Today being the winter1 solstice. I found myself explaining exactly what the solstices are, and how the earth’s inclination changes relative to the sun. During the course of this, I threw together a Celestia script to show earth from the sun’s perspective over the course of a year in one-day intervals in order to illustrate how the poles move relative to the sun. I thought maybe somebody else might find it useful.
This animation shows earth from the sun’s perspective over the course of a year, with the solstices and equinoxes noted.
The important thing to understand is that earth’s axial tilt doesn’t actually ever change2. The north and south celestial poles are fixed2 points in the sky, they don’t move over the course of a year. The inclination of the poles only changes from the Sun’s perspective because earth orbits around it, which means that on the summer solstice the south pole1 is pointed towards the sun and on the winter solstice the north pole1 is pointed towards the sun. But the north pole (and the big arrow in the animation) is always pointing at the star Polaris, and the south pole always points at the south celestial pole – a spot roughly in the center of a triangle defined by the two magellanic clouds and alpha centauri. At the summer solstice this animation is looking in exactly the opposite direction as it was at the winter solstice. This is why the background stars rotate – we’re tracking earth from the sun’s perspective and it’s rotating around the sun.
I’ve also done a variation which shows the earth’s rotation on the equinoxes and solstices:
1 Herp derp, if you’re in the northern hemisphere it’s the other way around, and the winter solstice is in December, and the summer solstice in June. This is written from the pespective of the southern (aka correct) hemisphere. Deal with it. I do, all the time.
2 OK, nerds, fine, yes – earth’s axial tilt has precession and the earth’s poles do actually change where they’re pointed – over long periods of time, but these timescales are measured in thousands of years, and are not relevant to what we’re talking about in the scale of a human lifetime – it’s totally fair for a human to say that the north pole always points in the same direction, with an implicit “within my lifetime” tacked on the end.