IN STARRY SKIES

Evening Post, Volume CIX, Issue 53, 5 March 1925, Page 16

IN STARRY SKIES

SOME QUESTIONS ANSWERED

THE SKY AS A CLOCK

(By -" Omega Centauri.")

In our* last note on " The Sky as a Clock " we intended to distinguish between constellations and individual stars by printing all the names of the latter in italics. Unfortunately this was not done, but -probably little confusion was caused, as most readers would understand that the names of the letters of the Greek alphabet always refer in astronomy to stars, never to constellations, and that these letters are assigned to the niember3 of each constellation, for the most part, in the order of their brightness. The most conspicuous star in any group is usually called Alpha, and the others receive the names Beta, Gamma, Delta, and so on, in the order of their decreasing brilliancy. A few stars have, in addition, proper names, such as Sirius, Bigel, or Canopus.

The list of hour-circles on the Meridian was given for 8 o'clock in the evening only, and for intervals of fifteen days • but there is no difficulty in using the table for any date and any hour. For example; at 8 p.m. on 20th February the hour-circle "VI. is on the' Meridian. By 9 p.m. the circle VII.' will have taken its place, and so on through .the night. After about fifteen days any particular, hour-circle will be on the Meridian an hour earlier than before. So ,far a date which is between any two of those given in the table it is only necessary to remember that the clock gains an hour in fifteen days, or, roughly, half an hour a week, or four minutes a day. The correspondent who asked about the stars which are on the Meridian added another question, which, however, he answered fairly completely himself. " And, further, he said, " will you kindly explain how one arrives at the apparent passage of the sun through the several signs of the Zodiac?" No doubt the explanation of the latter is simple enough when one knows it, so simple, presumably, that the books do not trouble to mention it. The only method that this parishioner has discovered is based upon knowledge of the face of a clock —the supposition that if Taurus' is overhead the sun is in the Scorpion,'and so on; but this seems inadequate and clumsy." Our correspondent's suggestion is perfectly right i as far as it goes, except that he should have specified the time. If Taurus is on the Meridian at midnight, the sun, being twelve hours from the Meridian— that is, on the opposite part of the same great circle—must be in the Scorpion. Now that there are observatories equipped with clocks and transit instruments in all parts of the world, the problem is quite simple. It must have presented much greater difficulty ;to ancient astronomers. The interval between the passage of the sun and . that of any known star across the Meridian gives the exact right ascension of the sun, allowance, of course, being made for the advance of the sun amongst" the stars,jwhich is due to the motion of the earth "in its orbits When instruments were few and' primitive the problem was of quite a different order. ANCIENT METHODS. Notwithstanding all the difficulties, the ancienta achieved some astonishing results. By such methods as watching the helical risings of the stars, or observing the shadows of the gnomoses of their sun-dials, they attained a marvellous insight into the structure of the solar sy6tem. The helical rising of a star, or of any heavenly body,, occurs when it rises above the horizon exactly ai sunrise. ■ Now, this is difficult or impossible to observe. But these great men of the past noticed the stars that rose just before they were hidden by the brightness of dawn, and those which set just after the 6un had disappeared. From these facts they drew their inferences. Again, they watched the shadow of a gnomon from day to day. In some cases the gnomon was a vertical pole whose shadow fell on a graduated horizontal plane. It is interesting to notice that the name gnomon meant originally "one that knows," for its use led to a great increase of knowledge. The length of the shadow each day was seen -to be least at noon, whilst the noon shadow was least at midsummer. By observing the annual changes for many seasons, the length of the year was determined. So accurately was this done that a difference was detected between the length of the year deduced from the gnomon, and that 'determined from the helical risings of the stars. Hipparchers thus discovered the procession of the equinoxes. He fgund^that the eclipse appears a fixed circle on the celestial sphere, but that the equator moved. Its plane remains at a constant angle to the ecliptic, but the points over the two circles cross are. for ever changing. They _ move along the ecliptic in the directipn oppoeite to the sun's motion, completing the whole circle in about 2580 years. This slow motion of about fifty seconds in the angular position of the equinox per year causes a difference of about twenty minutes in the lengths of the year as determined by the two methods juet described. The cause of the procession of the equinoxes-is the departure of the form of the earth from that of a perfect sphere, combined with the fact that its axis of' rotation is not at right angles to the ecliptic. The earth may be considered to be made up oi two portions : (i.), a perfect sphere with a diameter equal to the polar axis; (li.), an outer shell whose thickness increases from nothing at the poles to some thirteen miles at the equator. It is the attraction of the moon and the 6un on this tilted shell which causes the puzzling changes. The moon's orbit is inclined to the ecliptic at a little more than five degree* and the points where the orbits cross ■more westward at the rate of one revolution in nearly nineteen years. Two curious consequences follow. When the ascending side of the moon's orbitis at the vernal equinox the angle botween the moon's orbit and the equator as about 28* degrees, but about 9* years later, when it has moved to the autumnal equinox, the angle is only m degrees. The attractive force of the moon on the equatorial shell goes through a nineteen-year cycle, and the pole of the equator deviates repeatedly from the circle it would otherwise describe aboul the pole., of the ecliptic once in 25,800 years. It has thus a nodding, motion known as nutation.' One wave is superimposed on the circle every nineteen years, and about 1400 of these fill Hie whole circumference. These irregularities add great complications to the time problem. The stars provide a clock whose indications are easy to reauii' approximately, buL extremely difficult to master completely in all iheir minute and intricate variations.