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). 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 rotation of the Earth is given 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, on the ecliptic. We could instead use some star as reference point, since stars keep fixed positions on the celestial sphere: 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.

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. All other celestial objects are firmly placed and do not move, forming the "firmament."

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.

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.

Planets and the Zodiac

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.

Finding The Pole Star

Two bright constellations occupy opposite sides of the pole star--the Big Dipper and Cassiopeia. As the celestial sphere rotates (or appears to rotate), these constellations also march in circles around the pole . Depending on the hour of the night and the day of the year, one or the other may be low near the horizon where it is barely seen, or even hidden below the horizon. But when that happens the other constellation is sure to be high in the sky, where (weather permitting) it is easily seen.

The Big Dipper

The Big Dipper consists of 7 bright stars, forming a dipper, a small pot with a long handle. In England it is often called "the plough" (spelled "plow" in the US), and fugitive slaves before the Civil War knew it as "the drinking gourd", a signpost in the sky pointing the way north to safety, to Canada where slavery was outlawed. Astronomers name it "Ursa Major," Latin for "the big she-bear," and some other languages also refer to it as the Big Bear. In Greek, bear is "Arktos", and hence the far-north region where this constellation is usually overhead became known as "the Arctic."



When the territory of Alaska in 1926 decided to create a flag of its own, it asked citizens to submit proposed designs for the new flag. The winning design was that of Benny Benson, age 13, and is reproduced above.  It shows the 7 stars of the Big Dipper and Polaris, the north star.

The flag also shows how the north star can be found. Imagine a line connecting the two stars at the front of the "dipper", continue it on the side where the dipper is "open" to a distance 5 times that between the two stars (the flag shortens this a bit!), and you will arrive at (or very close to) the pole star. Because of their role in locating Polaris, these two stars are often called "the guides." And by the way--the last-but-one star in the handle of the "dipper", named Mizar by Arab astronomers, is a double star, whose components are readily separated by binoculars--or, some say, by very sharp eyes during good viewing conditions.

Cassiopeia

Cassiopeia was a queen in Greek mythology, and the constellation named for her is shaped like the letter W. Polaris is above the first "V" of this letter. If you draw a line dividing the angle of that "V" in half and continue along it, you will reach the vicinity of Polaris.

The name of Cassiopeia's husband, King Cepheus, goes with a nearby constellation, above the other "V" (the brighter one), but Cepheus is nowhere as striking as Cassiopeia. Her daughter Andromeda has another constellation, framed by a big undistinguished rectangle of four stars. An unremarkable constellation to the eye--but it contains a large galaxy, our nearest neighbor in space (not counting two dwarf galaxies in the southern sky), one which seems to resemble ours in size and shape.

Ursa Minor, the "Small Bear" or "Little Dipper" is a constellation somewhat resembling the Big Dipper, and Polaris is the last star in its tail. The "dipper" itself faces the tail of the Big Dipper, so that the two "tails" (or "handles") point in opposite directions. The two front stars of the "little dipper" (quite smaller and more square than the big one) are fairly bright, but other stars are rather dim and require good eyes and a dark sky.

 

THE PATH OF THE SUN, THE ECLIPTIC

 

Signs of the Zodiac

Even though the planets move on the celestial sphere, they do not wander all over it but are confined to a narrow strip, dividing it in half. Stars along that strip are traditionally divided into the 12 constellations of the zodiac. The name, related to "zoo," comes because most of these constellations are named for animals--Leo the lion, Aries the ram, Scorpio the scorpion, Cancer the crab, Pisces the fish, Capricorn the goat and Taurus the bull.

At any time, the Sun is also somewhere on the celestial sphere, and as the Earth turns, it rises and sets the same way as stars do.

Like the planets, the Sun, too, moves around the zodiac, making one complete circuit each year. Every month it covers a different constellation of the zodiac, which is the real reason why those constellations are 12 in number. Of course, during that month, this particular constellation is not seen, because the sky near the Sun is too bright for its stars to be seen (except, very briefly, during a total eclipse of the Sun).

One can however figure out where the sun is on the zodiac (as ancient astronomers have done) by noting which is the last constellation of the zodiac to rise ahead of the Sun, or the first to set after it. Obviously, the Sun is somewhere in between. In this manner each month-long period of the year was given its "sign of the zodiac."

Astrologers, who believe that stars mysteriously direct our lives, claim it makes a great difference "under what sign" a person was born. Be aware, however, that the "sign" assigned to each month in horoscopes is not the constellation where the Sun is in that month, but where it would have been in ancient times. The difference is due to the precession of the equinoxes.

