St. George Decoded - An Astronomy Lesson (contd.), page 9
It wouldn't take long for our farmer to notice that just before dawn on each equinox the same constellations of stars were in the eastern sky. That is, one constellation for the spring equinox, another for the autumn equinox. We say that the constellation 'carries' the sun into the equinox sky. No doubt in educating the next generation in farming lore, the farmer mentions these stars, draws a picture, or marks them on his alignment stone in some way.
A couple of generations down the line something is going wrong. The pattern of stars in relation to the sun has changed. This is not an imperceptible difference, it's an angle of about 1° over 72 years (actually 71.6, but 72 is close enough). That doesn't sound like much, but the full moon is about half a degree in width, so the position of the stars, in relation to the sun at spring equinox, will have changed by two widths of the full moon. This is not an insignificant amount.
Such a difference is noticeable over the generations if they bothered to draw pictures of the sun in relation to its equinox constellation. The fainter stars will now be blotted out by the increasing light from the rising sun, and the brighter stars will be rising later (see Appendix 3 for a simulation). If they had paired rising stars in the east with setting stars in the west, the eastern ones will one day disappear into the sun's light long before the western stars have set. In today's world, mariners who still use chronometers and star tables (showing rising and setting times of prominent stars) to fix longitude know that because of precession their star tables have to be republished from time-to-time with the latest times. The stars are rising later.
The sky shots in Figure 4 "Spring Equinox 100BC", and Figure 5 "Spring Equinox AD 500" below, from RedShift, show the movement of the background stars against the sun on the spring equinox just before the sun rose on the eastern horizon. The constellation is Pisces, shown at 100 BC and 500 AD. In relation to the sun, Pisces has changed position over 600 years by about 8°, or 16 times the width of the full moon.
Now look at the year 2000 AD, and we'll see a radical change in position, see Figure 6 "Spring Equinox 2000 AD" below. Pisces has now descended in the sky quite noticeably and the sun will soon have Aquarius carry it up into the dawn sky on the spring equinox. This is an effect of the Earth's wobble and is called the 'precession of the equinoxes'. It will take nearly 26000 years for the sky to look the same on any equinox dawn.