2.1: The Celestial Sphere

The Sun rules by daytime sky, but at night, especially if the Moon does not shine, the show belongs to the stars. Bright and dim, randomly distributed across the sky, with odd formations that catch the eye, their number seems huge. To ancient observers it seemed as if Earth was at the center of a giant star-studded “celestial sphere," which reinforced the belief, held for thousands of years, that we are at the center of the universe.

If you watch stars throughout the night, you will see that most of them also rise to the east of you and set west of you, like the Sun and Moon. Indeed, the entire celestial sphere seems to rotate slowly --- one turn in 24 hours --- and since half of it is always hidden below the horizon, this rotation constantly brings out new stars on the eastern horizon, while others to disappear beneath the western one.

We of course know that it is not the universe that rotates around us from east to west, but our Earth is the one rotating, (from west to east--see note at end). But it is still convenient to talk about “the rotation of the celestial sphere." That could also make the sky rotate the way it is observed to do.

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 (See the chapter “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 the chapter “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.

Note

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. Go to http://antwrp.gsfc.nasa.gov/apod/ap980912.html 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.