THE ECLIPTIC

The path of the Sun across the celestial sphere is very close to that of the planets and the moon. After clocks became available, it was a relatively straightforward job for astronomers to relate the path of the Sun in the daytime to the one of stars at night, and to draw it on their star charts. Because of its relation to eclipses, that path is known as the ecliptic.



The orbit of the Earth around the Sun.
This is a perspective view, the shape of the actual orbit is very close to a circle.

The significance of the ecliptic is evident if we examine the Earth's orbit around the Sun. That orbit lies in a plane, flat like a tabletop, called the plane of the ecliptic (or sometimes just "the ecliptic"). In one year, as the Earth completes a full circuit around the Sun (drawing above), the Earth-Sun line and its continuation past Earth sweep the entire plane. The far end of that line then traces the ecliptic on the celestial sphere; if you have a star chart handy (it is often included in an atlas), you will find the ecliptic traced there, too.

The Planets and the Moon

Planets seen in the sky are always near the ecliptic, which means that their orbits are never too far from the plane of the ecliptic. In other words, the solar system is rather flat, with all its major parts moving in nearly the same plane.

What about the connection between "ecliptic" and eclipses?

The moon's orbit cuts the ecliptic at a shallow angle, around 5 degrees, which means that on the celestial sphere the Moon, too, follows a path through the zodiac. Half the time the Moon is north of the ecliptic, half the time south of it. If the shadow of the moon hits the Earth, the Sun is eclipsed in the shadow area; if on the other hand the shadow of the Earth covers the moon, the moon goes dark and we have an eclipse of the moon.

Either of these can only happen when the Sun, Earth and Moon are on the same straight line. Since the Sun and Earth are in the plane of the ecliptic, the line is automatically in that plane too; if the moon is also on the same line, it must be in the plane of the ecliptic as well.

It takes close to a month for the Moon to go around the Earth ("month" comes from "Moon") and during that time its orbit crosses the ecliptic twice, as it goes from one side to the other. At the time of crossing, the Sun may be anywhere along the ecliptic; usually it is not on the Earth-Moon line, and therefore an eclipse usually does not take place. Occasionally, however, it is on that line or close to it. If it then happens to occupy exactly the same spot on the celestial sphere, we get an eclipse of the Sun, because the moon is then between us and the Sun. On the other hand, if it occupies the spot exactly opposite from that of the Moon, the Earth's shadow falls on the Moon and we have an eclipse of the Moon.

 

THE SEASONS

If the Earth's axis were perpendicular to the ecliptic, as in the drawings below, the Sun's position in the sky would be halfway between the celestial poles, and its daily path, seen from any point on Earth, would stay exactly the same, day after day.



Each point on Earth would be carried around the axis AB once a day. On the equator (point C) the sun would always rise until it was overhead, then again descend to the horizon. At the poles (A and B) it would always graze the horizon and never get away from it. Except at the pole, every point would be in the shadow half the time, when on the right of the line AB, and would experience night; the other half it would be in the sunlight, experiencing day. Because the motion is symmetric with respect to the line AB, day and night anywhere on Earth are always equal.

Actually, the axis of rotation makes an angle of about 23.5 degrees with the direction perpendicular to the ecliptic. That makes life a lot more interesting.

Equinox and Solstice



In particular (drawing above), the angle between the Earth's axis and the Earth-Sun line changes throughout the year. Twice a year, at the spring and fall equinox (around March 21 and September 22--the exact date may vary a bit) the two directions are perpendicular.

Twice a year, the angle is as big as it can get, at the summer and winter solstices, when it reaches 23.5 degrees. In the summer solstice (around June 21) the north pole is inclined towards the Sun, in the winter solstice (around December 21) it faces away from it.

Let us look at the summer solstice first, with the Sun on the left



Summer and Winter

The boundary AB between sunlight and shadow--between day and night--is always perpendicular to the Earth-Sun line, as it was in the example shown at the beginning.

But because of the tilted axis, as each point on Earth is carried on its daily trip around the rotating Earth, the part of the trip spent in daylight (unshaded part of the drawing) and in the shadow (shaded) are usually not equal. North of the equator, day is longer than night, and when we get close enough to the north pole, there is no night at all. The Sun is then always above the horizon and it just makes a 360-degree circuit around it. That part of Earth enjoys summer.

