Once again there was difficulty in the naniing. Bode's mythological family concept could not be carried on, for Uranus was the first god to come out of chaos and had no father. Some suggested the planet be named for Leverrier.

Wiser council prevailed. The new planet, rather greenish in its appearance, was named Neptune after the god of the sea.

(Leverrier also calculated the possible existence of a' planet inside the orbit of Mercury and named it Vulcan, after the god of fire and the forge, a natural reference to the planet's closeness to the central fire of the Solar System. However, such a planet was never discovered and undoubtedly does not exist.)

As soon as Neptune was- discovered, the English astron omer William Lassell turned his telescope upon it and dis covered a large satellite which he named Triton, ap propriately enough, since Triton was a demigod of the sea and a son of Neptune (Poseidon).

In 1851 Lassell discovered two more satellites of Uranus, closer to the planet than Herschel's Oberon and Titania. Lassell, also English, decided to continue Her schel's English folklore bit. He turned to Alexander Pope's The Rape of the Lock, wherein were two elfish characters, Ariel and Umbriel, and these names were given to the satellites.

More satellites were turning up. Saturn was already known to have seven satellites, and in 1848 the American astronomer George P. Bond discovered an eighth; in 1898 the American astronomer William H. Pickering discovered a ninth and completed the list. These were named Hy perion and Phoebe after a Titan and Titaness. Pickering also thought he had discovered a tenth in 1905, and named it Themis, after another Titaness, but this proved to be mistaken.

In 1877 the American astronomer Asaph Hall, waiting for an unusually close approach of Mars, studied its sur 73 roundings carefully and discovered two tiny satellites, which he named Phobos ("fear") and Deimos ("teffor"), two sons of Mars (Ares) in Greek legend, though obvi ously mere personifications of the inevitable consequences of Mars's pastime of war.

In 1892 another American astronomer, Edward E.

Barnard, discovered a fifth satellite of Jupiter, closer than the Galilean satellites. For a long time it received no name, being called "Jupiter V" (the fifth to be discovered) or "Barnard's satellite." Mythologically, however, it was given the name Amalthea by the French astronomer Camille Flammarion, and this is coming into more com mon use. I am glad of this. Amalthea was the nurse of Jupiter (Zeus) in his infancy, and it is pleasant to have the nurse of his childhood closer to him than the various girl and boy friends of his maturer years.

In the twentieth dentury,no less than seven more Jovian satellites were discovered, all far out, all quite small, all probably captured planetoids, all nameless. Unofficial names have been proposed. Of these, the three planetoids nearest Jupiter bear the names Hestia, Hera, and Demeter, after the Greek names of the three sisters of Jupiter (Zeus).

Hera, of course, is his wife as well. Undeithe Roman versions of the names (Vesta, Juno, and Ceres, respec tively) all three are planetoids. The two farthest are Posei don and Hades, the two brothers of Jupiter (Zeus). The Roman version of Poseidon's name (Neptune) is applied to a planet. Of the remaining satellites, one is Pan, a grandson of Jupiter (Zeus), and the other is Adrastea, another of the nurses of his infancy.

The name of Jupitees (Zeus's) wife, Hera, is thus applied to a satellite much farther and smaller than those commemorating four of his extracurricular affairs. I'm not sure that this is right, but I imagine astronomers under stand these things better than I do.

In 1898 the German astronomer G. Witt discovered an unusual planetoid, one with an orbit that lay closer to the Sun than did any other of the then-known planetoids. It inched past Mars and came rather close to Earth's orbit.

Not counting the Earth, this planetoid might be viewed as passing between Mars and Venus and therefore Witt gave it the name of Eros, the god of love, and the son of Mars (Ares) and Venus (Aphrodite).

This started a new convention, that of giving planetoids with odd orbits masculine names. For instance, the planet oids that circle in Jupiter's orbit all received the names of masculine participants in the Trojan war: Achilles, Hector, Patroclus, Ajax, Diomedes, Agamemnon, Priamus, Nestor, Odysseus, Antilochus, Aeneas, Anchises, and Troilus.

A particularly interesting case arose in 1948, when the German-American astronomer Walter Baade discovered a planetoid that penetrated more closely to the Sun than even Mercury did. He named it Icarus, after the mythical character who flew too close to the Sun, so that the wax holding the feathers of his artificial wings melted, with the result that be fell to his death.

Two last satellites were discovered. In 1948 a Dutch American astronomer, Gerard P. Kuiper, discovered an innermost satellite of Uranus. Since Axiel (the next inner most) is a character in William Shakespeare's The Tem pest as well as in Pope's The Rape of the Lock, free asso ciation led Kuiper to the heroine of The Tempest and he named the new satellite Miranda.

