Orion readies to fade as Jupiter joins Saturn and Mars in evening sky

Published on 30/03/2006 02:32

ORION stands in the south-west at nightfall at present but will soon be swallowed up in the twilight as the days lengthen and the sky turns westwards. By our star map times, only Betelgeuse at Orion's shoulder is visible above the western horizon as the southern sky is taken over by Leo and Virgo and the other groups of spring.

Saturn is sinking westwards by our map times, but the planet is still high in the south at nightfall, shining brightly against the dim stars of Cancer and ideally placed for evening observation. Mars is visible in Taurus in the west, but it is one twenty-fifth as bright as it was last autumn, and so much further away that its disk is too small for useful study through a telescope. The giant planet Jupiter, with its four bright moons, its cloud belts and Red Spot, is always worth a look, and it now rises in the east-south-east before midnight. Brighter than any star, it moves to the low south-western sky before dawn as the morning star, Venus, blazes very low down to the south of east.

The only bright planet not on show for us during April is Mercury. It lies too low in the morning twilight as seen from our northern latitudes, despite the fact that it stands 28 west of the Sun on the 8th.

The Sun climbs more than 10 higher in our noon sky during April as sunrise/sunset times for Edinburgh change from 06:44/19:51 BST on the 1st to 05:32/20:50 on the 30th. The duration of nautical twilight lengthens from 84 to 105 minutes. The Moon is at first quarter on the 5th, full on the 13th, at last quarter on the 21st and new on the 27th.

As it has done for a month or two, the young Moon makes another spectacular entry to our evening sky over the coming days as its northwards and eastwards motion against the stars presages that of the Sun over the coming months. It is only 33 hours old and the most slender of crescents when it stands low in the west as the evening twilight fades today. It should be high enough to see against a dark sky tomorrow evening, with the dark side of its face illuminated by the almost-full Earth in the lunar sky. Earthshine should also be obvious on Saturday when the Moon stands less than 3 below and right of the Pleiades star cluster in Taurus. Use binoculars for the best view and watch the Moon creep closer to the Pleiades as they sink towards the north-western horizon. In fact, as seen from the United States a few hours later, the Moon hides many of the cluster's stars.

Next Monday sees the Moon alongside Mars and even closer to Elnath, the star at the tip of the northern horn of Taurus. At magnitude 1.2, Mars is slightly brighter than Elnath and its reddish colour contrasts with the bluish tinge of the star. The arrow on our chart shows the eastwards motion of Mars during April as to moves to lie just 0.5, or one Moon-breadth, below and right of the star Mebsuta in Gemini on the 30th. By then it will have dimmed further to magnitude 1.5 and the Moon will be visible once again as an earthlit crescent close to Elnath.

The Moon lies near Pollux in Gemini on the night of the 5th and close to Saturn on the 6th. Now edging eastwards again, Saturn lies 3 to the west of the Praesepe cluster in Cancer, also known as the Beehive or as M44 after its position in Charles Messier's 18th-century catalogue (the Pleiades are placed next in the catalogue as M45). Binoculars show Praesepe better and may also reveal that Saturn is slightly elongated because of its rings. Through a telescope at mid-month, those rings spread across 42 arcseconds with their south face inclined at 20 to our view, while Saturn's creamy disk itself is 19 arcseconds wide.

As Saturn progresses around its 29-year orbit of the Sun, the tilt of the rings is decreasing and the planet is heading southwards and lower in our sky, so make the best of it now. In fact, the rings will lie edge-on to the Earth in 2009 when, because they are so thin, they disappear for most observers. Unfortunately, the edge-on phase in 2009 occurs with the planet too close to the Sun to be seen from our latitudes.

The Moon passes near Regulus in Leo on the 8th and close Spica in Virgo on the 13th. About 52 million light years away, and situated between Vindemiatrix, further north in Virgo, and Denebola in Leo's tail, lies the heart of the Virgo cluster of galaxies. Consisting of more than 1,300 galaxies, and including our Milky Way as an outlying member, the cluster includes several fuzzy objects that found their way into Messier's catalogue and are visible through small telescopes, though not when the Moon is bright.

The giant planet Jupiter stands 8 left of the Moon on the 14th-15th and a similar distance above the Moon on the next night. At magnitude -2.4 to -2.5, brighter than any star, Jupiter rises about one hour before our map times and is unmistakable as it crosses our low southern sky during the morning hours. At present the planet lies almost 3 east (left) of the wide double star Zubenelgenubi in Libra, but it tracks westwards during April to pass 1 north of the star late in the month. Its cloud-streaked disk appears 44 arcseconds across at mid-month.

Venus is a brilliant morning star (magnitude -4.3 to -4.1) that hugs the eastern to south-eastern horizon as the dawn twilight floods the sky. Even though it was furthest from the Sun in the sky last Saturday, it rises only 72 minutes before Edinburgh's sunrise tomorrow and a brief 51 minutes before by April 30.

The European Space Agency's Venus Express probe is due to enter orbit around Venus on the 11th. Its target is the most complete study to date of the deep Venusian atmosphere. Mostly carbon dioxide, this is responsible for a runaway greenhouse effect so that the surface roasts near 477C under a pressure 90 times greater than on Earth. Venus Express hopes to peer through Venus's clouds of sulphuric acid to discover whether volcanoes are still active on the surface and why Earth's sister planet should be so alien and hostile.