Observational Astronomy - Spring 2014



Celestial Coordinate Systems




Multimedia files

Here is an animation of the orbital motion and the sky of the planet Sideron. This strange planet has a precise 4:1 resonance between its orbital motion and its rotation rate, so that each year is comprised of 3 solar days and 4 sidereal days. This makes it much easier to see the difference between solar time and sidereal time than in the case of the Earth, where each year is 365.25 solar days and 366.25 sidereal days. Sideron also has four constellations which are geometric figures, imaginatively referred to as the Triangle, the Square, the Pentagon, and the Hexagon. These constellations are exactly 90 degrees apart on the sky, and all of these constellations are made up of red giants that are so bright they can be seen in the daytime. The left hand figure of the animation shows the orbital motion, and the right hand shows the sky visible to the observer (the little red stick-figure) when looking straight up. Note three things:

(1) During one year, the sun rises and sets three times - i.e there are three solar days.

(2) During the same year, the constellations rise and set four times - i.e. there are four sidereal days.

(3) While the sun and stars rise in the east and set in the west, the sun moves from west to east with respect to the stars.