Perseids meteor shower peaks under moonless skies
The revelations by New Horizons at Pluto were certainly the highlight for July, showing that even small ice-bound worlds far from the Sun can have an active and fascinating geology. No doubt we are in for further surprises as the data from the encounter are downloaded over the narrow-bandwidth link to the probe over the coming months.
August sees our attention return to Comet 67P/Churyumov–Gerasimenko which is due to experience its peak activity as it sweeps through perihelion, its closest to the Sun, on the 15th. We should enjoy a grandstand view courtesy of Europe’s Rosetta probe in orbit around the comet’s icy nucleus, but it is far from certain that Philae will be able to relay further measurements from the surface. The comet’s perihelion occurs 26 million km outside the Earth’s orbit so none of the icy debris being driven from its nucleus is destined to reach the Earth.
The Earth does, though, intersect the orbit of Comet 109P/Swift-Tuttle with the result that its debris or meteoroids plunge into the upper atmosphere to produce the annual Perseids meteor shower. Its meteors diverge from a radiant point in Perseus which lies in the north-east at our star map times and climbs to stand just east of the zenith before dawn. Note that the shower’s meteors appear in all parts of the sky, with many of them bright and leaving persistent trains in their wake as they disintegrate at 59 km per second.
According to the British Astronomical Association (BAA), the premiere organisation for amateur astronomers in Britain, the shower is active from July 23 until August 20 and, for an observer under ideal conditions, reaches a peak of 80 or more meteors per hour at about 07:00 on the 13th. This is obviously after our daybreak, but rates should be high throughout the night of the 12th-13th and particularly before dawn, and respectable on the preceding and following nights too. With the Moon new on the 14th and causing no interference, the BAA puts the Perseids’ prospects this year as very favourable, an accolade it shares with the Geminids shower in December.
The Sun dips 10° southwards during August as sunrise/sunset times for Edinburgh change from 05:16/21:21 BST on the 1st to 06:14/20:11 on the 31st. The duration of nautical twilight at dawn and dusk shrinks from 121 to 89 minutes. The Moon is at first quarter on the 7th, new on the 14th, at first quarter on the 22nd and full again on the 29th.
After the twilit nights during the weeks around the solstice, August should bring (if the weather ever improves!) a chance to reacquaint ourselves with the best of what the summer skies can offer. The Summer Triangle formed by Vega in Lyra, Deneb in Cygnus and Altair in Aquila stands high in the south at our star map times, somewhat squashed by the map projection used. After the Moon leaves the scene, look for the Milky Way as it flows diagonally through the Triangle, its mid-line passing between Altair and Vega and close to Deneb as it arches over the sky from the south-south-west towards Cassiopeia, Perseus and Auriga in the north-east.
The main stars of Cygnus the Swan are sometimes called the Northern Cross, particularly when the cross appears to stand upright in our north-western sky later in the year. The Swan’s neck stretches south-westwards from Sadr to Albireo, the beak, which is one of the finest double stars in the sky. A challenge for binoculars, almost any telescope shows Albireo as a contracting pair of golden and bluish stars.
The brightest star on the line between Sadr and Albireo is usually the magnitude 3.9 Eta. However, just 2.5° south-west of Eta is the star Chi Cygni which pulsates in brightness every 407 days or so and belongs to the class of red giant variable stars that includes Mira in Cetus. Chi is a dim telescopic object at its faintest, but it can become easily visible to the naked eye at its brightest. Last year, though, it only reached magnitude 6.5, barely visible to the naked eye. Now approaching maximum brightness again and as bright as magnitude 4.2 in late July, it may surpass Eta early in August, so is worth a look.
Venus and Jupiter have dominated our evening sky over recent months but are now lost in the Sun’s glare to leave Saturn as our only bright planet as the night begins. Although it dims slightly from magnitude 0.4 to 0.6, it remains the brightest object low down in the south-west as the twilight fades. Indeed, it stands only 5° or so above Edinburgh’s horizon at the end of nautical twilight and sets thirty minutes after our map times, so is now poorly placed for telescopic study. Catch it 2° below-right of the first quarter Moon on the 22nd when Saturn’s rings are tipped at 24° and span 38 arcseconds around its 17 arcseconds disk.
Jupiter reaches conjunction on the Sun’s far side on the 26th while Venus sweeps around the Sun’s near side on the 15th and reappears before dawn a few days later. Brilliant at magnitude -4.2, its height above Edinburgh’s eastern at sunrise doubles from 6° on the 25th to 12° by the 31st.
Also emerging in our morning twilight is the much dimmer planet Mars, magnitude 1.8. On the 20th and 21st it rises in the north-east two hours before the Sun and lies against the Praesepe or Beehive star cluster in Cancer. Before dawn on the 31st, Mars stands 9° above-left of Venus.
