06:17/20:07 on the 1st to 07:13/18:51 on the 30th as the duration of nautical twilight at dawn and dusk falls from 89 to 80 minutes.
The Moon is at first quarter on the 2nd, full on the 9th, at last quarter on the 16th and new on the 24th. As the full moon nearest to the equinox, the one on the 9th is also our Harvest Moon and, since it comes less than a day after the Moon is closest to the Earth, it is yet another supermoon.
Saturn stands about 11° high in the south-west and only 0.3° above the northern tip of the crescent Moon as the evening twilight fades on 31 August, with Mars another 4° below and to their left.
On 27 September, the young Moon returns to lie 6° to the right of Saturn which, by then, is 4° lower in the sky and becoming hard to spot in the twilight. Both planets begin the period at magnitude 0.6, but Mars dims slightly to magnitude 0.8 by the 29th when it stands 5° below the Moon and 3° above Antares. It is also 20° to the left of Saturn and drops below Edinburgh’s horizon at 20:51 BST. Viewed through a telescope, Mars is only 6 arcseconds in diameter at midmonth, while Saturn is 16 arcseconds wide within rings that span 36 arcseconds and have their north face tilted 22° towards us.
After Mars and Saturn set, the sky is devoid of bright planets until Jupiter rises more than five hours later. True, Neptune and Uranus are binocular objects at magnitudes of 7.8 and 5.7 in Aquarius and Pisces respectively, but we need better charts to identify them.
There is no mistaking Jupiter, though. The conspicuous giant planet rises at Edinburgh’s east-north-eastern horizon at 03:29 on the 1st and by 02:07 on the 30th. climbing well clear of the eastern to south-eastern horizon by dawn. As it brightens slightly from magnitude -1.8 to -1.9, it also tracks 6° eastwards, below and away from the Praesepe or Beehive star cluster in Cancer. Look for the waning earthlit Moon 6° below and right of Jupiter before dawn on the 20th. Viewed through a telescope on that morning, the cloud-banded Jovian disk is 33 arcseconds across.
Venus is also a morning object and, although it remains brilliant at magnitude -3.9, it is sinking deeper into the twilight as it approaches conjunction on the Sun’s far side in October. On the 1st, it rises 87 minutes before the Sun and stands 14° below and left of Jupiter as it climbs 12° above our eastern horizon by sunrise. Jupiter soon leaves it behind, though, so by the 30th it rises 32 minutes before the Sun and is only 6° high at sunrise. Viewed telescopically, its almost full disk is only 10 arcseconds across.
The other inner planet, Mercury, moves to lie 26° east of the Sun on the 21st, but hugs the western horizon at sunset and is not observable from our latitudes.
This is a slightly-revised version of Alan’s article published in The Scotsman on August 29th 2014, with thanks to the newspaper for permission to republish here.
Two brightest planets in closest meeting for 14 years
Our usual highlight for August is the return of the prolific and reliable Perseids meteor shower. Unfortunately, meteor-watchers have to contend with moonlight this year and it is just as well that we have other highlights as compensation. Foremost among them is the closest conjunction between Venus and Jupiter, the two brightest planets, since 2000 though they are low down in our morning twilight. Mars and Saturn rendezvous, too, and we have our best supermoon of the year.
Sunrise/sunset times for Edinburgh change from 05:17/21:20 BST on the 1st to 06:15/20:10 on the 31st. The spell of nautical twilight at dusk and dawn shrinks from 121 to 89 minutes. The Moon is at first quarter on the 4th, full on the 10th (see below), at last quarter on the 17th and new on the 25th.
The term supermoon has gained currency in recent years to describe a full moon that occurs when the Moon is near its closest in its monthly orbit. At such times, it can appear 7% wider and 15% brighter than an average full moon. In my view, the enhancement is barely perceptible to the eye and is less impressive than the illusion that always makes the Moon appear larger when it is near the horizon. As the media have discovered, though, supermoons provide a good excuse to feature attractive images of the Moon against a variety of landscapes, and if this encourages more people into astrophotography, so much the better.
This month, the Moon is full at 19:10 BST on the 10th, less than 30 minutes after its closest point (perigee) for the whole of 2014. On that evening, the supermoon is already 4° high in the east-south-east as the Sun sets for Edinburgh, so judge (and photograph?) for yourself.
