Monthly Archives: September 2015
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.