To understand the movements of the Moon, the planets, the Sun, the stars, and other objects through the sky, we'll need to define a coordinate system and become comfortable with a few terms describing various positions. Let's begin by identifying the primary causes for the apparent movement of objects across the sky.
Next let's consider the distance scales involved. Which ones matter most?
When considering the movements of the stars, the distance between two positions on the surface of the Earth, and the distance between two positions on the Earth's orbital path, are negligible. Why then do the stars appear different at the equator and the poles, or from winter to summer? The angle of Earth's rotational axis, and the direction you are looking at as you shift around the curved surface of the Earth, present views of different portions of the sky.
Imagine that you are standing on the equator, looking up at the night sky above your head. If you shift 10,000 kilometers northward, you'll end up at the north pole. The diorama of the stars overhead will have shifted considerably – Polaris, the North Star, used to lie on the northern horizon, but now it is directly overhead. If instead you shift 10,000 kilometers to one side or the other along a flat plane passing under your feet, however, you'll end up observing the same set of stars above. What is the difference? In both cases, you have moved by 10,000 kilometers. In the first case, however, you have also rotated your point of view by 90 degrees, while in the second case, you are still looking in the same direction. The linear shift in position of 10,000 kilometers is irrelevant when compared to the distance between stars (10,000 billion kilometers), but the angular shift plays a huge role in defining your view.
Let's now consider the movement of the stars, and define a celestial sphere, a transparent sphere with infinite radius which is centered at the center of the Earth. Like the Florentine poet Dante Alighieri with his crystalline spheres, we place, or project, the celestial objects upon this sphere. The sphere is fixed in place, so as the Earth rotates daily in a complete circle the celestial sphere appears to rotate once a day in the opposite direction.
We extend the rotational axis of the Earth far above the north pole and far below the South Pole, and define the points at which it intersects our celestial sphere as the north and south celestial poles (NCP and SCP). We then define the portion of the celestial sphere which lies in the same plane as our equator as the celestial equator (CE). We define coordinates on the surface of the Earth according to northern and southern latitudes (starting at zero at the equator and rising to 90 degrees at the poles) and longitudes east and west of the position of the Royal Observatory in Greenwich, England ranging around the Earth. On our celestial sphere, we similarly use declination (angles north and south of the celestial equator) and right ascension (the angle around the celestial equator, with the zero point corresponding to the constellation where the Sun is found at noon on the first day of spring).
On the surface of the Earth, one feels as though one is standing on a giant, flat plane stretching away toward the horizon in all directions, with half of the sky (and the celestial sphere) appearing above the horizon and half hidden below. If you drew a line from the center of the Earth through your upright body, and then extended it upwards all the way to the celestial sphere, this would mark the zenith (the point directly above your head in the heavens). At the other end of the line, hidden far below the horizon, lies the nadir (literally, the lowest point).
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| [NMSU, N. Vogt] | |
The figure shown above identifies landmarks used when observing, both in an observer's frame of reference (their local horizon, stretching off to the north and south, and east and west, and the sky above them), and along the celestial sphere that surrounds the Earth. The red line indicates the path of the Sun through the sky over the course of a day on the Spring Equinox (March 21, on the left), where the Earth is tipped neither toward nor away from the Sun, and three months later on the Summer Solstice (June 21, on the right), where the Earth's North Pole is tipped over by 23 degrees toward the Sun and the Sun thus appears to rise and set in the north, and to pass overhead and culminate (transit, or intersect the observer's meridian) higher in the sky.
At the north pole, the zenith aligns with the north celestial pole and the celestial equator lies on the horizon. As you shift southward toward the equator, what happens to the sky? The zenith tilts toward the celestial equator, as it rises overhead, and the north celestial pole descends toward the horizon.
If you stand at the north pole you can extend a line upward to the celestial sphere to Polaris, the North Star. You can draw a similar line northward from any other point in the northern hemisphere. You might think that this line would be tilted relative to the first line, because you have shifted your base point out from directly underneath the North Star. However, recall that shifting your position from one side of the Earth to the other is a shift of 0.000000001% of the distance to the nearest star (0.00000000001% of the distance to Polaris) – far too small a difference to matter. Because the Earth is so tiny compared to the distances to the stars, all lines pointing north from all over the surface are essentially parallel.
The Earth rotates from west to east on its own rotational axis (or counterclockwise, when viewed from above the north pole). Because of this, when we stand still on the Earth and observe the stars above we perceive them to be rising in the east and setting in the west, taking 24 hours to travel once around the entire Earth.
Our position with respect to the Earth's rotational axis controls the apparent movements of the stars during a night. If you stand at the north pole, then the entire northern sky appears to be rotating in a huge circle around the North Star above your head. (This is because you are turning around in a tiny circle in the opposite direction.) Over the course of one night the stars neither rise nor set; instead, the star in view travel in arcs (taking 24 hours to complete an entire circle).
If you stand at the equator, however, you are now aligned perpendicular to the Earth's rotational axis. From your point of view, a steady stream of stars are rising in the east, traveling across the sky overhead, and then setting in the west hours later. The difference is due to the fact that rather than standing in one place and spinning, you are now tracing out a giant circle around the Earth along the equator. Rather than looking up along the rotational axis toward Polaris, you are now looking up 90 degrees away, and finding the celestial equator overhead.
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| [NMSU, N. Vogt] |
Star that move in a circle which never drops below the horizon are called circumpolar, and the closer you are to the poles, the more of them you will observe. They have the largest declination values, and appear closest to the north and south celestial poles on the celestial sphere. In contrast, stars which lie near to the celestial equator rise in the east, transit overhead, and then set in the west. If at one month of the year they lie overhead at midnight, then six months later they will lie unseen behind the Sun at noon.
Your location on Earth determines how much of the sky will contain circumpolar stars.