Venus is brilliant at magnitude -3.9 as a morning star. Rising at Edinburgh’s north-eastern horizon at 03:10 BST on the 1st, it stands 15° high in the east-north-east at sunrise. By the 31st, it rises at 04:42 and is 12° high at sunrise. Between these dates it is caught and passed by Jupiter which emerges from the twilight below and to Venus’ left on about the 7th and stands only 0.2° below Venus before dawn on the 18th.
Jupiter is magnitude -1.8, one seventh as bright as Venus, but still outshines every star so the conjunction is a spectacular one, albeit at an inconvenient time of the night. In fact, the event occurs less than a degree south-west of the Praesepe or Beehive star cluster in Cancer, but this may be hard to spot in the twilight. Before dawn on the 23rd, the two planets lie to the left of the waning and brightly earthlit Moon. By the month’s end, Jupiter rises by 03:25 and stands 13° above-right of Venus.
Mars and Saturn have set by the map times but stand low in the south-west as our evening twilight fades. On the 1st, Mars is magnitude 0.4 and lies 10° to the left of Spica in Virgo. Saturn, only a little dimmer at magnitude 0.5 in Libra, is 13° to Mars’ left, and slightly higher. Look for the Moon to the right of Mars on the 2nd, between Mars and Saturn on the 3rd and to the left of Saturn on the 4th. Mars, meanwhile, tracks eastwards to cross from Virgo to Libra on the 10th and pass 3.5° below Saturn on the 24th. By the 31st, both planets have faded to magnitude 0.6, and Mars lies 5° below-left of Saturn with the Moon between them again and very close to Saturn.
After passing around the Sun’s far side on the 8th, Mercury is too low to be seen in our evening twilight.
Our chart depicts the bright stars Deneb, Vega and Altair high in our southern sky where they form the Summer Triangle. The centre of our Milky Way galaxy lies in Sagittarius on the south-south-western horizon but the Milky Way itself flows northwards through Aquila and Cygnus before tumbling down through Cepheus, Cassiopeia and Perseus in the north-north-east.
Use binoculars to seek out the star Mu Cephei high above familiar “W” of Cassiopeia. Dubbed the Garnet star by Sir William Herschel, Mu is one of the reddest stars we know and pulsates semi-regularly between magnitude 3.4 and 5.1. Some 6,000 light years away, it is so large that it would extend beyond the orbit of Saturn if it replaced the Sun and is sure to explode as a supernova within a few million years.
The Perseids are due to peak in the middle of the night on 12-13th August when we might have been able to glimpse more than 80 meteors per hour under ideal conditions. As it is, bright moonlight will ensure that meteor counts are well down, though we can still expect some impressive bright meteors that leave persistent glowing streaks, called trains, in their wake.
Decent rates may be seen from perhaps 10-15th August and, in fact, the shower is already underway as the Earth takes from 23 July to 20 August to traverse the stream of Perseid meteoroid particles laid down by Comet Swift-Tuttle. It is only appropriate that the resulting meteors are swift, too, as they disintegrate in the upper atmosphere at 59 km per second. Although they move in parallel through space, perspective means that they appear to diverge from a radiant point in Perseus, plotted on our northern star map below Cassiopeia. That point climbs through the north-east overnight to approach the zenith by dawn, but remember that the meteors can appear in any part of the sky and not just towards the radiant.
This is a slightly-revised version of Alan’s article published in The Scotsman on August 1st 2014, with thanks to the newspaper for permission to republish here.
Sun spotting in safety at solar maximum
With Scotland’s nights still awash with twilight, many people focus on the Sun during July. There are dangers involved, though, and I don’t just mean sunburn. Specifically, we must never look at the Sun directly through binoculars or any telescope. To do so invites serious eye damage. Instead, project the Sun’s image onto a white card held away from the eyepiece or obtain an approved solar filter to fit over the objective (rather than the eyepiece) end of your instrument.
The most obvious features on the solar disk are sunspots, cooler areas that are shaped by magnetic activity and last for a few hours to several weeks. Because the Sun rotates every 27 days with respect to the Earth, spots take two weeks to cross the Sun’s face, provided they survive as long.