In 1950 be discovered a second satellite of Neptune.

The first satellite, Triton, represents not only the name of a particular demigod, but of a whole class of merman-like demigods of the sea. Kuiper named the second, then, after a whole class of mermaid-like nymphs of the sea, Nereid.

Meanwhile, during the first decades of the twentieth century, the American astronomer Percival Lowell was searching for a ninth planet beyond Neptune. He died in 1916 without having succeeded but in 1930, from his ob servatory and in his spirit, Clyde W. Tombaugh made the discovery.

The new planet was named Pluto, after the god of the Underworld, as was appropriate since it was the planet farthest removed from the light of the Sun. (And in 1940, when two elements were found beyond uranium, they were named "neptunium!' and "plutonium" after Neptune and Pluto, the two planets beyond Uranus.)

Notice, though, that the first two letters of "Pluto,' are the initials of Percival Lowell. And so, ft0y, an astron omer got his name attached to a planet. Where Herschel and Leverrier had failed, Percival Lowell had succeeded, at least by initial, and under cover of the mythological conventions.

Any-one who writes a book on astronomy for the general public eventually comes up against the problem of trying to explain that the Moon always presents one face to the earth, but is nevertheless rotating.

To the average reader who has not come up against this problem before and who is inpatient with involved subtleties, this is a clear contradiction in terms. It is easy to accept the fact that the Moon always presents one face to the Earth because even to the naked eye, the shadowy blotches on the MooWs surface are always found in the same position. But in that case it seems clear that the Moon is not rotating, for if it were rotating we would, bit by bit, see every portion of its surface.

Now it is no use sm.Uing gently at the lack of sophistica tion of the average reader, because he happens to be right.

The Moon is not rotating with respect to the observer on the Earth's surface. When the astronomer says that the Moon is rotating, he means with respect to other observers altogether.

For instance, if one watches the Moon over a period of time, one can see that the line marking off the sunlight from the shadow progresses steadily around the Moon; the Sun shines on every portion of the Moon in steady pro gression. This means that to an observer on the surface of the Sun (and there are very few of those), the Moon would seem to be rotating, for the observer would, little by little'see every portion of the Moon's surface as it turned to be exposed to the suiidight.

But our average reader may reason to himself as fol lows: "I see only one face of the Moon and I say it is not rotating. An observer on the Sun sees all parts of the Moon and he says it is rotating. Clearly, I am more im portant than the Sun observer since, firstly, I exist and be doesn't, and, secondly, even if he existed, I am me and he isn't. Therefore, I insist on having it my way. The Moon does not rotate!"

There has to be a way out of this confusion, so let's think things through a little more systematically. And to do so, let's start with the rotation of the Earth itself, since that is a point nearer to all our hearts.

One thing we can admit to begin with: To an observer on the Earth,,the Earth is not rotating. If you stay in one place from now till doomsday, you will see but one portion of the Earth's surface and no other. As far as you are concerned, the planet is standing still. Indeed, through most of civilized human history, even the wisest of men insisted that "reality" (whatever that may be) exactly matched the appearance and that the Earth "really" did not rotate. As late as 1633, Galileo found himself in a spot of trouble for maintaining otherwise.

But suppose we had an observer on a star situated (for simplicity's sake) in the plane of the EartWs equator; or, to put it another way, on the celestial equator (see Chapter 3). Let us further suppose that the observer was equipped with a device that made it possible for Mm to study the Earth's surface in detail. To him, it would seem that the Earth rotated, for little by little he would see every part of its surface pass before his eyes. By taking note of some particular small feature (for example, you and I standing on some point on the equator) and timing its return, he could even determine the exact period of the Earth's rotation-that is, as far as he is concerned.

We can duplicate his feat, for when the observer on the star sees us exactly in the center of that part of Earth's surface visible to himself, we in turn see. the observer's star directly overhead. And just as he would time the periodic return of ourselves to that centrally located position, so we could time the return of his star to the overhead point.

The period determined wiU be the same in either case.

(Let's measure this time in minutes, by the way. A minute can be defined as 60 seconds, where I second is equal to 1/31,556,925.9747 of the tropical year.)

The period of Earth's rotation with respect to the star is just about 1436 minutes. It doesn't matter which star we use, for the apparent motion of the stars with respect to one another, ii viewed from the Earth, is so vanishingly small that the constellations can be considered as moving all in one piece.