1a. Celest. Sphere
1b. Pole Star
2. The Ecliptic
2a. The Sundial
3. The Seasons
4. The Moon (1)
4a. The Moon (2)
4b. Moon Libration
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. In the drawing to the left, the horizontal "belt" around the globe can be viewed as the horizon, while the sphere itself rotates around its axis.
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.
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.
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.
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.
The Mounting of a Telescope
As the drawing above makes clear, during the night we view the pole star from different positions (such as A and B). This however makes no noticeable difference in its place in the sky, because it is so distant from us. If the Earth rotated not around its axis but along a parallel line through A or B, the sky would not appear any different.
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.
To make such an adjustment easy, an astronomical telescope (pictured above) is mounted very differently from a surveyor's telescope (a "theodolite," pictured below). A theodolite usually has two axes--one allows it to scan all horizontal directions over 360 degrees, while the other adjusts its elevation and allows it to set its sights on reference marks higher than the viewer, such as mountaintops. On the other hand, a telescope for viewing stars (above) also has two perpendicular axes, but the main one (the "equatorial axis") is slanted to point at the pole star and is therefore parallel to the Earth's axis. As the celestial sphere rotates, a clockwork (or in cheap telescopes, the hand of the observer on a suitable knob) turns the telescope at a matching rate, keeping the same stars in the field of view.
Not all stars keep fixed positions on the sphere of the heavens. Even early sky-watchers noted that a few moved about: the ancient Greeks called them "planets", wanderers. The names we use today came from the Romans, who named them after their chief gods--Mercury, Venus, Mars, Jupiter and Saturn. Mercury and Venus are always close to the Sun and can only be seen shortly after sunset or before sunrise: Mercury is so close that most of the year it cannot be seen at all, because the bright sky drowns out its light. Venus is brighter than any other star (with appropriate conditions and looking right at it, you can see it even in the daytime) and Jupiter takes second place. When it is noon in New York, the Sun is almost overhead above "New York," but it is still only 9 in the morning in "San Francisco." Three hours later, the Earth has rotated and now it is noon at "San Francisco," with the Sun close to overhead. To get to this position, San Francisco must rotate to the position New York was in--from west to east.
Not all stars keep fixed positions on the sphere of the heavens. Even early sky-watchers noted that a few moved about: the ancient Greeks called them "planets", wanderers. The names we use today came from the Romans, who named them after their chief gods--Mercury, Venus, Mars, Jupiter and Saturn.
Mercury and Venus are always close to the Sun and can only be seen shortly after sunset or before sunrise: Mercury is so close that most of the year it cannot be seen at all, because the bright sky drowns out its light. Venus is brighter than any other star (with appropriate conditions and looking right at it, you can see it even in the daytime) and Jupiter takes second place.
When it is noon in New York, the Sun is almost overhead above "New York," but it is still only 9 in the morning in "San Francisco." Three hours later, the Earth has rotated and now it is noon at "San Francisco," with the Sun close to overhead. To get to this position, San Francisco must rotate to the position New York was in--from west to east.
On Polaris in the night sky: #1b Finding the Pole Star
Next Stop: #2 The Path of the Sun, the Ecliptic
Timeline Glossary Back to the Master List
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated: 12 Sept. 2004