We are at the highest point in London, gazing forlornly at the cloudy sky.

I have come to Hampstead Observatory, at the invitation of the promoters of the science fiction movie Stargate to have an astronomy lesson. Astrology, they said, but no matter. Local astronomer Douglas Daniels and I can only lament the light pollution and the rotten summer we're having.

August is supposed to be when our skies are the clearest. We could be enjoying the spectacular meteor shower caused by the passing of the comet Swift-Tuttle right now - if it weren't for these clouds.

But as Doug puffs his pipe and tries to explain Einsteinian relativity, a bank of clouds lift and a celestial body glimmers into view. "Oh look", I say, "there is one star out tonight".

Doug looks up. "That's no star... that's Jupiter! Quick, to the telescope!"

And we scramble back into the observatory, where the instrument is angled accordingly and I am afforded a 500x magnifcation of the Solar System's mightiest planet.

I am transfixed. I can discern the dust clouds that surround the gas giant, see its slightly squashed poles - caused by centrifugal force, for though Jupiter is 1317 times the volume of earth, its day is a mere ten hours long. Most amazingly, the four Galilean moons - volcanic Io (above), icy Europa, giant Ganymede and lonely, distant Callisto - are as clear as day. Amazing to think of them out there... and then the clouds envelop our own planet again and all is extinguished.

The observatory truly is a London gem. The telescope is a "six-inch Cooke refracting" model, reasonably hi-tech, but its home still has the cosiness of a hobbyist's shed. It belongs to the Hampstead Scientific Society, founded in 1899, during that great late-Victorian age of gentlemenly enquiry.

Enchanting to think of Doug (a bit of a gem himself) smoking in the adjoining room, comparing slides with fellow scientists on dark nights. The observatory is open to the public for the partial lunar eclipse on 16 August. You can have a go on the telescope for free. Outside summer time, on clear Fridays and Saturday nights, it is open 8pm-10pm. Doug noted sadly the thin trickle of school-children who now come to enjoy it. Surely outer space transcends generational differences? Apparently not.

Sadly, I was not thinking of Stargate, but of the ending to Mikhail Bulgakov's novel war novel, The White Guard. "Everything passes away - suffering, pain, blood, hunger and pestilence. The sword will pass away too, but the stars will remain when the shadows of our presence and our deeds have vanished from the earth. There is no man who does not know that. Why, then, will we not turn our eyes towards the stars? Why?"