A solar eclipse, the passage of the Moon across the face of the Sun, is a rare and exciting event. The Moon's shadow brings a sort of momentary night; the blue of the sky vanishes as sunlight is no longer scattered by the atmosphere; the stars come out. Daedalus now suggests a way of making an artificial solar eclipse.

His idea is to obstruct the Sun's disk with a high-flying circular shutter. To block most of the blue atmospheric scattering above a given point, it needs to be about 20 km across. It would subtend the Sun's diameter at some 2,000 km altitude — its upper limit of height. Clearly, it has to be in orbit. Daedalus recalls the scheme for a ‘solar sail’ spacecraft deploying a vast sail of aluminized polymer film. The pressure of solar radiation on its sail would propel it. His ‘Eclipsat’ will draw on much the same technology.

Eclipsat will resemble the inner tube for a bicycle tyre 20 km across, folded up small and immersed in a special viscous monomer fluid. Released in orbit, the toroidal tube will inflate under its internal pressure, and will slowly unfold into true circular form. The viscous monomer will be spread out like a soap-film as the tube unfolds, ultimately forming a perfect disk within it, 20 km across and a few micrometres thick. Solar radiation will soon polymerize it to a solid film. Sadly, no dye in the polymer could make such a thin film opaque. It will have to be coated with metal, possibly from a nearby pyrotechnic evaporator released from the same rocket.

The ideal orbital height for Eclipsat is perhaps 1,000 km. It will then produce a solar eclipse every 105 minutes along a track about 5 km across and maybe 8,000 km long. At any point on the track, each eclipse will last only 2 seconds; but their steady repetition will soon yield detailed information on solar prominences and their evolution with time, starlight shifts, and so on. Observers will see a giant shadow racing towards them at about 7 km per s, engulfing them in a brief night.

At other points of its orbit, the metallized disk of Eclipsat will act as a mirror. It will reflect the Sun down onto a narrow track on the dark side of the Earth, giving the novel phenomenon of an ‘anti-eclipse’. Observers will see a band of brightness racing through the night towards them; for 2 seconds it will be bright, dazzling, sunlit day; then darkness will descend again. It will be an awe-inspiring spectacle.