Sunspot numbers ebb and flow in a solar cycle of about 11 years, although the actual period varies from about 9 to 14 years. The last peak in the Sun’s activity occurred in 2000 and, following an unusually prolonged minimum between 2007 and 2010 when very few spots were seen, we are back near solar maximum though at a lower level than in 2000. This also means that solar flares, and the auroral displays that they can produce, are also more frequent even if they are hard to see given our summer twilight
As I warned last time, though, silvery or bluish noctilucent clouds are sometimes visible low down in the northern quarter of the sky and Scotland enjoyed a nice display on the night of 19-20 June. They are formed by ice crystals near 82km and more can be expected until mid-August or so.
The Sun tracks 5° southwards during July and from the 12th onwards Edinburgh enjoys at least a few minutes of official nautical darkness around the middle of the night. We need to wait a few days more for the bright Moon to leave the scene, but when it does the fainter stars should once again be visible.
If light pollution is minimal, the Milky Way may be seen arching high across the east at our star map times. Marking the central plane of our galaxy, with the greater density of distant stars, it stretches from Capella in Auriga in the north through the “W” of Cassiopeia in the north-east before flowing by Deneb in Cygnus in the east and downwards towards Sagittarius near the southern horizon. Where it passes through the Summer Triangle formed by Deneb, Vega and Altair it is split into two by obscuring interstellar dust, the Cygnus Rift.
The red star Chi Cygni, 2.5° or five Moon-widths south-west of Eta in the neck of Cygnus, pulsates every 13 months or so between a naked eye object of the fifth magnitude and a dim telescopic one near magnitude 13. It reached an unusually bright peak of better than magnitude four last year and should be near maximum again about now, though recent observations suggest it may not even hit magnitude six this time.
The Earth is 152,114,000 km from the Sun, and at its farthest for the year, on the 4th. Sunrise/sunset times for Edinburgh change from 04:31/22:01 BST on the 1st to 05:15/21:22 on the 31st when nautical darkness lasts for almost four hours around the middle of the night. The Moon is at first quarter on the 5th, full on the 12th, at last quarter on the 19th and new on the 26th.
Jupiter is barely 6° above the west-north-western horizon at sunset on the 1st and is unlikely to be visible as it heads for conjunction on the Sun’s far side on 24th.
Mars, to the right of Spica in Virgo and low down in the south-west at nightfall, sinks to set in the west-south-west at our map times. Fading from magnitude 0.0 to 0.4 this month, it tracks to the left to pass 1.3° above Spica on the 14th – the final and closest of three conjunctions between them this year.
The young Moon lies below Regulus in Leo low in the west on the evening of the 1st and close to Mars on the 5th. The 7th finds it close to Saturn and even closer to the double star Zubenelgenubi in Libra, the three making for a superb sight through binoculars. Saturn dims only slightly from magnitude 0.4 to 0.5 and hardly moves against the stars, appearing telescopically as an 18 arcseconds disk with rings 40 arcseconds wide.
A brilliant morning star at magnitude -3.9, Venus rises at about 03:00 BST and stands 12° to 14° high in the east-north-east at sunrise. As it tracks eastwards through Taurus, use it as a pointer to Mercury which is less than 8° below and left of Venus from the 10th to the 23rd as it brightens from magnitude 0.8 to -0.8. Set your alarm to catch Venus 8° to the left of the 7% illuminated waning earthlit Moon before dawn on the 24th.
While many stars are larger than our Sun, including the vast majority of stars visible to the unaided eye, there are billions that are smaller. Indeed, red dwarf stars, from about half the Sun’s mass to 1/13th as massive, are thought to make up 75% of the more than 100 billion stars in our galaxy. The smallest known star, and probably close to the smallest star possible, is a red dwarf in the constellation Lepus, just south of Orion. Smaller than Jupiter, but more massive, it has surface temperature of 1,800C and a luminosity of 1/8,000th of our Sun so that we need a large telescope just to see it even though it is only 40 light years away.
This is a slightly-revised version of Alan’s article published in The Scotsman on July 1st 2014, with thanks to the newspaper for permission to republish here.
The mysterious noctilucent clouds of summer
If we are prepared to do battle with June’s night-long twilight, and provided the weather improves at last, there is plenty of interest in our June sky. Saturn is the pick of the planets while the bright star Vega in Lyra leads the onslaught as the constellations of summer invade from the east at our star map times. We also need to be alert for noctilucent clouds as they make their seasonal appearance low in our northern sky.
