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This chapter is out of order. Traditionally, books on skygazing are supposed to start with the Moon, then advance to the planets and only then jump off to the far-off stars. Although there is a certain logic to starting from points close by and moving outwards, I started at the stars because they stay in one place, at least in regard to each other. Their positions are fixed on the invisible celestial sphere as the result of the vast distance between them and us skygazing from the Earth.
Scorpio is going to look like Scorpio, tonight, next month or a hundred years from now. Comforting, isn't it? You can go out and buy a sky map today and it will be good for a lifetime. Well, almost. Actually most of the better sky maps are dated in 50 year intervals called "epoch". We are currently using the epoch 2000.0. Nevertheless, the changes of star positions over a 50-year period are too small to be noticed with only the naked eye. The stars, for our purposes at least, remain in the same locations as always on the celestial sphere.
Already thousands of years ago, it was noticed that certain "stars" were not fixed. Over a period weeks and months they would wander back and forth along roughly the same path as the Moon and Sun – the ecliptic, and were appropriately called planets, from the Greek word for "wanderers". These five bright, restless stars were named Mercury, Venus, Mars, Jupiter and Saturn.
Then felt I like some watcher of the skies when a new
planet swims into his ken ...
John Keats