The Motion of the Sky

      As the Sun sets and the sky gradually grows darker, the stars begin to appear. They are always there of course, but while the Sun is shining the atmosphere is lit by its brilliance, and the stars' faint light is lost in its glare.
      The first objects to appear are the brighter stars and planets (presuming any planets happen to be up). Such bright objects are referred to as being of the first magnitude, meaning that their brightness puts them in first place among their peers. As the sky grows darker we can see fainter and fainter objects — second magnitude, third magnitude and so on — until, when it is very dark (something which those of us who live in cities never get to experience), the sky is filled with the faint light of thousands of fifth and sixth magnitude stars. Under those circumstances it is also possible to see a faint band of grayish white circling the heavens — the Milky Way.
      During the time that it takes the sky to gradually darken, a careful observer will notice that the positions of the stars are gradually changing. They do not change position relative to each other — in fact, their positions relative to each other are so permanent and unchanging that we sometimes refer to them as the fixed stars. They do, however, gradually move across the sky, rising in the east and setting in the west in the same way that the Sun does during the day. The motion that they have is not real, of course, but a mirror image of the rotation of the Earth. As we rotate around our axis of rotation, anything not attached to the Earth appears to move in the opposite direction from the motion of the Earth. The Earth's rotation is toward the east (this is how east is defined), so the stars move toward the west.
      Even though the stars' motion is an illusion caused by our own motion, it looks perfectly real, and it is easier to say that they rise and set than to talk about how the eastern horizon tilts downward and uncovers stars that were previously hidden, and the western horizon tilts upward and hides stars that were previously visible. As a result, throughout this online text, even though we know it is the Earth's motion that is real and the stars' motion is illusory, we will talk about the rising and setting of the stars, and discuss their motion as though it were real. It is only when we consider how our motion creates the apparent motion of the stars that we will talk about the reality behind their apparent movement.
      The Earth's motion to the east is very uniform (if it weren't, you would be able to feel some jerkiness as we speed up or slow down). As a result, the stars' mirror image of our motion is also very uniform. Their westward motion is so uniform and smooth (or more accurately, our eastward motion is so uniform and smooth) that it is impossible, in periods as short as one day, to detect any changes in the motion. As a result, throughout our discussion we will consider the positions of the stars to be fixed relative to each other, and their motions relative to us to be absolutely uniform.

Above: Star trails produced by the rotation of the North polar sky, shown by images spaced at 4 minute intervals over a period of 40 minutes. Stars further from the Pole move faster than those that are closer, because they cover larger (angular) distances in the same time. In an animated version of the star motion, the stars move counter-clockwise around the (North Celestial) Pole, since the viewer is facing north.
Below: Star trails in southern skies (centered on the South Celestial Pole, which is just out of frame to the left), produced by a 100-minute exposure at the Gemini Observatory in Chile. The circular gray smudges on the left are caused by the motion of the Magellanic Clouds. Although not obvious in the image, the stars move clockwise around the (South Celestial) Pole, since the viewer is facing south. (Elke Schulz, apod060901)