The nearest world to the sun is the little planet Mercury. It is not much bigger than our moon and very few people have seen it, although it shines quite brightly in the sky at certain times. In the British Isles and other northern countries it is best seen during the weeks of spring, when it comes for a few evenings into the western sky after sunset. It is always up there in the sky somewhere near the sun, but we can only see it when the sun is below the horizon and the planet is at its greatest distance from the dazzling sunshine.

Mercury travels round the sun every eighty-eight days. It turns the same face towards the sun always, and for this reason it is intensely hot on that side. On the other side, strangely, where there is endless night, it is the coldest world of the solar system. Mercury would be a very unfriendly world for space-travellers.

Venus is a much larger planet, very much like the earth in size, but there are many mysteries about it. It shines very brightly in the night sky, or in the early morning sky before sunrise. It is covered with a thick layer of cloud. We can never see the surface of Venus and we do not even know how long it takes to turn round on its axis. Some space-probes have already been sent towards Venus and these will soon be followed by others. We have great hopes that one day we shall know what kind of a world this beautiful but mysterious planet really is.