Countdown to the Great American Eclipse
With two eclipses and a major meteor display, August is 2017’s most interesting month for sky-watchers. Admittedly, Scotland is on the fringe of visibility for both eclipses while the annual Perseids meteor shower suffers moonlight interference.
The undoubted highlight is the so-called Great American Eclipse on the 21st. This eclipse of the Sun is total along a path, no more than 115km wide, that sweeps across the USA from Oregon at 18:17 BST (10:17 PDT) to South Carolina at 19:48 BST (14:48 EDT) – the first such coast-to-coast eclipse for 99 years.
Totality is visible only from within this path as the Moon hides completely the dazzling solar surface, allowing ruddy flame-like prominences to be glimpsed at the solar limb and the pearly corona, the Sun’s outer atmosphere, to be admired at it reaches out into space. At its longest, though, totality lasts for only 2 minutes and 40 seconds so many of those people fiddling with their gadgets to take selfies and the like may be in danger of missing the spectacle altogether.
The surrounding area from which a partial eclipse is visible even extends as far as Scotland. From Edinburgh, this lasts from 19:38 to 20:18 BST but, at most, only the lower 2% of the Sun is hidden at 19:58 as it hangs a mere 4° high in the west. Need I add that the danger of eye damage means that we must never look directly at the Sun – instead project the Sun through a pinhole, binoculars or a small ‘scope, or use an appropriate filter or “eclipse glasses”.
A partial lunar eclipse occurs over the Indian Ocean on the 7th as the southern quarter of the Moon passes through the edge of the Earth’s central dark umbral shadow between 18:23 and 20:18 BST. By the time the Moon rises for Edinburgh at 20:57, it is on its way to leaving the lighter penumbral shadow and I doubt whether we will see any dimming, It exits the penumbra at 21:51.
Our charts show the two halves of the sky around midnight at present. In the north-west is the familiar shape of the Plough while the bright stars Deneb in Cygnus and Vega in Lyra lie to the south-east and south-west of the zenith respectively. These, together with Altair in Aquila in the middle of our southern sky, make up the Summer Triangle. The Milky Way flows through the Triangle as it arches overhead from the south-west to the north-east where Capella in Auriga rivals Vega in brightness.
Of course, many of us have to contend with light pollution which swamps all trace of the Milky Way and we are not helped by moonlight which peaks when the Moon is full on the 7th and only subsides as last quarter approaches on the 15th. New moon comes on the 21st and first quarter on the 29th. The Sun, meantime, slips another 8° southwards during the month as sunrise/sunset times for Edinburgh change from 05:17/21:20 BST on the 1st to 06:15/20:09 on the 31st.
Meteors of the annual Perseids shower, the tears of St Lawrence, are already arriving in low numbers. They stream away from a radiant point in the northern Perseus which stands in the north-east at our map times, between Capella and the W-pattern of Cassiopeia. We spot Perseids in all parts of the sky, though, and not just around Perseus.
Meteor numbers are expected to swell to a peak on the evening of the 12th when upwards of 80 per hour might be counted under ideal conditions. Even though moonlight will depress the numbers seen this time, we can expect the brighter ones still to impress as they disintegrate in the upper atmosphere at 59 km per second, many leaving glowing trains in their wake. The meteoroids concerned come from Comet Swift-Tuttle which last approached the Sun in 1992.
Although Neptune is dimly visible through binoculars at magnitude 7.8 some 2° east of the star Lambda Aquarii, the only naked-eye planet at our map times is Saturn. The latter shines at magnitude 0.3 to 0.4 low down in the south-west as it sinks to set less than two hours later. It is a little higher towards the south at nightfall, though, where it lies below-left of the Moon on the 2nd when a telescope shows its disk to be 18 arcseconds wide and its stunning wide-open rings to span 40 arcseconds. Saturn is near the Moon again on the 29th.
Jupiter is bright (magnitude -1.9 to -1.7) but very low in our western evening sky, its altitude one hour after sunset sinking from 6° on the 1st to only 1° by the month’s end as it disappears into the twilight. Catch it just below and right of the young Moon on the 25th.
Venus is brilliant at magnitude -4.0 in the east before dawn. Rising in the north-east a little after 02:00 BST at present, and an hour later by the 31st, it climbs to stand 25° high at sunrise. Viewed through a telescope, its disk shrinks from 15 to 12 arcseconds in diameter as it recedes from 172 million to 200 million km and its gibbous phase changes from 74% to 83% sunlit.
As Venus tracks eastwards through Gemini, it passes below-right of the star cluster M35 (use binoculars) on the 2nd and 3rd, stands above-left of the waning earthlit Moon on the 19th and around 10° below Castor and Pollux as it enters Cancer a few days later. On the 31st it stands 2° to the right of another cluster, M44, which is also known as Praesepe or the Beehive.
