How different would the night sky have looked in 40,000 B.C.?
What was the most striking difference in the summer night sky in Central Europe in around 40,000 B.C. compared to today's? Was there a noticable precessional shift or any other phenomena around that time?
You are right that one thing that would make a difference is the precessional motion of the Earth which has a period of 26,000 years. The Earth spins on its axis, a bit like a spinning top. If you watch the motion of a spinning top you may notice that the axis that it spins along moves slowly round in its own little circle. This is called precession and is completely analogous to the precession of the Earth. The effect to star gazers is that the position of the north celestial pole wanders in a big circle across the sky (with a period of 26,000 years this is of course not noticeable in any one person's life time). Since 40,000 BC (which is 42,000 years ago) it's gone around about one and a half times! 26,000 years ago (24,000 BC) the skies would have looked the same as they do now (ignoring the proper motions of stars which I'll mention below). 16,000 years before that, the skies would be about half way through the precessional cycle so the north celestial pole would be close to Vega. What this means is that the stars would appear to rotate around Vega during the course of the night (as opposed to going round Polaris like they do now). This also has an effect on which constellation the Sun is in at a given time of year and so which constellations can be seen at night at the different times of year.
Proper motions of stars (as mentioned above) would also cause a change. The stars with the largest known proper motion is Barnard's star which goes 10.29 arcseconds a year. So it would have gone about 120 degrees across the sky in 42,000 years and would be in a completely different area of the sky. A more typical proper motion is about 0.1 arcsecs/year, which would move the typical star about 1 degree in 42,000 years - that's still 2 times the size of the full moon so would make a fairly noticeable change to the familiar constellations.
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