The Sky in April: Venus and Mercury prepare to team up in evening twilight

If you have been paying attention, you will know that Venus is now a brilliant evening star low in the western sky at nightfall. Indeed, given an unobstructed view in that direction, you could hardly miss it.

The sight of such a bright light low in the sky is likely to generate reports of a UFO and some of the media, as usual, are all too happy to propagate these, often accompanied by shaky and/or out-of-focus images and video of the "mysterious craft". Even some supposed exports in UFO phenomena appear puzzled by the sightings, though whether this is through ignorance or mischievousness is unclear.

We are stuck with Venus low in our western to west-north-western sky for another four months or so, so we had better keep our skeptical hat handy. Just now, though, Venus is providing an ideal pointer to the other evening star, Mercury, as the little innermost planet puts on its best evening show of 2010.

Venus's diameter of 12,104 km is only 652 km less than that of the Earth, but its scorching hot volcanic surface, crushing atmospheric pressure, and sulphuric acid clouds make it totally hostile to life. Mercury airless moonlike globe is only 4,879 km wide, which makes it the smallest of the solar system's (now) eight planets.

Looking west 40 minutes after sunset tonight, and if the weather cooperates, Venus appears 9 high with Mercury 4 away to its lower-right. At magnitude -3.9, Venus should be obvious but Mercury is considerably fainter at magnitude -1.0 and it may be best to pick it up through binoculars until the twilight has faded enough for it, too, to be visible to the naked eye. Over the coming weeks Venus climbs very slowly higher from night to night while Mercury climbs to stand alongside Venus before falling back towards the Sun and inferior conjunction on the Sun's near side on 28 April. All the time, though, Mercury is fading, so the sooner we glimpse it, the better.

The two evening stars are closest on the 3rd when Mercury is magnitude

-0.6 and 3 to the right of Venus. A week later, on the 10th, Mercury is less than half as bright at magnitude 0.4 and lies 4 right of Venus. By the 15th, Mercury has dimmed to magnitude 1.4 and lies 7 below-right of Venus and 2 above-left of the very young and slender crescent Moon. Both Mercury and the Moon set almost two hours after sunset but, because of Mercury's faintness and their low altitude, we will probably need binoculars to glimpse the two.

Mercury continues to dim sharply and will soon be lost, but the Moon, Earthlit and impressively thin, climbs to stand 5 above Venus and 7 below-right of the Pleiades in Taurus on the 16th.

Taurus and Orion are slipping below the western horizon at our star map times, but both are on view in the south-west at nightfall at present as Mars transits high in the south, a few degrees below and left of Castor and Pollux. By 23:00 BST Regulus and the Sickle of Leo are crossing the meridian as the Plough sails overhead.

Saturn, creeping westwards in the western part of Virgo stands due S at the map times and is the last of our night's planets, at least until Jupiter, magnitude -2.1, begins to emerge from our bright morning twilight in the east towards the month's end.

Our chart has Mars in the west-south-west, with an arrow showing it tracking 10 eastwards to pass 1.2 north of the Praesepe star cluster in Cancer on the 16th. As the night progresses, Mars sinks through the western sky and sets in the north-west more than five hours later.

The planet recedes from 152 million to 192 million km and fades from magnitude 0.2 to 0.7 during April, dipping below Procyon in brightness for the first time since last October. Its small disc contracts from 9 to 7 arcseconds in diameter so that telescopic views of any surface markings are increasingly difficult. Look for Mars near the first quarter Moon on the 21st, with Praesepe between them.

The Moon was full this morning and reaches last quarter on the 6th, new on the 14th, first quarter on the 21st and full again on the 28th. The Sun, meanwhile, climbs 10.6 northwards in the sky as sunrise/sunset times for Edinburgh change from 06:44/19:51 BST on the 1st to 05:32/20:50 on the 30th. Nautical twilight at dawn and dusk grows in duration from 84 to 105 minutes as the Sun makes an increasingly shallow angle with the horizon as it rises and sets.

Saturn stood directly across from the Sun in the sky at opposition on 22 March, so it is well placed throughout the night. As the brightest object in the lower south-eastern sky at nightfall, it climbs into the south by map times where we find it 23 above and right of the Moon tonight, with the latter alongside Virgo's leading star Spica.

Saturn lies a little to the east of Zavijava, also called Beta Virginis even though it is only the fifth brightest star in the constellation; Spica, not surprisingly, is Alpha.

Spica is magnitude 1.0 while Saturn is slightly brighter, though fading from magnitude 0.6 to 0.8. Some of this fall is due to it receding from

1,275 million to 1,307 million km, but more is down to that fact that, as seen from the Earth, the rings close up from a tilt of 2.8 to only 1.9. At midmonth, the rings extend for 44 arcseconds but are a mere 2 arcseconds thick around Saturn's 19 arcsecond globe. Look for Saturn directly above the Moon late on the evening of the 25th.