Science: Stars and Planets: June

THE BRILLIANT "Evening Star" in the north west after sunset is the planet Venus. For months Venus has been a celestial beacon in the evening sky, but it reaches its greatest distance from the Sun this month, and then plunges rapidly down towards the horizon. By mid-July it will no longer be visible.

So take this opportunity to observe Venus while you can. A small telescope will show its phase, looking like a half-moon. Like all the planets (and the Moon), Venus is lit up by the Sun and its shape seems to change depending on how much of the illuminated region we see. Venus is appearing larger and larger all month as it approaches the Earth, and by the end of June its shape should be visible in good binoculars.

Even though Venus is the closest planet to Earth, even the most powerful telescope will reveal little more than Venus's phase. That's because the planet is shrouded in an unbroken veil of clouds, made of sulphuric acid droplets.

Russian and American scientists have spent billions of dollars sending space probes to Venus, to check out what lies beneath the clouds. Orbiting craft carrying radar sets have mapped its hidden surface, while landers have checked out the terrain in detail.

They have revealed what is a hell of a world: hundreds of thousands of volcanoes erupt streams of lava that fill all the low-lying plains, and also belch carbon dioxide and sulphurous fumes into the atmosphere. The dense carbon dioxide "air" creates an intense greenhouse effect, which raises the surface temperature to 465C - higher than the hottest domestic oven.

But new research suggests that Venus may sometimes expose its normally hidden surface. Mark Bullock and David Grinspoon, of the University of Colorado at Boulder, calculate that the components of the sulphuric acid clouds gradually dissipate; the sulphurous gases react with carbonate rocks and are absorbed into Venus's surface, while water from the acid disappears into space. If the volcanic activity that creates the clouds were to lull for 400 million years, Venus's modest veil would disappear altogether.

Any hypothetical inhabitants of Venus would then have a chance of viewing the Universe beyond their planet - including their sister planet, the Earth. But without clouds to reflect much of the Sun's heat back into space, Venus's greenhouse surface would be a hundred degrees hotter even than it is today.

The thickness of Venus's current cloud layers indicates that it's been active for much of the past 30 million years, so we must wait hundreds of millions of years before the veils are likely to lift again.

But the last major outburst of vulcanism was about 500 million years ago. It covered most of Venus with fresh lava, burying older geological features. When this outburst died down - and before the present episode of vulcanism - perhaps Venus exposed her surface briefly to view. If there had been any astronomers on Earth in the era of the dinosaurs, they might have seen Venus as a red-hot planet, literally glowing with its own heat in the darkness of space.

What's up this month

Venus lies in Cancer, and in 13 June passes close to the star cluster Praesepe (the beehive) - a fine sight as seen with binoculars. Mercury makes a rare appearance this month. It always sets soon after the Sun, and only near the summer solstice can it be above the horizon at 11pm, the time of the chart. But the summer twilight will make Mercury difficult to spot; make sure you have a clear north-west horizon, and use Venus and Castor and Pollux (see chart) to locate Mercury.

The "red planet" Mars lies to the south, and is gradually fading as the Earth draws away.Another red giant, Arcturus, lies directly above Mars; it's the principal star in the constellation Bootes (the herdsman). In the morning sky, look out for brilliant Jupiter and the fainter Saturn.

Heather Couper and Nigel Henbest