The Sun is furthest north at 11:51 BST on the 21st, the instant of our summer solstice. On that day, the Sun dips only 10.6° below Edinburgh’s northern horizon in the middle of the night, so that our sky remains bathed in twilight throughout the night while from further north in Scotland the sky is brighter still. This obviously impedes our ability to see the dimmer stars and “faint fuzzies” such as galaxies and nebulae. On the other hand, it means that satellites remain sunlit whenever they pass overhead. Indeed, the International Space Station is conspicuous two or three times each night until 10 June as it transits from west to east across Scotland’s southern sky – visit heavens-above.com for predictions customised for your location.
The Sun’s shallow sweep below our northern horizon overnight also allows us occasional views of noctilucent or “night-shining” clouds. Composed of tiny ice crystals in a thin layer at a height near 82 km, they catch the sunlight long after our usual low-level clouds are in darkness and can appear like chaotic banks of electric-blue cirrus, sometimes in a herringbone pattern. Their preferred direction follows the Sun around the horizon, so they are more commonly seen low in the north-west after nightfall and towards the north-east before dawn. They occur from mid-May to mid-August but why they are more frequent than they were a century ago remains a mystery. Could the rise be due to global warming, increased industrial pollution or even particles from rocket launches?
Sunrise/sunset times for Edinburgh change from 04:35/21:47 BST on the 1st to 04:26/22:03 on the 21st and 04:31/22:02 on the 30th. The Moon is at first quarter on the 5th, full on the 13th, at last quarter on the 19th and new on the 27th.
At magnitude -1.9, our brightest evening planet continues to be Jupiter, but we must look lower into the west to catch it below Pollux in Gemini as the twilight fades. Shining at magnitude -1.9, it stands 9° above-right of the Moon on the 1st. Jupiter sinks to set in the north-west almost three hours after the Sun as June begins but by the 30th it is only 6° high at sunset and may already be lost from view.
Mercury lies 18° below and to the right of Jupiter on the 1st but is one twentieth as bright at magnitude 1.4 and fading rapidly as it moves to pass through inferior conjunction between the Sun and Earth on the 19th.
The bright star Arcturus in Bootes stands high on the meridian at nightfall but has moved to the middle of our south-western sky by the map times. This leaves our high southern sky devoid of bright stars until we come to Vega in Lyra high in the east-south-east. Directly below Vega is Altair in Aquila while Deneb in Cygnus, almost due east, completes the Summer Triangle. The arc from Vega to Arcturus cuts through Hercules and Corona Borealis, the pretty semi-circular Northern Crown whose main star has the dual names of Alphecca or, perhaps more appropriately, Gemma.
Mars fades from magnitude -0.5 to 0.0 as it tracks eastwards in Virgo towards Spica. It also recedes from 119 million to 148 million km during the month as its small disk contracts from 12 to 9 arcseconds if viewed through a telescope. Look for its reddish light about 26° high in the south-west at nightfall and catch it above the Moon on the 7th. Our maps show it sinking towards the west where it sets two hours later.
Saturn, magnitude 0.2 to 0.4, stands almost 20° high in the south at nightfall at present and continues to creep westwards in Libra almost 4° above-left of the double star Zubenelgenubi. After standing close to Spica on the 8th, the Moon lies near Saturn on the 10th when the planet appears 18 arcseconds wide, its disk set within rings that span 41 arcseconds and have their north face inclined 21° towards us. Don’t miss an opportunity to observe it this month for it will soon be following Mars lower into the south-west at nightfall, and it stands even further south in our summer sky during every year until 2022.
Continuing as a brilliant morning star of magnitude -4.0 to -3.9, Venus rises above Edinburgh’s east-north-eastern horizon 61 minutes before the Sun tomorrow and in the north-east 102 minutes before sunrise on the 30th. Before dawn on the 24th, it lies 5° left of the slender waning Moon and 6° below the Pleiades in Taurus.
Last month, I reported the prediction that the Earth would slice through streams of particles from Comet 209P/LINEAR on the morning on 24 May and that the resulting meteor shower might be spectacular. In fact, it appears that the encounter occurred as forecast, but that the resulting display was disappointing with only a few bright meteors, even for observers in the Americas for whom the timing of the outburst was ideal. Radar studies suggest that the vast majority of meteoroids were unusually small and their meteors too dim to be seen by the unaided eye.
