WITH SMALL PLANETS

Taranaki Daily News, 13 April 1935, Page 13 (Supplement)

WITH SMALL PLANETS

THE SIZE OF MOUNTAINS

(By

the Rev. B. Dudley, F.R.A.S.)

So many asteroids or minor planets are discovered annually that Mr. G. F. Chambers facetiously remarked at a meeting of a London Astronomical Association a few years ago when he was its President: “Members who discover any more of these little bodies ought to be summarily excommunicated.” Each new addition to an already numerous host has to be followed up and watched closely. Once discovered, it is necessary to name, or number it, and keep in touch with its movements. This means a great deal of work forth ose concerned. As it is, some of these objects are lost, and subsequently rediscovered. As far back as 1772 a curious relation was found to subsist between the distances of the planets from the sun. A Wittemberg astronomer, J. D. Titius, took the numbers 0,3,6,12,24,48,96 and to each of them added the number 4, thus obtaining the series 4,7,10,16,28,52,100. If now the distance of the earth from the sun is represented by 10, then the distances of the other planets known, at that time will be—Mercury 3.9, Venus 7.2, Mars 15.2, Jupiter 52, Saturn 95.4. It was found that these numbers were in remarkable agreement with those suggested by Titius. Another astronomer, Johann Elert Bode of Hamburg, afterwards director of the Berlin observatory, suggested that a search should be made for unknown planets between the orbit of Mars and that of Jupiter, so as to fill in the gap represented by the number 28. He thought it possible to extend the numbers to 196, 388, and sb on. Hence it is that the arithmetical relation is known as “Bode’s Law.” There proved to be a breach in the regularity of the operation of this law Ln the case of such distances as those of Saturn and Uranus and a complete breakdown in the case of Neptune; yet the series has been useful on at least two important occasions in the history of astronomy. It led,- for example, to the discovery of the minor planets whose name, as wo have seen, is legion. On January 1, 1801, the first day of the 19th century, Piazzi detected a tiny body to which he gave the name of Ceres. Since then the list has grown continually, and the total now reaches 1300. The reason why they are known as asteroids is simple enough. It is because of their similarity to a star (“aster” in Greek) in that they show no appreciable disc.

The larger planets in a general way keep to one plane in their motion about the sun. The asteroids adhere to it less closely. For this reason searchers for such objects usually confine their operations to a narrow zone of the sky. At first discoveries were made mainly by comparing the stars seen in the field of a telescope with those mapped out in star charts. An innovator was detected by the fact of its motion amongst' the known stars. This laborious method has been improved upon by photography. The camera attached to the telescope was kept focussed upon the same region of the sky by clockdriven action, enabling the astronomer to get a long exposure. In this way the real stars came out as dots, whereas an asteroid (should there be one) appeared’ on the plate as a white streak. A better method, now generally adopted, is : to impart to the telescope a motion such that it would follow, not the stars, but the asteroid, in the event of there being one in the field. Thus the plate would show the stars as streaks and the asteroid as a dot or. line, which could not fail to be recognised. This improvement lies in the fact that this method; permits the astronomer to pick up very faint objects such as \asteroids generally are. Work on these bodies has shown that they are not absolutely limited to the space between Mars and Jupiter. There are a few fugitives which stray from the path of rectitude. One of the asteroids, Eros, comes at times actually within the orbit of Mars, while the remotest of them goes' slightly beyond the distance of Jupiter. The average distance, however, agrees with the space relation given by Bode’s Law. At times when the earth and Eros are on the same side of the sun their distance apart is small. Such occasions afford an opportunity to astronomers to more accurately determine the sun’s distance from us. Favourable appositions of this kind happened in 1901 and 1931, when Eros was closely observed by mathematicians throughout the world. It lends itself to this purpose not only because of its relative nearness to the parth, but also because its star-like image is an advantage, in determining its own position accurately. In 1931 Eros was' only 16,200,000; miles -from the earth. No other planet had ever approached us so nearly, f; : - About two years ago two more of these tiny planets were, discovered and found to come even closer to the earth than Eros and they are likely to be still more valuable than it for determining the sun’s distance. One of them is known as Delporte, named after the Belgian astronomer who first glimpsed it. Ten days after its discovery this asteroid was found to be only 10,900,000 miles from us. A little later still Reinmuth of Heidelburg made another happy discovery. This was the asteroid which now bears his name and which, on reaching its minimum distance, as it did in May, 1933, was only about 6,000,000 miles from the earth. The naked eye is incapable of seeing any of the asteroids. At its closest approach “Delporte,” for instance, would have to be 625 times as bright as it then was to attain naked-eye visibility. Needless to state, therefore, these interesting objects are very small. The largest known is considered to have a diameter of 480 miles. The smaller ones range from about 10 to 50 miles in diameter. The two exceptional bodies, Delporte and Reinmuth, stand alone almost for smallness, being not more than a mile in diameter. In fact, as someone has put. it, they are just fair sized mountains. The brightness of most of the asteroids varies. Nor is this due alone to, their changing distance from us. Evidently their different -sides have different reflective power. They are not necessarily round, and as their several “faces’ are turned toward us, they might easily show a varying brilliancy. How they originate can be guessed at only. Did a planet fail in the making? Or has a planet at some time in its history exploded, leaving us only the fragments? If the latter theory’—the more favoured one—be true, the exploded world must have been a small one, unless there are countless hosts of fragments, such as Eros and Delporte, yet undiscovered. The total mass of all known asteroids is not more than a two thousandth part of the mass of the earth.