ASTRONOMICAL NOTES

Press, Volume LXXII, Issue 21746, 31 March 1936, Page 9

ASTRONOMICAL NOTES

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APRIL, 1936

The planet Mercury will be in superior conjunction with the sun on April 10; it will not be visible in the western sky until the end of the month. Venus will rise on April 1 at 4.42 a.m. and on April 15 at 5.18 a -™-> Mars will set on these dates at 6.58 p.m. and 6.28 p.m. respectively. Jupiter will rise on April 1 at 9.54 p.m. and on April 15 at 8.59 p.m.; Saturn will rise on these dates at 4.37 a.m. ana 3.52 a.m. respectively. The Date of Easter An enquiry as to the mode of calculating the date on which Easter Sunday falls in any given year reached me some time ago, but I thought it better to postpone consideration ot tne matter until I was writing the astronomical notes for the month in which Easter Sunday falls. It may be well to state clearly what is the rule for determining the date of Easter Sunday. According to Dr. Morgan, as set forth in the Companion to the Almanac, 1845, the Nicene Council decreed in 325 A.D. that the Roman practice in computing the d;ate < of Easter should be followed, and this practice required that Easter Sunday should be the first Sunday after the first full moon next following the vernal equinox—full moon being assumed to occur on the fourteenth day from the day of the preceding new moon (though, as a matter of fact, it occurs on an average after an interval of rather more than 142 days), and the vernal equinox being assumed to fall on March 21 (though, as a matter of fact, it falls sometimes on March 22). When, in 1582, Pope Gregory XIII. corrected the Julian calendar and introduced the one we now use, he retained the above rule and two assumptions on the ground that it was inexpedient to alter a rule with which so many traditions were associated. In the British Act of Parliament which adopted the Gregorian calendar, the explanatory clause which defines full moon is omitted, but practically full moon has been interpreted to mean the Roman ecclesiastical full moon; hence the Anglican and Roman rules are the same. At one time the German Lutheran states employed the actual sun and moon, but I do not know if they continue to do so. How to Find Easter Anyone who has studied the rules for finding Easter prefixed to the Book of Common Prayer of the Church of England is aware of the complicated nature of the calculations which have to be made. These rules are, I believe, due to Clavius, to whom the work of framing the new calendar was entrusted; he computed them on the supposition that the year contained, on the average, 365.2425 days; it actually contains, on the average, 365.2422419 days, _ and hence the Gregorian calendar is in error by one day in about 3600 years. There is a much simpler method than that due to Clavius for computing the date of Easter. As originally propounded by Gauss it was not quite accurate, but the requisite correction was supplied by pelambre. It involves some algebraic notation, but it is of so elementary a character that it cannot cause any distress, even to the non-mathematical mind. Once mastered any reader has at his command a means of computing the date of Easter for any year between 1900 and 2099. The steps are as follows: (i) Divide the given year by 4, 7, 19 and let the respective remainders be a, b, c; (ii) divide 19c •+■ 24 by 30 and let the remainder be d; (iii) divide 2a + 4b + 6d + 5 by 7 and let the remainder be e. Then, if d + e is not greater than 9, Easter Sunday will be op the (22 4- d -4e)th day of March; if d + e is greater than 9, Easter Sunday will be on the (d -f- e—9)th ddy of April, unless that day falls on the 25th or 26th, in which case Easter Sunday will be on the 18th or 19th as the case may be. At first sight it looks rather formidable, but it is not so; it is merely tedious. To make sure that my readers. do not let themselves be discouraged I will apply this procedure to 1936; we know beforehand that the answer has to be April 12. Dividing 1936 by 4, 7, and 19, the remainders are respectively 0, 4. and 17; i.e., a=o, b=4, c=l7 and hence 19c -f- 24 is 347; this when divided by 30 leaves a remainder of 17, and so d=l7. We have therefore that 2a + 4b + 6d -f- 5 =0 + 16 + 102 •+- 5=123; on dividing this by 7 the remainder is 4, and therefore e=4 and d + e=l7 4- 4= 21. Hence since d 4- eis greater than 9, the date of Easter. 1936, is April (21—9), i.e., April 12. Q.E.D. A Delicate Point The late W. W. Rouse Ball writing on the date of Easter draws attention to a possible difficulty which may arise in fixing this date. “Assuming that the Gregorian calendar and tradition are used, there still remains one point in this definition of Easter which might lead to different nations keeping the feast at different times. This arises from the fact that local time is used. For instance, the difference of local time between Rome and London is about 50 minutes. Thus the instant of the first full moon next after the vernal equinox might occur in Rome on a Sunday morning (say, at 12.30 a.m.), while in England it would still be Saturday evening (11.40 p.m.), in which case our Easter would be one week earlier than at Rome. Clavius foresaw the difficulty, and the Roman communion all over the world keep Easter on that day of the month which is determined by the use of the rule at Rome. But presumably the British Parliament intended time to be determined by the Greenwich meridian, and if so the Anglican and Roman dates for Easter might differ by a week; whether such a case has ever arisen or been discussed I do not know, and I leave to ecclesiastics to say how it should be settled.” Extra-Galactic Nebulae

During the last five years the University of Harvard has been studying at its branch observatory at Bloemfontein, South Africa, the distribution of extra-galactic nebulae in the southern skies by means of long-exposure photographs taken by the Bruce telescope. These galaxies are systems of stars similar in character to the universe to which our sun belongs. Dr. Harlow Shapley, the director, has published recently an account of the work done during the last five years in mapping the nebulae discovered in the southern constellation Horologium, and it appears that in this region is a remarkable aggregation of these objests, an area less than 1 per cent, of the entire sky yielding no less than 7889 galaxies; these have been classified according to position, brightness, diameter, form, and structure; they are in almost all cases fainter than the fifteenth magnitude. This constellation is, according to Dr. Shapley, “a congested area” as the galaxies here occur with a density which is twice as great as that of space in general, while in certain parts of this cloud there are clusters of nebulae when the concentration is three times as much as that found elsewhere. By intensive work of this kind is gradually being built up a fund of information from which we may hope ultimately to obtain some considerable insight into the structure of that part of the universe which our most powerful telescopes can explore. The 200-Inch Telescope About a year ago I referred in these notes to the second attempt then being made to secure a suitable lens of pyrex-glass on which to figure the mirror for the projected 200-inch tele-

scope. The period required for cooling ended a short time ago, and it is very satisfactory to know that the casting of this huge mass of glass has been successful. The next step is the transference of the disc to California, where the grinding and polishing will take place. The preparation of the glass for the coating of aluminium which will be the reflecting medium employed will occupy about five years. At the finish it is hoped that the mirror will be dimensionally true to one-millionth of an inch. This means, as one writer puts it, "if the 20-ton disc were enlarged to cover the British Isles, the rise and fall on its gigantic skating floor would nowhere exceed threequarters of an inch." The estimated cost of the mirror is placed at £l.2oo.ooo—making it easily the most expensive piece of glass in the world. The telescope will be erected at Mount Palomar, San Diego, California, a locality about 100 miles south of Pasadena. The new observatory will be about 6126 feet above sea-level, or about 300 feet higher than the Mount Wilson Observatory—the site of the Hooker telescope, of 100-inch diameter, which, during the 15 years of its existence, has made additions to our knowledge which may,be said to have revolutionised our conceptions of the universe.