ASTRONOMICAL NOTES.

Press, Volume LXVI, Issue 19942, 31 May 1930, Page 11

ASTRONOMICAL NOTES.

FOR JUNE, 1930. (»P*CIALT.* TOMTTM fOB THB PBXSS.) [By E. G. Hogg, M.A., F.R.A.S.] The sun will enter thp zodiacal sign Cancer on June 22nd; this will be the shortest day of the year in the Southern Hemisphere. The meridian altitude of the sun on that day at Christchurch will be only 23 deg. 2 mins.; the distance of the earth from the sun will be about 94,530,000 miles. The planet Mercury will rise on June Ist at 5.52 a.m. and on June 15th at 5.36 a.m., and Venus will set on these days at 6.18 p.m. and 6.46 p.m. respectively. Mars will rise on June Ist at 3.50 a.m. and on June 15th at 3.49 a.m.; Jupiter will set on June Ist at 5.22 p.m.; it is rapidly approaching the'sun, with which body it will be in conjunction on June 20th,. and it will not be visible in the eastern sky until next month. Saturn will rise on June Ist at 6.39 p.m., and on June 15th at 5.40 p.m. The New Planet.

The references to the new planet in the scientific periodicals which have reached New Zealand do not throw much additional light on the subject, but it is interesting to learn that the detection of the planet was not due to a fortunate chanco but was the result of a systematic search which has been carried on for a long time at the Flagstaff Observatory, with the Lowell Lawrence telescope. A young assistant, Mr C. W. Tombaugli, had been conducting a photographic survey of the zone of the ecliptic and had worked his way round nearly half the circle when on January 21st success crowned his efforts and the long-souglit-for body was caught on one of the plato«. Photographs taken on January 23r.i and 29th confirmed tho discovery but it was not until the body had been under observation for nearly seven weeks that any announcement was made.

It would seem that the early estimates made of the planet's size were considerably in error and later opinion leans to the view that its diameter is not more than 4000 miles, so that instead of being comparable in volume with Saturn it may only just exceed Mercury in size. It is unlikely that the planet's mass will be known accurately for a long time; this feature of a planet can bo most easily deduced from the motions of the satellites which accompany it, but in the present caso we cannot hope that any satollite it may have will ever be visible to us and the planet's mass will have to be learnt from the observed deviations in tho motions of Uranus and Neptune which can be rightly attributed to the attraction of tho hew member of the solar system.

The Total Eclipse of October 21st, 1930. It would appear that New Zealand is not to be the only country, sending an expeditionary party to Niuafou or "Tin Can Island" to observe the total eclipse of the sun, which takes place on October 21st, as, from a recent issue of "Science," wo learn that the United States Naval Observatory proposes to be represented there by a body of American astronomers under the leadership of Dr. S. A. Mitchell, Director of the Lcander McCormick Observatory of the University of Virginia —a veteran in these matters as he has already seen no fewer than seven total eclipses of tho sun. This party, starting from San Francisco, will proceed to Pago Pago in American Samoa, whence it will be carried by a naval vessel to Niuafou.

The arrangements for the dispatch of the New Zealand expedition are progressing satisfactorily, and we hope to know soon its personnel and the details of its scientific equipment. Its Director, Dr. Adams, and his co-work-crs will, we may be sure, welcome the news that they will have with them during their long stay on the island, companions from whose experience in eclipse matters they will doubtless, be able to profit so much. There are few men who can be so well acquainted with the details of this work as Dr. Mitchell, who, in his book "Eclipses of the Sun," published in 1924, wrote "The author has travelled more than forty thousand miles to witness four total eclipses of the sun. The total time afforded him for scientific observations during these four eclipses has been a t>"riort of less than eleven minutes." The Niuafou trip will add very considerably to his mileage travelled, but will only increase the sum of his times of totality by 95 seconds.

Minor Planet 1929. The search for these aiminutive members of the solar system is being carried on with unabated zeal; 142 asteroids were discovered in 1928, but this record was beaten last year, when the number rose to 150, Herr K. Keinmuth, of the Konigstuhl Observatory, Heidelberg, with 73 to his credit easily heading the list of. successful observers. The magnitudes of those detected last -year ranged from 11 to 15.5, the average being nearly 14; none of those found, however, appears to be of any special interest. There are now 1114 of these objects to which either names or permanent members have b«en assigned, the procedure which constitutes the definite recognition of their status as minor planets. The number at the close of 1928, was 1083, hence only 31 were added to the list last year, from which we may infer that a large majority of those caught on the photographic plates had been previously discovered.

