Night, our window on the universe…

The night is our window on the universe. In the daytime the blueness of the sky prevents us from seeing into space. The blue light is caused when the intense sunlight is scattered by oxygen molecules in the upper atmosphere. Beneath this atmospheric layer, which is a few tens of miles deep, the situation is somewhat like looking through net curtains: if a searchlight were illuminating the fabric, it would be impossible to see anything apart from the searchlight itself. By day we can see only the Sun and the Moon (if it has risen), which is so much fainter by comparison that many people think incorrectly that it is invisible during the day. But when we switch off the searchlight, the surrounding landscape can be discerned: the stars, the planets…

If the Earth did not have an atmosphere, the stars would be visible in broad daylight. The dozen Apollo astronauts who landed on the Moon experienced just such a spectacle, despite the dazzle of sunlight. However, things could be worse. The planet Venus is shrouded in a dense atmosphere and the surface pressure is one hundred times that of the Earth's atmosphere. This blanket is so thick that not a single star is visible at night; even in daytime the Sun itself is invisible and only a feeble glimmer of light reaches the surface.