How the Sky Moves

The North Star, Polaris, lies very near the rotation axis of the celestial sphere, right above the Earth’s north pole.  Since it’s almost right on the north celestial pole, Polaris appears to stay nearly fixed in the sky all night and all year, just as the Earth’s north pole stays fixed as the rest of the Earth’s surface moves around it.  Any other star on the celestial sphere south of Polaris rotates in circles of increasing diameter about the rotation axis.  It’s the same with the south celestial pole.  Stars above the Earth’s equator trace out the largest circles around the sky during their daily motion across the celestial sphere as the Earth turns.  And south of the equator, stars trace out circles with smaller apparent diameters as they lie closer to the south celestial pole.   The image below gives you a better idea of how the stars appear to rotate during the day.

Star trails caused by the apparent rotation of the celestial sphere around a celestial pole

Star trails caused by the apparent rotation of the celestial sphere around a celestial pole (from http://antwrp.gsfc.nasa.gov/apod/ap051220.html)

Like the stars and planets, the Sun also appears to move on the celestial sphere.  If you measure the time when the sun is highest in the sky, you will find it takes exactly 24 hours for the Sun to move all the way around the celestial sphere and return to its highest point.  In fact, that’s how we define a “day”, or what astronomers formally call a solar day.

How about the stars?  If you go out at night and select a star to observe, and measure its position on the celestial sphere, you will find it takes 24 hours to move all the way around the sky and get back to the same spot.

Well, almost 24 hours.

If you measure accurately, you’ll find it takes only 23 hours and 56 minutes for a star to get back to the same position in the sky as it was the night before.  That’s because, during the day, the Earth revolved around the Sun by 1/365 of its orbit.  So each day, you look in a slightly different direction in space, and this causes every star to appear to rise 4 minutes earlier each night.  In two weeks, the star rises about an hour earlier; in one month the star rises 2 hours earlier, and in 12 months, it appears to move all the way around the sky back to the position at which you first saw it the previous year.

This apparent motion where the stars rise a little earlier each night, which is caused by the Earth’s revolution around the Sun, explains why the stars you see in the night sky in each season are different than the stars you saw during the last season.

Now that you have some idea of the layout of the sky and how it appears to move each day and during the year, let’s have a look at what you can actually see in the night sky each season.

As mentioned in the introduction to this guide, your first pass through this section may not be completely clear.  It takes time and a little thought and experience to figure this out.  Be patient.  As you learn the sky, these concepts will become clearer to you.