For thousands of years, people have wondered about the stars. What are they? How far away are they? What is the meaning of their arrangement in the sky? How do their locations in the sky change over time, and why?
This last question turns out to be the easiest to answer, so it’s a natural place to begin our study of astronomy. As you’ll see, the answer ends up telling us only a little about the stars, but a great deal about the earth.
The Stars from Utah
Here’s a time-exposure photo that vividly illustrates the motion of the stars through a portion of our sky:
Stars rising in the east, as viewed from Ogden, Utah. The camera was mounted on a tripod and the shutter was left open for ten minutes, to show the stars’ apparent motion. The foreground mountains were illuminated by city lights.
As time passes, the stars rise in the east (just like the sun). But notice that they rise diagonally, not straight up. The diagonal goes from north (left) to south (right). After a few hours, these same stars will appear high in the southern sky.
Turning to face south, we see that the stars there are moving from left (east) to right (west):
Ten-minute time exposure facing south and slightly west, from the same location as the previous photo. The stars are moving from left (east) to right (west) across the field of view. Despite the annoying light pollution, you can barely make out part of the Milky Way, right of center.
By now you can probably guess that stars set in the western sky, again along a diagonal:
Ten-minute time exposure facing west, from the same location as the previous two photos. The stars are setting along a diagonal, from south (left) to north (right). The bright star at the lower-right is Arcturus.
And in the north, the motion is most interesting. Stars rise in the northeast and set in the northwest, moving in counter-clockwise circles around a point that's high above the northern horizon:
Half-hour time exposure facing north and slightly west, from the same location as the previous three photos. The stars are tracing counter-clockwise circles, centered on a point near the prominent North Star (Polaris). Notice the Big Dipper at the lower-left.
The magestic motions of the night sky were intimately familiar to ancient people. Today this familiarity has been lost (except by astronomy geeks), so you'll need to make a special effort to remember and visualize the patterns. It helps to stand under the night sky and point with your hands, tracing out the paths of different stars. In summary:
Some stars rise directly east, heading to the right, then cross the high southern sky, and eventually set directly west.
Other stars rise in the southeast and follow shorter, lower arcs across the south before setting in the southwest.
And in the north, there are many stars that never rise or set at all; these “circumpolar stars” follow counter-clockwise circles. (In fact, it is the center of these circles that defines what we mean by “north”.)
Besides direct observation, you can get accustomed to these motions by playing with the Sky Motion Applet that I've created for this purpose. A variety of other useful resources are listed at the bottom of this page.
Constellations
Orion the Hunter is one of the brightest and most familiar constellations of the night sky. The row of three stars near the middle is