This is a slightly-revised version of Alan’s article published in The Scotsman on July 31st 2017, with thanks to the newspaper for permission to republish here.
Total eclipse of the harvest supermoon on the 28th
There is no shortage of stellar interest in our September evening sky but anyone willing and able to observe later in the night may see the largest full moon of the year turn dull and red as it is totally eclipsed on the morning of the 28th. There is also a nice grouping of planets in the east before dawn.
Our chart depicts the sky in our late evening at present and shows the star Deneb in Cygnus almost at the zenith as the Summer Triangle it forms with Vega and Altair begins to topple westwards. The Square of Pegasus is climbing in the south-east in a rather sparsely populated region of the sky.
The Pleiades in Taurus glimmer low in the east-north-east as they begin their climb to the high meridian by dawn. They stand above the Moon on the Friday night of the 5th/6th as the Moon draws closer to Aldebaran, the leading star of Taurus, eventually to occult the star during the morning twilight. As seen from Edinburgh the star winks out as it disappears behind the sunlit eastern edge of the Moon at 05:51 BST on Saturday, 6th.
Our sole bright evening planet, Saturn, hovers only 10° above Edinburgh’s south-western horizon as darkness falls and sinks a little lower each evening as it creeps eastwards against the stars of eastern Libra. Telescopically, its disk appears 16 arcseconds broad while the rings are 37 arcseconds across with their north face inclined at 24°, but it is past its best as a target and, since it sets 30 minutes before our map times, we need to be quick to catch it. Look for it to the left of the young Moon on the 18th.
The Sun slips southwards over the celestial equator at 09:21 BST on the 23rd, marking the autumnal equinox in our northern hemisphere. Meanwhile, sunrise/sunset times for Edinburgh change from 06:16/20:08 BST on the 1st to 07:13/18:52 to the 30th as the duration of nautical twilight at dawn and dusk shrinks from 89 to 80 minutes.
The Moon is at last quarter on the 5th and new on the 13th when a partial solar eclipse is visible from southern Africa and Antarctica. Following first quarter on the 21st, the full moon on the 28th is a much more intriguing event because not only is it totally eclipsed, but it coincides with the Moon’s closest approach to the Earth (at perigee) in the whole of 2015. As a result, the Moon appears 7% wider than it does on average and, while the enlargement is not startlingly obvious to the casual observer, it has led to near-perigee full moons being called supermoons.
As the full moon closest to the equinox, this is also the harvest moon, a title that comes from the fact that, for several nights in a row, the bright Moon hangs low in our eastern sky as the night begins and extends the period by which the harvest may be gathered. The illusion that makes the Moon appear larger than usual when it stands low in the sky is also pronounced around this time and can only be enhanced by the supermoon circumstance.
The eclipse on the morning of the 28th, though, begins at 01:12 BST with the Moon well up in our southern sky, in the constellation Pisces and below the Square of Pegasus. The Moon meets the outer edge of the Earth’s penumbral shadow at that time, but little darkening may be noticed for another 30 minutes or more.
The dark shadow of the umbra beings to invade the lunar disk at 02:07, while from 03:11 until 04:23 the Moon is totally eclipsed as it crosses the southern part of the umbra. Here the Moon is only illuminated, usually with a reddish hue, by light scattered around the edge of the Earth. Varying atmospheric conditions, clouds and volcanic dust, on our home world mean that the appearance and brightness of the eclipsed Moon is always of interest. The Moon leaves the umbra behind at 05:27 and stands only 7° above Edinburgh’s western horizon when it exits the last of the penumbra at 06:23.
Venus, already a spectacular morning star, rises at Edinburgh’s east-north-eastern horizon 100 minutes before the Sun on the 1st and more than four hours before sunrise at the month’s end. Improving from magnitude -4.4 to -4.5, it shows a slender but dazzling crescent through binoculars though it shrinks in diameter from 51 to 43 arcseconds as it recedes from 48 million to 76 million km.
The giant planet Jupiter lags some 20° below-left of Venus and emerges from the Sun’s far side by mid-month to shine at magnitude -1.7, just brighter than Sirius which is low in the south-east at the time.
Our third morning planet, Mars, is much fainter at magnitude 1.8 and stands 9° above-left of Venus on the 1st. Also still on the far side of its orbit, it slips down and to the left in the direction of Jupiter and passes 0.8° north of Leo’s leading star Regulus on the 24th. The blue-white of Regulus outshines the orange tinted Mars and the contrasting pair make an interesting sight roughly half-way between Venus and Jupiter for a few mornings around that day.
The little innermost planet, Mercury, stands furthest east of the Sun (27°) on the 4th but is much too low in our western evening twilight to be seen this month. After sweeping around the Sun’s near side on the 30th, it is due to make its best appearance of the year as a morning star in October.
This is a slightly-revised version of Alan’s article published in The Scotsman on September 1st 2015, with thanks to the newspaper for permission to republish here.