This is a slightly-revised version of Alan’s article published in The Scotsman on July 31st 2015, with thanks to the newspaper for permission to republish here.
New Horizons to be first at dwarf planet Pluto
Since our nights are still awash with summer twilight, we may be excused if our attention this month turns to Pluto as it is visited by a spacecraft for the first time.
Pluto was still regarded as a the solar system’s ninth planet when NASA’s New Horizons mission launched in 2006, but it was officially reclassified as a dwarf planet later that year. We now recognise it as one of several icy worlds, and not even the largest, in the Kuiper Belt of such objects beyond the orbit of Neptune.
Pluto is 2,368 km wide and has a system of five moons, two of them only discovered while New Horizons has been en route. The largest moon, Charon, is 1,207 km across and, like Pluto itself, will block the probe’s signal briefly as New Horizons zooms through the system at a relative speed of almost 14 km per second on the 14th. The closest approach to Pluto is due at 12:50 BST at a range of some 12,500 km from Pluto and 4,772 million km from the Earth. It will not hang around, though, since this is a flyby mission and the probe will speed onwards, perhaps to encounter another still-to-be-identified Kuiper Belt world before the end of this decade.
A conjunction of an altogether different type is gracing our western evening sky as July begins. The two most conspicuous planets have been converging over recent weeks and stand at their closest on the 1st when the brilliant Venus passes within 21 arcminutes, or two-thirds of a Moon’s breadth, of Jupiter.
On that evening, Venus is just below and left of Jupiter and sinks from 14° high in the west at sunset to set in the west-north-west about 100 minutes later. Shining at magnitude -4.4 from a distance of 76 million km, Venus appears 33 arcseconds in diameter and 33% illuminated, its dazzling crescent visible through a small telescope or even binoculars. Jupiter, 911 million km away on that evening, is 32 arcseconds wide but only one eleventh as bright at magnitude -1.8.
Over the coming days, Venus slips to the left with respect to Jupiter as both planets drop lower into the twilight. By July 15, Venus is only 7° high at sunset and sets within the hour, while Jupiter is 5° to its right and slightly higher, but unlikely to be seen without binoculars and a clear horizon. By then, too, Venus has closed to 61 million km and its crescent is taller but narrower, 41 arcseconds and 22% sunlit.
It is perhaps surprising that the Earth reaches aphelion, the farthest point from the Sun in its annual orbit, on the 6th. We are then 152,093,481 km from the Sun, 4,997,277 km further away than we were at perihelion on January 4.
Sunrise/sunset times for Edinburgh change from 04:31/22:01 BST on the 1st to 05:14/21:23 on the 31st. The Sun tracks 5° southwards during July to bring the return of nautical darkness for Edinburgh from the night July 11/12. By the last night of the month this official measure of darkness lasts for almost four hours.
The Moon is full on the 2nd, at last quarter on the 8th, new on the 16th, at first quarter on the 24th and full again on the 31st – there is a notion, sadly mistaken, that a second full moon in a month should be termed a “blue moon”.
The white star Vega climbs from high in the east at nightfall to dominate our high southern sky at the star map times. It stands at a distance of 25 light years, being the third brightest star ever seen in Scotland’s night sky after Sirius, which is currently out of sight, and Arcturus in Bootes which stands low in the west by the map times.
Vega is twice as massive as our Sun and 40 times more luminous but only one tenth as old. It has an extensive disk of dusty material, although observations hinting that this contained a planet appear not to be supported my more recent studies.
Vega is the leader of the small box-shaped constellation of Lyra the Lyre, representing a small harp from classical times. It is also the brightest star in the Summer Triangle that includes Deneb in Cygnus, high in the east at the map times, and Altair in Aquila, in the middle of our south-east. Far to the south of Vega is the so-called Teapot in Sagittarius which seems to be pouring to the right as it sits on Scotland’s southern horizon. We have much clearer views of this region of sky, rich in stars and star clusters, if we view them higher above the horizon under darker skies further south.
The red supergiant Antares in Scorpius glowers to the right of the Teapot and lies 13° below-left of Saturn which is twice as bright at magnitude 0.3 to 0.4. Indeed, after the Moon, it is the brightest object low down in the south at nightfall, moving to the south-west by our map times and setting less than two hours later. Currently creeping westwards in eastern Libra, it shows an 18 arcseconds disk through a telescope, set within glorious rings that stretch across 41 arcseconds and have their north face inclined towards us at 24°. Catch the Moon to the right of Saturn on the 25th and to its left a day later.
Of the other planets, Mars has yet to emerge from the Sun’s glare while Mercury hides low in our bright morning twilight as it moves towards superior conjunction on the Sun’s far side on the 23rd.