A mirror-image situation exists south of the equator. Nights are longer than days, and the further one gets from the equator, the larger is the imbalance--until one gets so close to the pole that the sun never rises. That is the famous polar night, with 24 hours of darkness each day. In that half of the Earth, it is winter time.

Half a year later, the Earth is on the other side of the Sun, that is, the Sun's position in the above drawing should be on the right, and the shaded part of the Earth should now be on the left (light and dark portions in the drawing switch places). The Earth's axis however has not moved, it is still pointed to the same patch of sky, near the star Polaris. Now the south pole is bathed in constant sunshine and the north one is dark. Summer and winter have switched hemispheres.

A big difference between summer and winter is thus the length of the days: note that on the equator that length does not change, and hence Spring, Summer, Fall, and Winter do not exist there (depending on weather patterns, however, there may exist a "wet season" and a "dry season"). In addition (as the drawing makes clear), the Sun's rays hit the summer hemisphere more vertically than the winter one. That, too, helps heat the ground.

At equinox, the situation is as in the first drawing, and night and day are equal (that is where the word "equinox" comes from)

THE MOON



The Month

 

The monthly cycle of the moon (we won't capitalize the word here) must have mystified early humans--"waxing" from thin crescent ("new moon") to half-moon, then to a "gibbous" moon and a full one, and afterwards "waning" to a crescent again. That cycle, lasting about 29.5 days, gave us the word "month"--related to "moon," as is "Monday."

The civil year, January to December, no longer ties its months to the moon, but some traditions still do and their terms for "month" reflect the connection--in Arabic, "shahr", in biblical Hebrew "yerach" and also "chodesh" from "new," since it was reckoned from one new moon to the next. Jericho (pronounced Yericho), one of the oldest cities on Earth, took its name from "yerach," and of course, legends tell of many moon-gods and goddesses, e.g. Artemis and Diana.

Early astronomers understood the different shapes of the moon, noting that each was linked to a certain relative position between moon and Sun: for instance, full moon always occurred when moon and Sun were at opposite ends of the sky. All this suggested that the moon was a sphere, illuminated by the Sun.

The moon's path across the sky was found to be close to the ecliptic, inclined to it by about 5 degrees. Eclipses of the Sun always occurred when moon and Sun were due to occupy the same spot in the sky, suggesting that the moon was nearer to us and obscured the Sun. Eclipses of the moon, similarly, always occurred at full moon, with the two on opposite sides of the Earth, and could be explained by the shadow of the Earth falling on the moon.

Lunar eclipses allowed the Greek astronomer Aristarchus, around 220 BC, to estimate the distance to the moon. If the moon and the Sun followed exactly the same path across the sky, eclipses of both kinds would happen each month. Actually they are relatively rare, because the 5-degree angle between the paths only allows eclipses when Sun and moon are near one of the points where the paths intersect.

The cycle from each new moon to next one takes 29.5 days, but the actual orbital period of the moon is only 27.3217 days. That is the time it takes the moon to return to (approximately) the same position among the stars.

Why the difference? Suppose we start counting from the moment when the moon in its motion across the sky is just overtaking the Sun; we will call this the "new moon," even though the thin crescent of the moon will only be visible some time later, and only shortly after sunset. Wait 27.3217 days: the moon has returned to approximately the same place in the sky, but the Sun has meanwhile moved away, on its annual journey around the heavens. It takes the moon about 2 more days to catch up with the Sun, to the position of the next "new moon," which is why times of the new moon are separated by 29.5 days.

The Face of the Moon

The visible face of the moon has light and dark patches, which people interpreted in different ways, depending on their culture. Europeans see a face and talk of "the man in the moon" while children in China and Thailand recognize "the rabbit in the moon." All agree, however, that the moon does not change, that it always presents the same face to Earth.

Does that mean the moon doesn't rotate? No, it does rotate--one rotation for each revolution around Earth! The drawings on the left, covering half an orbit, should make this clear. In them we look at the moon's orbit from high above the north pole, and imagine a clock dial around the moon, and a feature on it, marked by an arrow, which initially (bottom position in each drawing) points at 12 oclock.



In the top drawing the marked feature continues to point at Earth, and as the moon goes around the Earth, it points to the hours 10, 8 and 6 on the clock dial. As the moon goes through half a revolution, it also undergoes half a rotation     If the moon did not rotate, the situation would be as in the bottom drawing. The arrow would continue to point in the 12-oclock direction, and after half an orbit, people on Earth would be able to see the other side of the moon. This does not happen.