Mercury joins two brightest planets in evening sky
Spring may have arrived, but the leading constellation of our winter sky, Orion, is still on view in our early evening sky, if not for very much longer. Look for it in the south-west at nightfall, with the three stars of his Belt lying almost parallel to the horizon. Stretch their line to the left to reach Sirius in Canis Major, our brightest nighttime star, and to the right towards Taurus, with its bright star Aldebaran and the Pleiades star cluster. High in the south-south-west is Gemini, with the twins Castor and (slightly brighter) Pollux.
To the south of Gemini is Procyon, the Lesser Dog Star in Canis Minor. Together with the true Dog Star, Sirius, and the distinctive red supergiant Betelgeuse at Orion’s top-left shoulder, Procyon completes an almost-isosceles triangle which we dub “The Winter Triangle”. At present, in fact, it forms a similar but smaller triangle with Pollux and the conspicuous planet Jupiter which dominates our southern sky at nightfall. Leo stands to the left of Jupiter with its leading star, Regulus, in the handle of the Sickle.
As April’s days lengthen, our whole sky-scape shifts further westwards each evening until, by the month’s end, Orion is setting in the west as the sky darkens
The Sun climbs more than 10° northwards during April and sunrise/sunset times for Edinburgh change from 06:44/19:50 BST on the 1st to 05:32/20:49 on the 30th. Meanwhile, the duration of nautical twilight at the start and end of the night stretches from 84 to 105 minutes.
The Moon is full on the 4th when a total lunar eclipse is visible from the Pacific and surrounding areas but not from Europe. In fact, totality, with the moon just inside the northern part of the Earth’s dark umbral shadow lasts for a mere 4 min 34 sec centred at 13:00 BST, making this the briefest total lunar eclipse for 486 years. By comparison, totality lasts for 72 minutes during the next total lunar eclipse which is visible from Scotland on the morning of 28 September this year. The Moon’s last quarter on the 12th is followed by new moon on the 18th and first quarter early on the 26th.
By nightfall on the 26th, that first quarter Moon lies 6° below Jupiter in the south-south-west. Jupiter, itself, dims a little from magnitude -2.3 to -2.1 and is slow-moving in Cancer 5° to the left of the Praesepe cluster in Cancer. The star cluster is best seen through binoculars which also show the changing positions of the four main Jovian moons as they swing from side to side of the planet. The giant planet progresses into the south-west by our star map times and sets in the north-west more than five hours later.
Even though Jupiter is twice as bright as Sirius, it pales by comparison with the evening star Venus which blazes brilliantly in the west at nightfall, and sets at Edinburgh’s west-north-western horizon by 23:38 BST on the 1st and in the north-west as late as 01:08 on the 30th. This month Venus approaches from 180 million to 150 million km and swells in diameter from 14 to 17 arcseconds, its dazzling disk appearing gibbous through a telescope as its phase changes from 78% to 67% illuminated.
Venus also speeds eastwards during the period, moving from Aries to Taurus where it passes 2.7° south of the Pleiades on the 11th to end April 3° south of Elnath at the tip of the Bull’s northern horn. There should be an impressive sight on the evening of the 21st when Venus lies 7° above-right of the earthlit crescent Moon which, in turn, is 2.5° above-left of Aldebaran.
Mars, magnitude 1.4, is low and hard to spot in our western evening twilight, becoming lost from view later in the month as it tracks towards the Sun’s far side. However, after passing beyond the Sun at superior conjunction on the 10th, Mercury emerges in our twilight to begin the innermost planet’s best evening apparition of the year.
On the 19th, Mercury shines at magnitude -1.3 and stands 4° high in the west-north-west forty minutes after sunset. Mars lies 2.8° above and to its left while the sliver of the earthlit Moon is 12° high and to their left. By the month’s end Mercury is 10° high forty minutes after sunset and shines at magnitude -0.4 22° below and to the right of Venus and only 1.7° below-left of the Pleiades. Binoculars may help to pick it out at first but it should emerge as a naked-eye object as the twilight fades and it sinks to the north-western horizon by 23:00.
Saturn is on show during the second half of the night though it does not climb far above our horizon so is not well placed for the sharpest views of its stunning ring system. For Edinburgh, it rises in the south-east at 00:46 on the 1st and by 22:40 on the 30th, reaching its highest point of 15° in the south four hours later before dawn.
Shining at magnitude 0.3 to 0.1, Saturn lies in Scorpius where it creeps 1.5° westwards above the double star Graffias. As it lies below and left of the Moon on the 8th, a telescope shows the planet’s rotation-flattened disk to be 18 arcseconds wide, within rings that span 41 arcseconds and have their northern face tilted Earthwards at 25°. Although they show an amazing complexity on the small scale, the main rings, dubbed A and B, are separated by the relatively empty dark arc of the Cassini Division. B, the brightest of the rings, has A outside it and the dusky C ring within.