This is a slightly-revised version of Alan’s article published in The Scotsman on May 30th 2014, with thanks to the newspaper for permission to republish here.
Saturn’s opposition in the Balance
Our days lengthen during May until the period of true nighttime darkness dwindles to almost nothing by the month’s end. You might think that astronomers would be tempted to mothball their telescopes, but if they did they would miss the year’s best views of Saturn.
The beautiful ringed planet comes to opposition at a distance of 1,331 million km on the 10th when it lies in Libra, the Balance or Scales, and stands in the south in the middle of the night. The ochre deserts and white north polar cap of Mars are also observable, as are all the other brighter planets at one time or another. There may also be a spectacular meteor shower that has never been seen before.
Look overhead at nightfall to find the Plough and extend a curving line along its handle to reach the star Arcturus shining brightly in Bootes well up in the east-south-east. Continue that line, still bending, into the south-east where Mars is conspicuous and reddish in Virgo, above-right of Virgo’s leading star Spica. By our star map times, the Plough has moved to stand high in the west, Arcturus is high in the south, and Mars is in the south-west.
Following its own opposition on 8 April, Mars is now receding from us, from 96 million to 119 million km during May, and although it halves in brightness from magnitude -1.2 to -0.5 it still outshines Arcturus. Viewed telescopically, its disk shrinks from 15 to 12 arcseconds and only in moments of steady “seeing” can we discern its surface detail. The Red Planet’s slow westerly progress below the famous binary star Porrima halts on the 21st when it reaches a so-called stationary point before tracking eastwards again.
Saturn, creeping westwards in the middle of Libra and bright at magnitude 0.1, stands close to the horizon and beneath Arcturus at nightfall. By our map times, though, it is almost due south at an altitude of nearly 19° as seen from Edinburgh. This is 12° lower than Mars when it transits the meridian, so we see it through more of the Earth’s atmosphere and the seeing is likely to be worse. On the other hand, Saturn’s disk is bigger at 19 arcseconds while its superb ring system spans 42 arcseconds and has its north face tipped 22° towards us. This is a good time to look for the Cassini Division, the 4.800 km gap between the two main rings.
Binoculars show the star Zubenelgenubi, 5° to the west of Saturn, to be an obvious double star, while Zubeneschamali, to Saturn’s north, is held (perhaps mistakenly) by some observers to be one of the few greenish-hued stars in the sky. The Arabic names for these stars mean Southern and Northern Claw respectively and date from an era when they were also associated with the brighter nearby constellation of Scorpius the Scorpion. Use binoculars to scan 11° north of Zubeneschamali for the fuzzy blob of M5, a globular cluster of up to 500,000 stars at a distance of about 25,000 light years. Some observers rate it more highly than the more familiar M13 globular in Hercules and M3 in Canes Venatici, 12° to the north-west of Arcturus.
Jupiter remains prominent, and brighter than any star, in the west at nightfall but is close to setting in the north-west by our map times. At magnitude -2.0, it is tracking eastwards in the middle of Gemini, below Castor and Pollux, and shows a 34 arcseconds disk at midmonth.
Mercury is an evening star as it climbs to stand furthest east of the Sun, 23°, on the 25th. Between the 13th and 29th it stands about 10° high in the west-north-west forty minutes after sunset though it may be hard to spy without binoculars in the slowly-fading twilight. It dims from magnitude -0.6 to 1.0 between these dates. Venus is a brilliant morning star on magnitude -4.1 which rises in the east fifty minutes before the Sun on the 1st and one hour before sunrise on the 31st.
Sunrise/sunset times for Edinburgh change from 05:29/20:52 BST on the 1st to 04:36/21:45 on the 31st. Nautical twilight at dusk and dawn lasts for 105 minutes on the 1st and for all but the middle 24 minutes of the last night of May.
The Moon is at first quarter on the 7th, full on the 14th, at last quarter on the 21st and new on the 28th. The Moon is strongly earthlit when it stands just above Aldebaran in Taurus on the 1st evening. Catch it again below-left of Jupiter on the 4th, near Mars on the nights of the 10th and 11th and Saturn on the 13th and 14th.