We need to go aboard a spaceship and fly halfway around the Moon before we get a view of its other side--as did the Apollo astronauts who took the picture below.



This strange rotation of the moon is maintained because the moon is slightly elongated along the axis which points towards earth. To understand the effect we look at the motion of a body with a much more pronounced elongation--an artificial satellite with the shape of a symmetric dumbbell (see below).



It can be shown that if the forces on the dumbbell (or indeed on a satellite of any shape) are unbalanced, it rotates around its center of gravity. That point will be defined later, but in a symmetric dumbbell with two equal masses marked A and B, the center of gravity is right in the middle between them.

Both masses A and B are attracted to the Earth, and if the attracting forces were equal, their tendencies to rotate the satellite ("rotation moments" or "torques") are equal and cancel each other, so that no rotation occurs. If however A starts closer to the center of Earth, the force on it is just a little stronger. Therefore the satellite will rotate until A is as close to Earth as it can be, which is a possible position of equilibrium. Of course, it may then overshoot its equilibrium position, and end up swinging back-and-forth like a pendulum, only slowly (like a pendulum) losing energy and settling down. The elongated moon acts like a dumbbell too.

The rotating force which lines up the moon or an orbiting dumbbell is the difference between the pull on A and on B. It depends not on how strongly gravity pulls these masses, but on how rapidly the pull of gravity changes with distance--on the "gravity gradient." Near Earth that is a gentle force, though still strong enough to line up elongated satellites. Among those was Triad, deliberately shaped like a long dumbbell with an additional payload in the middle, the first satellite to map the electrical currents associated with the polar aurora.

Near a black hole or pulsar, though, the gravity-gradient force can be fierce enough to rip a spacecraft apart.

Actually, the long axis of the Moon does not always point exactly to the center of the Earth, but swings back and forth around that direction, a motion known as libration. Most of this is caused because the Moon rotates around its axis with a fixed period, while its motion around its orbit slows down far from Earth and speeds up close to it. This speeding up and slowing down is the result of Kepler's 2nd law, discussed in section 12, and is a rather small effect, since the moon's orbit is very close to circular.

Because of libration, even though at any time only half the Moon is visible, over time 59% can be seen, since it lets astronomers look at the Moon from slightly different viewing directions.

Earthshine

At times when only a narrow crescent of the Moon is seen (e.g. a "new moon"), one can also see the rest of the Moon faintly outlined. The Sun now shines on almost all of the side of the moon turned away from Earth (those calling that "the dark side of the moon" are quite wrong!) and therefore it also illuminates most of the side of the Earth facing the moon. If you were standing on the moon at that time, a "full Earth" would shine brightly in your sky, and the faint "earthshine" of the darker part of the moon is just the reflection of some of that bright earthlight.

Earthshine is of interest to scientists, because its brightness is contributed by all the factors which turn back sunlight before it manages to heat the Earth--light reflected from the ground and from clouds, and light scattered back by dust and small particles ("aerosols") in the atmosphere. In a time when atmospheric scientists are trying to assess heating of the Earth by the greenhouse effect, earthshine measures a process which works in the opposite direction, reducing the heat our planet receives.

The fraction of light reflected is hard to estimate theoretically, but earthshine allows it to be measured. According to recent reports ("The Darkening Earth", Scientific American August 2004, p. 16), this fraction has been growing, reducing the amount of sunlight received by Earth and canceling about 1/3 of the greenhouse heating.

 

RIGHT ASCENSION AND DECLINATION

The globe of the heavens resembles the globe of the Earth, and positions on it are marked in a similar way, by a network of meridians stretching from pole to pole and of lines of latitude perpendicular to them, circling the sky. To study some particular galaxy, an astronomer directs the telescope to its coordinates.

On Earth, the equator is divided into 360 degrees, with the zero meridian passing Greenwich and with the longitude angle φ measured east or west of Greenwich, depending on where the corresponding meridian meets the equator.

In the sky, the equator is also divided into 360 degrees, but the count begins at one of the two points where the equator cuts the ecliptic--the one which the Sun reaches around March 21. It is called the vernal equinox ("vernal" means related to spring) or sometimes the first point in Aries, because in ancient times, when first observed by the Greeks, it was in the zodiac constellation of Aries, the ram. It has since then moved, as is discussed in the later section on precession.

