Phobos is more effective in illumination, both because it is larger and because it is less distant. At the Martian equator, its brightness is equal to 1/60th that of our moon, but beyond 69° of latitude it is permanently shut out from view by the curvature of the globe.
THE PLANETOIDS.—Camille Flammarion
On the first day of the last century (January 1, 1801), Piazzi, an astronomer devoted to the sky, was observing at Palermo the small stars of the constellation Taurus, and noting their exact positions, when he remarked one which he had never seen before. The following evening (January 2) he directed his telescope again toward the same region of the sky, and remarked that the star was no longer at the point where he had seen it the day before, and that it had retrograded by 4′. It continued to retrograde up to the 12th, stopped, and then moved in the direct way—that is to say, from west to east. What was this moving star? The idea that it might be a planet did not immediately occur to the mind of the observer, and he took it for a comet, as William Herschel had done in 1781, when he discovered Uranus.
However, the skilful Sicilian observer was a member of an association which had for its special object the search for an unknown planet between Mars and Jupiter. From the earliest times of modern astronomy Kepler had described the disproportion, the void which exists between the orbit of Mars and that of Jupiter. If we omit, in fact, the orbit of the small planets or asteroids, we notice that the four planets, Mercury, Venus, the earth, and Mars, are in some measure crowded quite close to the sun, while Jupiter, Saturn, Uranus, and Neptune extend far into immensity. The law of Titius indicates a number, the number 28, as not being represented by any planet. It was in 1772 that this savant published this relation in a German translation which he had made of the Contemplation de la Nature of Charles Bonnet. Bode, Director of the Berlin Observatory, was so astonished at the coincidence that he announced this arithmetical relation as being a real law of nature, and spoke of it in such a way that it is generally known only by his name. He even organized an association of twenty-four astronomers to explore each hour of the Zodiac and search for the unknown. This systematic exploration had not yet produced any result when, by the merest chance, Piazzi saw his moving star, and at first believed it to be a comet. But on receipt of the news, Bode was convinced that this was the looked-for planet.
The new planet was found to be at the distance 2.77, and to revolve within a few days of the predicted period. Piazzi gave to the new body the name of Ceres, the protecting divinity of Sicily in the “good old times” of mythology.
The gap being thus filled up at the distance 28 by the discovery of Ceres, no one thought that other planets might exist there; and if Piazzi had supposed so, he might have at once discovered a dozen of the small bodies which revolve in this region. An astronomer of Bremen, Olbers, observed this planet on the evening of March 28, 1802, when he perceived in the constellation of the Virgin a star of the seventh magnitude which was not marked on Bode’s chart, which he used. The following day he found it had changed its place, and recognized by this fact that it was a second planet. But it was much more difficult to give citizenship to it than to its elder sister, because, the gap being filled up, it was not required, and it was more inconvenient than agreeable. They looked upon it, then, as a comet until its motion proved that it revolved in the same region as Ceres at the distance 2.77, and in 1,685 days (the period of Ceres is 1,681 days). They gave it the name of Pallas.
The unexpected discoveries of Ceres and Pallas led astronomers to revise the catalogues of stars and celestial charts. Harding was of the number of the zealous revisers. He was soon rewarded for his trouble. On September 1, 1804, at ten o’clock in the evening, he saw in the constellation of Pisces a star of the eighth magnitude which was not noted in the Histoire Céleste of Lalande. On September 4, he found it had perceptibly changed its place: it was a new planet. It received the name of Juno. Its distance from the sun is expressed by the number 2.67, and its revolution is performed in 1,592 days.
After these three discoveries, Olbers, noticing that the orbits of these planets crossed each other in the constellation of the Virgin, advanced the hypothesis that they might be nothing else but fragments of a large shattered planet. Mechanics show that, in this case, the fragments would again pass every year—that is to say, at each of their revolutions—through the spot where the catastrophe took place. Olbers then set himself to explore the constellation Virgo carefully, and found on March 29, 1807, a fourth small planet, to which he gave the name of Vesta. Its distance is but 2.36, and its revolution only 1.326 days. This is the brightest of the small planets, and it is sometimes seen with the naked eye (when we know where it is), like a star of the sixth magnitude.
It seems surprising that after these brilliant beginnings thirty-eight years should then have passed without the discovery of a single planet, for it was only in 1845 that the fifth, Astræa, was discovered by Hencke (who should not be confused with the astronomer Encke), a simple amateur astronomer, postmaster at Berlin, who amused himself by constructing charts of the stars. The principal reason for this must be attributed to the want of good star-charts, for to find these little moving points the first thing necessary is to have a very precise chart of the region of the Zodiac which we observe, in order to see whether one of the stars observed is in motion. The earliest good Zodiacal charts are those which the Academy of Berlin commenced to publish in 1830, taking as a basis the zones of Bessel continued by Argelander. Those of the Paris Observatory, which are more perfect, were only begun in 1854.