The morning of the 24th may see slow meteors streaming away from a radiant point in the dim constellation of Camelopardalis the Giraffe, see north map. The prediction is made by analysts who have back-tracked the motion of a small comet whose official name is Comet 209P/LINEAR. Discovered as recently as 2004, its path carries it between the orbits of the Earth and Jupiter every 5.1 years and it is to pass harmlessly only 8,290,000 km from the Earth on the 29th, the ninth closest approach by a comet on record.
Only a few days earlier, it is thought that the Earth may encounter several streams of particles that were released by the comet between 1803 and 1924. Meteor rates could hit many hundreds per hour, if not storm force, though the peak of activity is predicted between 08:00 and 09:00 BST on the 24th, during daylight for Britain but ideal for observers in N America. Our pre-dawn hours could still be interesting, though.
This is a slightly-revised version of Alan’s article published in The Scotsman on April 29th 2014, with thanks to the newspaper for permission to republish here.
This telescope, hanging on a wall in the Meridian Building at Greenwich, was used to make two of the greatest discoveries of all time in the field of celestial dynamics. But before I introduce these discoveries, I need to create an image of the stage onto which they came.
Measuring the speed of light began with Galileo’s failed attempt of the early 1600’s where he tried to arrive at a result by timing light over about two miles. Moving on, the next person to become involved, in 1676, was Ole Romer, a Danish astronomer who was studying the orbit of Io, the innermost satellite of Jupiter. Romer correctly deduced that difficulties he had in timing this orbit were related to light taking longer to reach earth when Jupiter was further away in its orbit. Working backwards from the observational data he arrived at a speed of light of 200,000 km/ sec. Although this figure is around 33% incorrect – attributed later to the imprecise dimensions of the Solar System known at the time – Romer became the first person in history to measure the speed of light.
Meanwhile, the creation of Greenwich Royal Observatory was underway, and precision measurements of the celestial sphere were being developed. Stellar Parallax was of particular interest as, although Copernicus hypothesised a helio-centric universe in 1543, to date no one had found any proof that this was correct. Existence of Stellar Parallax would confirm this. The effects of Atmospheric Refraction mean that small movements of celestial objects are best measured in the zenith where there is no refraction to interfere with the observational data. As a result, Gamma Draconis, which is in the zenith over London, was a prime candidate for measuring any small movements that a star may make.
It was Robert Hooke in 1669 who invented the Zenith Telescope to study Gamma Draconis in more detail, and he noted that the star “moved by more than 1/100th of a degree” between early July and late October that year. Although compelling evidence for parallax, until a whole year’s data is obtained the question remains open. Hooke went on to design the Monument to the Great Fire in London as a 202ft Zenith Telescope, it being hollow from the basement to a trapdoor at the top that could be opened to reveal the sky. But, even at this time, the monument vibrated under the influence of London traffic and he was unable to carry out any scientific experiments. John Flamsteed in Greenwich built a Zenith Telescope into an existing well in 1679 but the records state he made only one observation.
The problem lay dormant for the better part of 50 years until it was again addressed, this time by two individuals called Samuel Molyneux and James Bradley. Molyneux was a rich “amateur” astronomer, and Bradley was a trained observational astronomer. Molyneux funded a 24ft Zenith Telescope that was attached to a chimney of his mansion at Kew and Bradley ran the experiment.
After one year’s observation Gamma Draconis was shown to indeed move with time – in an almost cyclical manner – but this could not be mathematically related to parallax – something else was happening.
The diagram above shows the movement of Gamma Draconis, in green, in 1726. The red curve is the path that Bradley calculated the star would take if they were observing parallax. Although they seem to superimpose quite well, there is a phase shift in their timing and it is not possible for the green curve to represent stellar parallax.
There is also a vertical green line in the image. To measure the horizontal (Right Ascension) movement of the star a precision Sidereal Clock is required, but Molyneux and Bradley did not have one. So they had to be content with only observing and recording the north/ south movement of the star.
Bradley took stock and realised that the telescope they were using was not suitable to carry the experiment forward. It had so little travel at its lower end that only two stars could be observed. Using a star catalogue Bradley established that if the lower end of the telescope could be moved by 5 degrees in each direction over 70 stars could be observed. He further noted that he would be able to see Capella – one of the brightest stars in the sky – if the movement was over 6¼ degrees.