The celestial globe, however, uses terms and notations which differ somewhat from those of the globe of the Earth. Meridians are marked by the angle α (alpha, Greek A), called right ascension, not longitude. It is measured from the vernal equinox, but only eastward, and instead of going from 0 to 360 degrees, it is specified in hours and other divisions of time, each hour equal to 15 degrees.

Similarly, where on Earth latitude goes from 90° north to 90° south (or -90°), astronomers prefer the co-latitude, the angle from the polar axis,equal to 0° at the north pole, 90° on the equator, and 180° at the south pole. It is called declination and is denoted by the letter δ (delta, Greek small D). The two angles (α, δ), used in specifying (for instance) the position of a star are jointly called its celestial coordinates.

PRECESSION

The priests of ancient Babylonia and Egypt were also pioneer astronomers. They studied the heavens, mapped their constellations, identified the path of the Sun and estimated the periods of the Moon and Sun as they moved across the sky.

But it was a Greek astronomer, Hipparchus of Nicea, who made the first major new discovery in astronomy. Comparing observations more than a century apart, Hipparchus proposed that the axis around which the heavens seemed to rotate shifted gradually, though very slowly.

Viewed from Earth, the Sun moves around the ecliptic, one full circuit each year. Twice a year, at equinox, day and night are equal and the Sun rises exactly in the east and sets exactly in the west. Ancient astronomers had no good clocks and could not tell when the day and night had the same length, but they could identify the equinox by the Sun rising exactly in the east and setting exactly in the west. At those times the Sun's position is at one of the intersections between the ecliptic and the celestial equator.

Around the year 130 BC, Hipparchus compared ancient observations to his own and concluded that in the preceding 169 years those intersections had moved by 2 degrees. How could Hipparchus know the position of the Sun among the stars so exactly, when stars are not visible in the daytime? By using not the Sun but the shadow cast by the Earth on the moon, during an eclipse of the Moon! During an eclipse, Sun, Earth and Moon form a straight line, and therefore the center of the Earth's shadow is at the point on the celestial sphere which is exactly opposite that of the Sun. 

"The Dawning of the Age of Aquarius"

Hipparchus concluded that the intersection marking the equinox slowly crept forward along the ecliptic, and called that motion "the precession of the equinoxes. " The rate is about one full circle in 26 000 years. In ancient times the intersection marking the spring equinox was in the constellation of Aries, the ram, and for that reason the intersection (wherever it might be) is still sometimes called "the first point in Aries."

Around the year 1 it moved into the constellation of Pisces (pronounced "pie-sees" in the US) and currently it is again in transition, to the constellation of Aquarius, the water carrier. If you ever heard the song "The dawning of the age of Aquarius" from the musical "Hair," that is what it is all about. To astronomers precession is mainly another factor to be taken into account when aiming a telescope or drawing a star chart; but to believers in astrology, the "dawning of the age of Aquarius" is a great portent and may mark the beginning of a completely new and different era.

The Precession of the Earth's Axis

What does this motion tell us about the Earth's motion in space? If you ever had a spinning top, you know that its axis tends to stay lined up in the same direction--usually, vertically, though in space any direction qualifies.


Precession of a spinning top: the spin axis traces the surface of a cone.

Give it a nudge, however, and the axis will start to gyrate wildly around the vertical, its motion tracing a cone (see above). The spinning Earth moves like that, too, though the time scale is much slower--each spin lasts a day, and each gyration around the cone takes 26 000 years. The axis of the cone is perpendicular to the plane of the ecliptic.

The cause of the precession is the equatorial bulge of the Earth, caused by the centrifugal force of the Earth's rotation. That rotation changes the Earth from a perfect sphere to a slightly flattened one, thicker across the equator. The attraction of the Moon and Sun on the bulge is then the "nudge" which makes the Earth precess.

Through each 26 000-year cycle, the direction in the sky to which the axis points goes around a big circle, the radius of which covers an angle of about 23.50. The pole star to which the axis points now (within about one degree) used to be distant from the pole, and will be so again in a few thousand years (for your information, the closest approach is in 2017). Indeed, the "pole star" used by ancient Greek sailors was a different one, not nearly as close to the pole.

Because of the discovery mady by Hipparchus, the word "precession" itself no longer means "shift forward" but is now applied to any motion of a spin axis around a cone--for instance, the precession of a gyroscope in an airplane's instrument, or the precession of a spinning satellite in space.

