the stars : Twilight zone

Although the sky is never truly dark in summer, now is the time to spot the beautiful and elusive noctilucent clouds.

Stargazing is a far more pleasant activity at this time of year - but there's one big disadvantage: it never gets truly dark. Although the Sun dips below the horizon, it is never far away. Strictly speaking, the period between mid-May and mid-July is one of "astronomical twilight", which is defined as a time when the Sun is never more than 18 degrees below the horizon.

The brightest stars and planets, however, are still easy to pick out. Look for Venus setting, about an hour after the Sun, towards the northwest. Mars - target of Nasa's Mars Pathfinder spaceprobe mission on 4 July - is also visible after sunset, looking like a reddish "star" in the constellation of Virgo. Jupiter is putting in an appearance too, rising at about 22.30 BST mid-month.

Mars and Jupiter flank a fascinating part of the sky which unfortunately never rises high enough in our northern latitudes to be seen to its advantage.

Look due south towards the constellations of Scorpius and Sagittarius, and you are facing the densely-packed downtown regions of our star-city: the Milky Way galaxy.

Sagittarius looks less like a valiant archer than a teapot, complete with "steam" emanating from a "spout". The "steam" is in fact a huge cloud of stars - one of several you will see if you look towards this most heavily populated region of the Galaxy. Next door Scorpius is dominated by blood red Antares ("rival of Mars"), but you have to travel south to Mediterranean latitudes at least (where the twilight is also less of a problem) to see the constellation in all its glory. From there, you will see that Scorpius is one of the only constellations that looks anything like its namesake - a huge, curling scorpion with a mighty sting in its tail.

Twilight actually enhances one of the phenomena you only get to see in summer months - noctilucent clouds. These feathery clouds look delicate and beautiful against an eggshell blue sky, illuminated by the low angle of the Sun. Appearing late at night (hence their name), they cannot be confused with cirrus clouds or star-clouds in the Milky Way; their silvery blue colour and pearly glow make them look quite unlike anything else. Noctilucent clouds form very high up in the atmosphere, some 80km (50 miles) above the ground. Like conventional clouds, they are the result of the condensation of water vapour on to tiny nuclei - but rather than terrestrial dust, the nuclei for noctilucent clouds are minuscule fragments of meteors that have entered the atmosphere.

There have been stunning displays of noctilucent clouds over the past two summers, and we should be due for more in 1997. However, this may be the last year for a good showing. The clouds form when temperatures in the upper atmosphere are sufficiently low for water vapour to condense - and solar activity can prevent this. As the Sun builds towards its stormy "sunspot maximum" over the next three or four years, its enhanced output of short-wavelength radiation will raise temperatures in the upper atmosphere to an extent that noctilucent clouds cannot form. So now is your chance to spot one of the rarest, most beautiful and most elusive of sky sightsn

Diary (all times 24-hour clock, BST)

July

4 19.40 New Moon; Earth at aphelion (furthest from Sun in its elliptical orbit: 152 million km). Pathfinder due to land on Mars.

12 22.44 Moon at first quarter

20 04.21 Full Moon

26 19.29 Moon at last quarter

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