He therefore had a new, 12½ ft Zenith Telescope made, but he had this installed in the house in which he was living with his aunt, in Wanstead. He wanted to make copious measurements and by having it where he lived he would not have the difficulty of travel to Kew. He began using this telescope in August 1727. He measured over 50 stars and at least 10 on a very regular basis.
Unfortunately, Samuel Molyneux, whilst in attendance at the House of Commons where he was an MP, fell seriously ill in April 1728 and he died a few days later. This is particularly sad as Bradley was on the cusp of making one of the greatest discoveries in the history of celestial dynamics.
Bradley came to a conclusion on his discovery in late 1728 when sailing with a friend on the Thames. By comparing the constant wind on the Thames with the constant light stream from Gamma Draconis, the movement of the yacht with that of the earth in its orbit, and the slight movements of a vane on the yacht mast with a telescope trained on Gamma Draconis (having to be moved as the earth changed direction in its orbit) Bradley came up with a theory now known as the “Aberration of Light”. He correctly deduced that when the earth is moving in a non parallel manner to the light from a star that the telescope would have to be tilted slightly to receive the light from that star.
He calculated that the largest angle that the telescope would have to be tilted by – when the earth was moving perpendicularly to the light from the star – was 20.2 seconds of arc. This value is called the Constant of Aberration and is denoted by the Greek letter Kappa. Its currently accepted value is 20.49552 seconds of arc, less than 1.5% different from Bradley’s initial result. Knowing this angle, Bradley found the speed of light to be 20,210 times the speed of the earth in its orbit. Bradley, however, did not give a value for the speed of light in his 1729 paper. It relies on a knowledge of the radius of the orbit of the earth. Perhaps Bradley was not sufficiently convinced that the value of the radius known at this time was of sufficient accuracy for him to quote a speed of light. But what is evident is that, given an accurate radius, Bradley’s data yields a result within 1.5% of the currently accepted value.
Bradley noted that the movements of Gamma Draconis were more complex than could be explained by the Aberration of Light. Indeed had the star movements been solely due to the aberration of light the above green curve that the star took in 1726 would have been a closed curve. So he kept his experiment going – indeed he ran it until 1747 – over 20 years since he started out with Molyneux in 1725. In 1747 Bradley published a second paper on his experiment as the secondary effects on the movement of the star were found to be related to “Nutation” – a wobbling of the earth’s axis due to the gravitational pull of the moon. One of the cycles of the moon is 18.6 years long – hence the length of time Bradley needed to take observational measurements to see this effect. The behaviour of the star over this period is demonstrated in the diagram below.
These were not his only accomplishments, however. Bradley became Savilian Professor of Astronomy at the University of Oxford in 1721, and the third Astronomer Royal at Greenwich in 1742, both of which he held to his dying day in 1762. In both of these he made further significant astronomical observations and discoveries. His telescope still hangs today on a wall in the Meridian House at Greenwich where it can be seen by the public.
In conclusion, I feel that the current exhibit at Greenwich does not do justice to a telescope that made two of the greatest discoveries of all time in the field of Celestial Dynamics.
This article is an adaptation of Bruce Vickery’s talk of the same name, which was given to the Astronomical Society of Edinburgh on the 7th of March 2014. Bruce is a member of the Society and one of his personal interests is the development of software to demonstrate the dynamic behaviour of the celestial sphere. This software was used to graphically illustrate the principles involved in the discovery process throughout his talk, and examples of this are included in the article above. Our thanks to Bruce for sharing his knowledge of this subject with us in such an interesting and informative manner.
Mars shines brightly at opposition in Virgo
Six years have passed since Mars was as close and bright as it is this month, but two other planets outshine it and a fourth, Saturn, will soon be at its best for the year. There are also two of 2014’s four eclipses but, as with the second pair in October, neither is of much interest for observers in Scotland.
For the moment, our evening sky retains a flavour of stellar feast we enjoyed over the winter. Orion is still on show in the south-west at nightfall below the conspicuous planet Jupiter. Orion’s Belt now lies almost parallel to the horizon, a line along it pointing to the left towards Sirius, our brightest nighttime star, and to the right towards Aldebaran and the Pleiades in Taurus. By our star map times, though, Orion has all but sunk below our western horizon.
