Note: The text above--and sections that follow--gives the period of rotation of the Earth as 24 hours. That is not exactly true: 24 hours is the mean length of a solar day, the average time that passes from noon to the next noon. Noon is always defined by the position of the Sun--when it passes exactly to the south (to viewers in Europe and the US, at least), and is at its greatest distance from the equator.
Using the Sun for reference, however, gives a shifting reference point in the sky. Between one noon and the next, the Sun too moves slightly in the sky, as part of its annual circuit around the celestial sphere, discussed in the next section, on the ecliptic. We could instead use some star as reference point, since stars keep fixed positions on the celestial sphere (see further below): for instance, define as "sidereal day" (sidereal--related to stars) the time between one passage of Sirius (the brightest star) to the south, and the next passage. That would be the true rotation period of the Earth, shorter than 24 hour by nearly 4 minutes--more accurately, 235.9 seconds.
(If you wish to calculate the difference: 24 hours are equal to 86400 seconds, and the average year contains 365.2422 solar days (see section on the calendar, where this point is also discussed). Actually, however, the Earth completes 366.2422 rotations in that time, so the real rotation period is just (365.2422/366.2422) of 86400 seconds. You should be able to figure out the rest.)
Most stars keep fixed positions relative to each other, night after night. The eye naturally groups them into patterns or constellations ("stella" is Latin for star), to which each culture has given its own names. The names we use come from the ancient Greeks and the Romans, e.g. Orion the hunter, accompanied by his two faithful dogs nearby. Other names evoke animals, whose Latin names are used--Scorpio the scorpion, Leo the lion, Cygnus the swan, Ursa Major the Big Bear (better known as the "big dipper") and so forth.
The Sun slowly moves through this pattern, circling around it once a year, always along the same path among the stars ("the ecliptic"). The ancients distinguished 12 constellations along this path, and since most are named for animals, they are known as the zodiac, the "circle of animals." The Sun spends about one month inside each "sign of the zodiac." The Moon moves close to the Sun's path, but only takes about a month, and a few conspicuous stars also move near it, the planets. We will come back later to all these: all other celestial objects are firmly placed and do not move, forming the "firmament."
Like the globe in the drawing, the sphere of the sky has two points around which it turns, points that mark its axis --the celestial poles. Stars near those poles march in daily circles around them, and the closer they are, the smaller the circles (they do not rise and set). At any time, only half the sphere is visible: it is as if the flat ground on which we stand sliced the celestial sphere in half--the upper half is seen, the lower half is not. Because of that, only one pole is seen at any time, and for most of us, living north of the equator, that is the north pole.
(If you mount a camera on a dark night in a way that the pole is in the middle of its field of view, open the shutter and take a time exposure, the image of each star will be smeared into part of a circle, and all the circles will be centered on the pole. Click here to see such a picture.)
Just as the globe of the Earth has an equator around its middle, halfway between the poles, so the sphere of the sky is circled by the celestial equator, halfway between the celestial poles. As the sky rotates, stars on the equator trace a longer circle than any others.
Of course, we know well (as the priests in Babylon didn't) that the stars are not attached inside a huge hollow sphere. Rather, it is the Earth which rotates around its axis, while the stars are so distant that they seem to stand still. The final effect, however, is the same in both cases. Therefore, whenever that is convenient, we can still use the celestial sphere to mark the positions of stars in the sky.
Polaris, the Pole Star
By pure chance, a moderately bright star is seen near the northern celestial pole--Polaris, the pole star (or north star). Polaris is not exactly at the pole, but its daily circle is very small and for many purposes one can assume it is at the pole, a pivot around which the entire sky rotates.
![[IMAGE: Polaris, the pole star]](Sfigs/Spolaris.gif)
All this looks much clearer if one remembers that it is the Earth that rotates, not the sky. The axis around which the Earth spins points in a certain direction in the sky, and that is also the direction of the pole star (or more accurately, the northern celestial pole). As the Earth turns, even though the observer moves with it (for instance, from point B in the drawing to point A), that direction always makes the same angle with the horizon and is always to the north. Hence the pole star is always in the same spot--north of the observer, and the same height above the horizon.
If on a clear night you find yourself lost in the wilderness or at sea, the pole star can tell you where north is, and from that you easily deduce east, west and south. Any other star is unreliable for determining direction--it will move across the sky, and may even set--but not this one. For instructions on finding the pole star at night, click here.
The closer you are to the equator, the closer is the pole star to the horizon, and at the equator (point C) it is on the horizon, and probably not easy to see. Further south, at points such as D, it is no longer visible, but now you can see the southern pole of the sky. Unfortunately, no bright star comparable to Polaris marks that position. The existence of a bright star near the north celestial pole is just a lucky accident, and as will be seen, it wasn't always so, and will not be a few thousand years from now.
To the eye the rotation of the sky is very, very slow (it is most noticeable when the Sun or Moon are rising or setting). A telescope however greatly magnifies the rotation rate, and any star observed with it quickly drifts to the edge of the field of view and then disappear, unless the direction of the telescope is constantly adjusted. That is usually done automatically, by turning the telescope around an axis parallel to the Earth's rotation, for as explained above, a parallel shift does not change the apparent rotation of the stars.
![[IMAGE: An old sky globe]](Sfigs/Sglobe.gif)
![[IMAGE: The equatorial mounting of a telescope]](Sfigs/Seqmount.gif)
![[IMAGE: Theodolite]](Sfigs/Stheodol.gif)
