ASTRONOMICAL NOTES FOR MAY.

Lyttelton Times, Volume CXX, Issue 14982, 30 April 1909, Page 6

ASTRONOMICAL NOTES FOR MAY.

The planet Jupiter will continue to be visible in the evenings during the whole of May, but it is much more toward the west than it was a month ago. On May 1 the planet passes the meridian at 7.53 p.m., and sets at 1.12 the following morning. On. May 31 it passes the meridian at 6 p.m., and sets at 11.21 p.m.. Its position changes very slowly, and during the whole of the month it will be seen a few degrees oast of Rogulus, and quite close to a fourth magnitude star Rho Leonis. As a contrast to Jupiter, Mars may be observed moving rather rapidly among tlie stars. At the beginning of the month it is in Ca.pricornus, near the star Theta Capricorni, hut at the end of tho month it will have moved into Aquarius, and its position Will ho about twonty degrees east of this star. On May 1 the planet rises at 11.18 p.m., and its time of rising varies only about twenty - minutes during the month. On May 12 it will he close to the moon. The little planet Pallas is visible at present. Its position is in tho constellation Hercules, about twenty-one degrees north of the equator. This is a very unusual place for a planet, because in this right ascension the sun’s is about twenty-three degrees south of the .-equator; so that it is evident that Pallas must be quite forty-

four degrees north of the ecliptic. The explanation is that the orbit of Pallas has a very high inclination, and on May 10 the planet will be at its greatest height above the plane of the ecliptic. The latitude measured from the sun is then equal to the inclination, namely; 34deg 43min, but the planet is nearer to the earth than to the sun, so the geocentric .latitude is greater than the lieiiocentric latitude, and amounts to 44-dg 16min. Another of the brighter minor planets, Vesta, is in opposition to the sun this month, hut its position is much farther south in that part of the constellation Ophiuchus which crosses the ecliptic. Vesta is brighter than any of the other minor planets, and may sometimes be seen without a telescope. Its apparent magnitude this month is 5.9. The diameter of its disc has been measured at the Lick Observatory, and it is very much less than that of Ceres and Pallas, so the reason of its superior brightness must bo caused by its surface reflecting sunlight better than the others. About May 6 or 6 the earth will be close to the track of Halley’s comet. There is a recognised meteor-stream which is seen at this time every year, but it yields only a few meteors, apparently radiating from the constellation Aquarius, and nothing resembling a conspicuous shower has ever been reported. But if these meteors are really associated _ with Halley’s comet, there is a possibility that tlie shower may ho more plentiful this year, because the comet is only about a year’s journey distant. This is pointed out by Mr Denning, who has made a speciality of studying meteoric radiants'. He admits that there are no records of rich showers of Aquarid meteors in 1759 or 1835, when Halley’s comet previously returned to perihelion, but he argues that they may easily have escaped notice like many other showers. Hia letter concludes: “It now remains for observers in immediately ensuing years to determine whether, like the Leonids and Andromedids, this meteor stream develops unusual activity near the time of the return of the parent comet.” _ A number of photographs of the eighth satellite of Jupiter were obtained at Greenwdch during February and March. It Was at its greatest elongation west of Jupiter, and the apparent distance between planet and satellite was a little more than two degrees. Tills distance is represented by about five inches in the focus of the large reflecting telescope at Greenwich, and as the plates used are only six inches long it follows that photographs showing the satellite near the centre of the plate do not show Jupiter at all. The method •adopted was to expose another plate upon Jupiter, and then to measure the positions of both planet and satellite with reference to the known stars photographed near them on the two different plates. According to Crommelin and Cow r eU’s elements the satellite was at its least distance from Jupiter last October, and will be at its greatest distance next October, the two distances being nine million miles and twenty million miles. It has appeared to the west of Jupiter all the time it has been under observation this year, but it will be in superior conjunction with the planet next December, and so when the opportunity conies to observe it again next year it will he time to search on the east of the' planet. A report recently published by the Cape of Good Hope Observatory describes an experiment made with a new method of determining the sun’s distance hy means of _ the spectroscope. When a star is moving directly toward or away from the earth, certain lines in its spectrum are moved, to one side or the other, and the rate of approach or recession can be calculated from the measured displacement. Now suppose a spectroscope to be used upon a star situated not far from the piano in which the earth is moving round the sun. There must bo one day in the year on which the earth is moving directly toward the star, and another day about six months later on which the earth is moving directly away from it. The star’s proper motion is assumed to he constant, but the rate at which its distance from the earth is changing must vary throughout the year because of the varying direction of the earth’s motion. If the greatest and least velocities can he measured correctly the difference between them represents twice the velocity at which the earth is moving, and from this the earth’s distance from the sun can be calculated according to the laws of gravitation. Instead of expressing the distance in millions of miles, it is usual to state the apparent semidiameter of the earth if viewed from this distance. This angle is called, the solar parallax, and the spectroscopic method makes its mean value 8.800 seconds, which is very nearly the same as that given by several other methods. It is almost certain that this value cannot he more than one hundredth part of a second in error, but it is hoped that a result with a “probable error” of not more than one thousandth of a second may be obtained in the year 1931, when the minor planet Eros will make an exceptionally close approach to the earth. ’ -