Venus and Jupiter witnessed in dawn rendezvous
WITH no planets on view, and with large areas of the southern sky devoid of bright stars, the evening sky at our star map times may not be the most exciting of the year. Contrast it, for example, with our February evenings when Orion stands proudly in the heart of a southern sky peppered with bright stars and sparkling constellations.
As a taster of things to come, Taurus and the Pleiades are already climbing in the east at the map times, and even Orion is clambering above the eastern horizon. Wait just six hours more until the morning hours and that same vista from February is spread out above us in glorious preview. And more besides, for as the sky turns towards dawn we have the spectacle of the two brightest planets in close conjunction in the eastern sky.
Venus, continuing as a brilliant morning star of magnitude -4.0, meets up with and overtakes the giant planet Jupiter as both track eastwards against the stars of Virgo. Venus stands 3above-right of Jupiter tomorrow, the latter’s magnitude of -1.7 putting it a shade brighter than the brightest star Sirius. On Friday morning, Venus passes only 0.6, a little more than one Moon-width, above-left of Jupiter as the pair rise almost due east just before 04:00 as viewed from Edinburgh. They climb to stand 25 high in the south-east at sunrise when Venus, and perhaps Jupiter too, might still be glimpsed by the naked eye in a crystal-clear sky. Viewed through a telescope on that morning, Venus shows a uniformly dazzling gibbous disk, only 13 arcseconds in diameter and 81 per cent illuminated, while the cloud-banded disk of Jupiter is 32 arcseconds wide.
Following their conjunction, Venus races ahead of Jupiter by about one degree each day to pass 4above-left of Virgo’s brightest star Spica on the 17th and end the period and end November 26below-left of Jupiter. By then, Venus will have pulled to within 4 of the fainter planet Mars, glowing with its usual reddish hue at magnitude 1.7. Tomorrow, though, it is Mars that lies 3above-left of Spica as the two (Spica being the brighter) lie deeper in the brightening twilight some 18below-left of Jupiter and Venus. Telescopically, Mars appears as a tiny spot less than 4 arcseconds across. As the slender crescent of the waning Moon sweeps through the field, it stands above-right of Jupiter on the 9th, between Venus and Spica on the 10th, and just below Mars on the 11th.
The first planet to appear each night, however, is Saturn, which brightens from magnitude 0.1 to -0.1 and stands below, and almost in line with, Castor and Pollux in Gemini. It rises in the north-east less than ten minutes after our map times and climbs to pass high on the meridian more than eight hours later. It stands alongside the Moon on Wednesday night when a telescope shows it as a slightly squashed disk 19 arcseconds across, surrounded by rings 43 arcseconds wide, with their southern face inclined towards us by 22. The Moon lies above Saturn, and close to Pollux, again on the 30th. Mercury reaches its greatest distance east of the Sun, 22, on the 21st but lies too deep in our evening twilight to be seen from our latitudes.
The years of spectacular displays by the Leonids meteor shower are probably behind us for a few decades. The shower may still produce a few swift meteors between the 15th and 20th as they slash away from the Sickle of Leo which rises in the north-east late in the evening and climbs high into the south by dawn. Peak activity is expected on the 17th, but there are predictions of bursts of activity on the 19th as the Earth cuts through ribbons of Leonid meteoroids laid down by the stream’s parent comet as long ago as 1333 and 1733.
Sunrise/sunset times for Edinburgh change from 07:20/16:32 GMT today to 08:18/15:45 on the 30th as the duration of nautical twilight at dawn and dusk increases from 83 to 93 minutes. The Moon reaches last quarter on Friday, is new on the 12th, at first quarter on the 19th and full on the 26th. The evening sky may be lacking in spectacle this month, but at least one of the celestial showpieces is at its best, almost overhead. The famous spiral galaxy in Andromeda, M31, glows as a third magnitude oval smudge to the unaided eye in a reasonable sky, and is usually obvious using binoculars. To locate it, first find Alpheratz, the top-left star of the Square of Pegasus, and then carry a line to the left to the equally bright Mirach and Almach (see our south map). Star-hop a couple of fainter stars above Mirach and M31 lies alongside the second of the two. If at first you do not see it, glance slightly away and it may well pop into view in your more sensitive peripheral vision. Photographs show M31, named for its place in Charles Messier’s catalogue, to measure some 3 by 1, though only the brighter central regions are visible to the naked eye.
Following a recalibration of the extragalactic distance scale by the Hipparcos satellite, M31 is now thought to lie 2.9 million light years away, be some 250,000 light years in diameter and have a mass 1.23 trillion times that of our Sun. As such, it may be about two-thirds as massive as our own Galaxy, the Milky Way, which it is approaching at about 100 km per second.
A sister galaxy, another member of the Local Group of galaxies to which the Milky Way belongs, is M33 in Triangulum. A smaller, less inclined and more open spiral, it is a sixth magnitude fuzz that is hard to spot in anything but ideal conditions. Look for it 40 per cent of the way from Mirach towards Hamal, the brightest star in Aries. M33 is thought to be three million light years distant, just beyond M31.
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Weather for Edinburgh
Saturday 25 May 2013
Temperature: 6 C to 17 C
Wind Speed: 13 mph
Wind direction: West
Temperature: 9 C to 16 C
Wind Speed: 14 mph
Wind direction: South west