Precession of a spinning scientific payload (also known as its "coning"--from "cone"--or its "nutation") is an unwelcome feature, because it complicates the tracking of its instruments. To eliminate it, such satellites use "nutation dampers," small tubes partially filled with mercury. If the satellite spins as it was designed to do, the mercury merely flows to the part of the tube most distant from the spin axis, and stays there. However, if the axis of rotation precesses, the mercury sloshes back and forth in the tube. Its friction then consumes energy, and since the source of the sloshing is the precession of the spin axis, that precession (very gradually) loses energy and dies down.

THE PLANETS

Most stars we observe form fixed constellations in the sky, undergoing daily motion (e.g. rising and setting) but maintaining fixed positions relative to each other--like the stars of Orion, or the Big Dipper. The ancients however noted that 5 stars constantly moved--all following close to the paths of the Sun and Moon across the heavens, i.e., close to the ecliptic. The Greeks called them planets, i.e. wanderes, a name still used.

Venus and Mercury

The five planets known to the ancients were named after principal Greek gods, later replaced by their Roman equivalents: Mercury, Venus, Mars, Jupiter and Saturn. They were relatively bright--Venus and Jupiter can be brighter than any fixed star--though their brightness seemed to vary. Venus and Mercury never appear far from the Sun and (outside the polar regions, at least) are only visible just after sunset or before sunrise, suggesting that those planets were confined near the Sun. The Greeks called Venus "Hesperus" when it appeared as the evening star and "Phosphorus" when as morning star it rose before sunrise, though they realized both were the same object. Mercury, which is fainter and closer to the Sun, is particularly hard to detect by eye and this only when its visible position is far from the Sun's.

All planets seemed to move among the stars in the same direction as the Moon (and of the Sun)--with one strange variation: sometimes their apparent motion is temporarily reversed ("retrograde motion"). That is most evident with Mercury and Venus, which shuttle back and forth across the position of the Sun. As the Sun moved among the stars--along the constellations of the zodiac--these planets sometimes move the same way and add their motion to that of the Sun, but sometimes their apparent motion opposes the one of the Sun, causing them to seem to move backwards or "retrograde."

Mars, Jupiter and Saturn

The other three planets visible to the eye can be seen anywhere along the ecliptic--even at midnight, directly opposite the Sun, which was when they appear brightest. Mars seems to move the fastest, Jupiter next, and Saturn the slowest. But all exhibit that puzzling quirk--near the point of their celestial path exactly opposite the Sun ("opposition"), their motion among the stars temporarily turns around.



Schematic drawing of the apparent reversal of motion (retrograde motion) observed with Mars.
Positions 1...7 of the Earth correspond to positions 1...7 of Mars, which moves more slowly.
As the Earth overtakes Mars (positions 4 and 5) the planet's position in the sky moves backwards. 

Today we undestand all that very well (see image above). Planets are spherical objects like Earth--Venus, Mercury and Mars are smaller, Jupiter and Saturn much bigger. Earth is a planet too and others exist as well (too faint to be seen without a telescopes), all orbiting the sun on or near the plane of the ecliptic. Their speed however varies--the closer to the Sun, the faster. Therefore, when the three outer planets are near opposition, the Earth orbiting closer to the Sun overtakes them, and they seem to move backwards.

The retrograde motion of the two inner planets has a similar cause. Being closer to the Sun, they overtake the Earth in their motion. 

Summary of what is now known about Planets

- Major planets, in order of distance from the Sun--Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. All but the inner two have satellites, and all four outer ones have rings as well, composed of small orbiting chunks of matter.

- Asteroids or minor planets, most but not all between Mars and Jupiter. Ranging in diameter up to about 500 km.

- The "Kuiper Belt" of icy objects outside the orbit of Neptune, of which the best known (though as of now only the second largest) is Pluto, discovered in 1930 and about the size of our Moon. The belt is named after the Belgian astronomer Gerard Kuiper, may extend to twice the distance of Neptune and is estimated to consist of as many as 100,000 objects (about 1000 of them identified so far), many only 100 km across or smaller.

- Comets, traditionally divided into "non returning" (official name, "long period comets") and "periodic" ones." Non-returning comets are believed to come from the "Oort cloud," a huge near-spherical collection of frozen chunks on the distant fringes of the solar system. They are loosely bound to the Sun, and now and then the gravity of a distant star is believed to slightly change the motion of some and send them sunwards. They become visible as comets when sunlight evaporates some of their surface to create the comet's glow and tail.

- Periodic comets were once believed to have started as non-returning ones but to have been diverted by the pull of one of the larger planets. They are now widely held to arrive from the Kuiper belt as a class of objects known as Centaurs.