Jupiter, however, continues as our brightest evening object bar the Moon. As it slips 3.5° or seven Moon-widths eastwards in the middle of Gemini during April, it fades a little between magnitude -2.2 and -2.0 and its telescopic diameter shrinks from 38 to 35 arcseconds. The earlier in the night that we catch it, the higher it stands and the sharper the view of its cloud-banded disk. By our map times Jupiter is some 30° high in the west and on its way to setting in the north-west four hours later.
The month begins with impressive views of the young earthlit Moon in the west at nightfall. It is only 5% illuminated on the 1st as it stands 14° high forty minutes after sunset. Look for it below the Pleiades on the 2nd, below the Aldebaran-Pleiades line on the 3rd and 6° below Jupiter on the 6th as it nears first quarter.
Mars reaches opposition on the 8th when it lies 93 million km away and shines at magnitude -1.5 so that its orange-red beacon rivals Sirius in brightness if not in colour. By definition, it stands opposite the Sun in the sky so that we find it climbing from the eastern horizon as the evening twilight fades to pass 28° high on Edinburgh’s meridian two hours after our map times. As the arrow on our south map shows, Mars tracks 10° westwards in Virgo during April, from 5° above the magnitude 1.0 Spica today to lie 1.6° below-left of the famous binary star Porrima as the month ends.
Often the day of opposition is when a planet is closest to us but Mars is approaching the Sun in its orbit and is 450,000 km closer to us on the 14th than on the 8th. Through a telescope, its ochre disk is 15 arcseconds wide and shows dusky markings and the dwindling white smudge of its north polar ice cap, tipped about 22° towards us.
The full Moon lies below Mars on the evening of the 14th and is approaching Spica as it sets for Edinburgh at 06:08 BST on the 15th. Only 14 minutes before this, and while it is less than 2° above the west-south-western horizon in the twilight, it begins to enter the outer shadow of the Earth, the penumbra. Sadly, we have no hope of seeing any dimming of the lunar disk before it sets. Observers in the Americas are much better placed to view the resulting total eclipse of the Moon which is total from 08:07 until 09:26 BST (03:07 to 04:25 EDT).
Sunrise/sunset times for Edinburgh change from 06:44/19:51 BST on the 1st to 05:32/20:50 on the 30th while the duration of nautical twilight at dawn and dusk stretches from 84 to 105 minutes. After first quarter on the 7th, the Moon is full during the eclipse on the 15th, at last quarter on the 22nd and new on the 29th when a small area of Antarctica and perhaps a few penguins experience an annular eclipse of the Sun. A partial solar eclipse is visible from Australia and the southern Indian Ocean.
On course to reach opposition in May, Saturn rises at Edinburgh’s east-south-eastern horizon at 23:31 on the 1st and only 36 minutes after sunset by the 30th, climbing to pass 18° high on the meridian four hours after our map times. Improving from magnitude 0.3 to 0.1, it edges westwards in Libra and draws ever closer to the Moon overnight on the 16th-17th when Saturn’s disk is 18 arcseconds wide while its stunning rings span 42 arcseconds.
Mercury is hidden in the dawn twilight until it passes around the Sun’s far side on the 26th. Venus, brilliant as a morning star, rises in the east-south-east seventy minutes before sunrise on the 1st and in the east only 51 minutes before the Sun on the 30th. Dimming from magnitude -4.3 to -4.1, its gibbous disk shrinks from 22 to 17 arcseconds in diameter.
It is less than a month since results from NASA’s WISE spacecraft appeared to rule out any Jupiter or Saturn-sized planet lurking unseen in the outermost solar system. Now we learn that a new dwarf planet, dubbed 2012 VP113, has been found to have an orbit that comes no closer to the Sun than 80 times the Earth’s distance, further than any other known object in the solar system. Thought to be a ball of rock and ice perhaps 450 km wide, it may be six times further away at its farthest, and take perhaps 5,000 years to complete each orbit.
Surprisingly, 2012 VP113’s orbit is similarly orientated to those of some other remote bodies, including the only other comparable object, Sedna. There is speculation that this is because they are influenced by a larger undiscovered world, perhaps a super-Earth, even further out.