CURIOSITIES OF THE SKY

Evening Post, Volume CIII, Issue 141, 17 June 1922, Page 9

CURIOSITIES OF THE SKY

SMALL BUT REMARKABLE

MERE CHUNKS OF ROCK.

(Writen for The Post),

The fact that Mars is "in opposition," is an important event for astronomers. The phrase means simply that the earth is directly between Mars and the sun. 'When this condition is fulfilled, the two planets: are approximately at their closest Ito each other. Actually their .. closest positions, on this approach, occur tomorrow. The condition of opposition happens at intervals of about two years, but the times of unusually close ap--proach are about 15 or 17 years apart. Such occasions give specially favourable opportunities for the study of the planet with powerful telescopes, and it was at such an opportunity that it was found that the planet has two satellites, or moons. They are only to be seen in very powerful telescopes, and under the best conditions. IN REVERSE DIRECTIONS. It is a truly extraordinary fact about these satellites that, whereas one of them rises, like our moon, in the east, and sets in the west, the other rises in tho west, and sets in the east. Yet they both in reality travel in the same direction round the planet, and that direction is the same as that of the planet's own rotation. The apparent motion of the satellite, as viewed from the planet, is due to the time of rotation of the planet, and that of the revolution of the satellite round it. If the "day" of the planet is shorter than tho satellite's period of revolution, the satellite will behave normally; if the. satellite gets round in. less than a day, its apparent ■motion will be reversed. The period of revolntion of a satellite is governed by its distance from the planet, and the "backwards" satellite of Mars is remarkably close to the planet. The distance is less than 6000 miles, whereas the distance of the moon from the earth is nearly 240,000 miles. This closeness gives it a very short period of revolution. Mars itself is 4316 miles in diameter, a little more than half the diameter, and about one-sixth the bulk, of the earth. It rotates on its axis in about half-an-hour longer than an earthly day —24 hours 37 minutes 23 seconds, "to be almost exact. MONTHS OF HOURS ONLY. The outermost ol" the two moons, named Deiuios, revolves about Mars in a "month" of only 30 hours 18 minutes. This planet would, therefore, appear to a Martian to rise in the east and set in the west, and would also traverse the heavens at a very slow rate, tho time hctween rising and setting being about two days and a-ha]f. The other satellite, Phobos.is, so far as is known, unique in the solar system, because it revolves round tho planet in much less than one day—actually in 7 hours 40 minutes. The result is, that it is running ahead of the planet all the time, and instead of appearing in tho east as the result of the west-to-east rotation of Mars, it rises in the west and rushes across the sky, making more than threecomplete revolutions in each Martian day. This has the offect of making Fhobos rise and set about twice' each day. Another remarkable fact about these moons''is their"diminutive size. ' TOO SMALL TO MEASURE. The only way of estimating their size is by their brightness, because, US' they are never less than about 35,000.000 miles from the earth, they are mere j points of light. It is guessed, therefore, ! that Deimos is about ten miles in. ! diameter and Phoboa ie a, little larger. As, however, Phobos is l only about ! 6000 miles from Mars, it would have a j presentable appearance to a. Martian, ' though it would still be much smaller than our mocn looks' to us. j Until 1877 it was supposed'—in tho absence of visual proof to the contrary —that, unlike all the other planels'whose orbits are outside that of the earth, Mars had no satellites. In that year, however, the planet was unusually near the earth, and both moons were, by acombination of favourable circumstances, discovered by Professor Asaph Hall, of the Washington Observatory. Having regard to the small size of the two satellites, it is not safe to assume | that they are spherical. The force | which causes the spherical shape of a planet is its own gravitation, and it is conceivable that the moons of Mars, having only a small insrinfiic' gravitation, may be merely irregular lumps of rock. SWIFTS WONDERFUL GUESS. Sir Robert Ball, the famous astronomer, draws attention, in one of his books, to a truly remarkable prophecy made by Dean ' Swift in "Gulliver's Travels." The astronomers on the imaginary flying Island of Laputa, in. the famous romance, were recorded by Swift as. having discovered that Mara had two satellites, of which one revolved about the planet in ten hours. The author had thus-, not only made ai correct guess at the number of the satellites, but had also actually statedl the periodic time of the more remarkable of them with considerable accuracy. The closeness of the guess is the more astonishing from the fact that, prior to the actual truth being known, any astronomer would have regarded a tenhour satellite as an absurdity.