It may be mentioned here that observations made during the year on the spectrum of Vesta—the only asteroid visible to the naked, eve—lead to the conclusion that this body rotates on its axis in the short period of 5 hours 55 minutes—a result which agrees w'th the variations noticeable in its brightness.

Proxima Centauri. We are indebted to the Johannesburg "Star" for the following account of tho circumstances which led up to the discovery of our nearest stellar neighbour —Proxima Centauri. It would appear that about 1915 Dr. Innes, Director of the Union Observatory, Johannesburg, being impresseu with the increasing number of cases in which a star —single or binary—was found to be attended by one or more faint companions, decided to utilise th? services of his observatory to see whether the double-star

Alpha Centauri might not be added to the list. Ho possessed photographs of this region of the sky taken it Johannesburg in 1910; his assistant, Mr Wood, took in 1915 aother photograph of the region with the same telescope, and under the same conditions as the 1910 one. On comparing the two sets of plates Dr. Innes at once detected a faint star, of photograph magnitude 13.5, which had changed its position relative to the other stars during the interval of five years, and further investigation showed that its motion was nearly equal and parallel to that of Alpha Centauri. The connexion was further established when the parallax of the faint star was found to be almost exactly equal to that of Alpha and thus at approximately the same distance as it from the earth; the name Proxima Centauri was given to our new neighbour.

Becent Work on Proxima. We would premise that by the parallax of a star is meant the angle which would be formed if two straight lines were drawn from the star, one to the sun and the other to the earth, i.e., it is the angle subtended at the star by the line joining the earth to the sun; it.is in all cases a very minute quantity and can only be determined by the exercise of the greatest skill and care. Some new measurements of the parallaxes of Alpha and Proxima Centauri, made by Dr. Aldcn at the Yale station at Johannesburg, and presumably by the telescope which might have come to New Zealand had we been prepared to receive it, have recently been published; it appears that the parallaxes of Alpha and Proxima are respectively 0.765 and 0.783 seconds of arc, showing clearly that Proxima is the nearer of the two to the earth, its distance from us being 24.5 billion miles, while that of Alpha is 25.1 billions. The mean distance of Proxima from Alpha is 1.1 billion miles or more than twelve times that which separates the earth from the sun, and its period of revolution about Alpha is about a million years. It is a dwarf red star, with a temperature much below that of the sun; it is estimated that it is a little smaller in size than the planet Saturn.

Alpha Centauri. Tho binary nature of this star was discovered by the gifted Abbe Lacaille in 1752, at the Cape of Good Hope, whither he had travelled for the purpose of studying the stars of the Southern Hemisphere —a field _ of work which had not been touched' since Halley's visit to St. Helena in 1677-78. A great amount of information about this double-star—the most striking in the heavens—has accumulated by now, to j which we will briefly refer. _ The brighter component A is very similar to the sun, weighing about 14 per cent, more than that body, and being about j 12 per cent, more luminous; the other j member B is considerably redder than the sun, and its surface temperature is only about 4400 deg. centigrade, against the sun's 6000 deg. centigrade; it has 97 per cent, of the sun's weight, but j only about one-third of its luminosity,' and its diameter is 22 per cent, greater than that of the sun. The two to- j gether form a visual binary whose apparent magnitude is 0.3. They describe their orbit—an elongated ellipse—in 81.18 years, and we see it nearly edgewise consequently as they move round it the angular distance between them is continually changing. According to a paper contributed some years ago to the " Astrophysical Journal" by Dr. Lunt, of the Cape Observatory, the two stars were 17 seconds of arc apart in 1919; the distance between them is slowly decreasing at present, and in August, 1937. will be reduced to 4 seconds; after this they will separate out until in 1951 they will be 10 seconds from

each other; they will then close up, and in 1959 the angular distance between them will reach ita minimum value—just under 2 seconds. The maximum angular distance between them, which is just over 22 seconds, occurred in the middle of 1889, and will be reached again in 1980; on these occasions they are about 850 thousand million miles from each other.

Alpha Centauri is moving across the sky at the rate of about 3.68 seconds of arc per annum, and it also is receding from us at the rate of about 14 miles a second; by combining these movements Mr C. L. Stearns recently determined the actual motion of Alpha relative to the sun, and he found that it made its nearest approach to the earth about 28,000 years ago, when it was about three-tenths of a magnitude brighter than it is now. It will,, however, long remain the most brilliant spectacle of its kind in the heavens; telescopic equipment will doubtless improve vastly in the centuries to come, and we may expect that it will appeal tb our far-off descendants as powerfully as it does to us.