How the sky moves

Rotating Earth


The earth spins on a tilted axis much like a top. therefore;

  • Stars like the sun rise from the east and go west
  • Moving East or west doesn't change the stars you see. however moving North and South greatly changes the stars.
  • An accurate diagram of the earth moving against the stars isn't very helpful in figuring out Where the stars are because, if your looking AT the earth you don't share the same perspective as people ON the earth do.
  • Rather you need a different perspective.

The celestial sphere; a different perspective


The celestial sphere is an imaginary rotating sphere of "gigantic radius", concentric and coaxial with the Earth. All objects in the sky can be thought of as lying upon the sphere. Projected from their corresponding geographic equivalents are the celestial equator and the celestial poles.

This sphere can be reduced to two dimensional maps; much in the same way that we can map a three dimensional map of the earth into a two dimensional map.

  • If the map shows all the stars, the northern reaches will be highly distorted
  • If we break the map into segments we can minimize distortion.

Most importantly, from our perspective the Earth doesn't seem to spin. Rather The celestial sphere seems to spin. it appears to spin (in the North) about the bright star polaris (which is very close to the north celestial pole). Thus we need to orient our two dimensional map to account for the movement of the earth. Keep in mind that our lattitude, north and south will determine how much of the celestial sphere we see.

if we live between the equator and the north pole, (most of the worlds population) in the far north we actually see the north stars after they have spun below the north star. Because of this and depending on how far north we live, we might see some of the constellation as the spin around the north star. These constellations are circumpolar (or visible all year round). (see activity).

people living further south will see more stars since they can see stars near the south celestial hemisphere.

How the sky rotates during time and date


The celestial sphere rotates every 23 hours 56 minutes. The sun rotates every 24hours, thus moving against the backdrop of the stars. Thus both time and date come into play when determining what the stars will look like.

As the night get later the sky will rotate about the north star. Each night the sky will look four minutes later each night, with the evening sky swallowing early rising constellations and the morning sky, retreating and allowing you to see new constellations.

Thus when we're learning constellations. we first start by learning the North constellations, that in the most populate area of the world is visible year round. Next we learn each season; spring, summer, fall and winter. These will be the constellations visible shortly after dusk (when most people observe). If you still up later, you can begin to see the next season's stars. The advantage of staying up late to see the next season's stars is that you can see the same sight for a while 3-6months.