ROUGH ASTRONOMICAL NOTES.

Evening Star, Issue 4190, 1 August 1876, Page 3

ROUGH ASTRONOMICAL NOTES.

August, 1876. The Sun is now moving rapidly eoutlw-.rd. About the 20th his north declination diminishes at the rate of 20 min. of arc per day. His meridian altitude, meridian passage, rising and setting, at Dunedin, on three days of the month, are as follows:

The Moon will be in Apogee on 2nd at 7h. p.m., in Perigee on 18th at Bh. a.m., and in Apogee again on 30th. at Bh. a.m. She attains her greatest S. Dec. on Ist at midnight, her greatest N. Dec. on 16th at 3h; a.m., and her greatest S. Dec. again on 29th at, Bh. r.m. Her phases occur as follows, New Zealand Mean time:— Full Moon, sd. 6h. Bm. p.m. Last Quarter, 13d. 9h. 2dm. a.m. New Moon, 19d. lib. 56m. p.m. First Quarter, 27d. sh. 47m. a.m. The Moon will be in conjunction with Saturn on 7th at 4.30 p.m., with Venus on 17th at 4 a.m., and with Jupiter on 26th at 2 p.m. Mercury, in superior con j auction with the Sun on Sth at noon, will be invisible the greater part of the month, but; may be seen towards the end, about half an hour, after sunset.

Venus will bo a fine object in tbe mornings during the month, for those who can get up to view her. She' will U» stationary on the Sth, at noon, and at her greatest brilliancy on 20th, at which time she appears,. when viewed through a good telescope, like a young moon, four days old, but much more brilliant, Venus is best observed iu the daytime, because her brightm ss produces so much irradiation when the sky is dark that it is difficult to get a good view of her figure. In daylight or in twilight this difficulty disappears. Those possessed of a transit instrument, or an equatorial, will have no difficulty in finding her on the meridian by the following table

Mars is in conjunction with the Sun on the 13th, and too near tne Sun for observation, all the month. Jupiter is in Libra, and well situated for observa. tion, . The N.Z. time of re-appenrance of the first satellite, in those eclipses which are visible in New Zealand, is as follows;

d. hj, m. a. d. hj. m. Aug. 7 8 18 46 p.m. Jupiter sets 8 142 a.m. 14 10 13 32 „ 15 i 17 22 0 8 17 a.m. 22 0 53 !! 23 6 36 59 p.m. 24 0 46 .. 30 8 81 45 „ 31 0 23 ” The tame of re-appeamr.ce given is when the satellite is half out of the shadow of Jupiter. A powerful telescope will show it some seconds sooner, and an inferior one some seconds later. Prom the time when it begins to emerge until it is endrelyontof the shadow, about three and a-half minutes elapse, which is the time required to travel its own diameter in its orbit round Jupiter, A good approximation may be made to the longitude of a place by observing the eclipses of this satellite in the following way Make a series of tions of the local time of disappearance before Jnpiter's opposition, and a series of observations of the local time of reappearance after opposition, using the same telescope on both occasions. Then the time errors will be about equal, and in opposite directions, so that they correct one another. The mean difference between the observed times and the Greenwich times, taking each series as one observai ion, will be the longitude of the place nearly. Saturn,, this month and next, is better situated for observation tbas he will bo again for several f 6O -™- Be w ill he on the meridian at lb. 6m. a.m. on 15th; ait., 61deg. 57mia. In two years more the nn? he presented to us edgewise, and will be invisible by means of ordinary telescopes. Afterwards, the other side will be shown, but then Saturn will be in the northern half of Ids orbit, and muoh lower than ho is at present. The ring ap pears now as an ellipse, whose major axis Is to minor axis ns 7 to 1. Uranus will be in conjunction with the Sun on 13 th, Neptune is in Arles, and not well situated for observation. . MEAU nr ifSITT OF THE CKITERSE If we suppose a sphere having the bun for its centre, and whose diameter is 100 million* of miles, it will contain the Sun and Alpha Ceutauri, witi.' their attendants. If these wore united into one globe, of the same density os onr Sun, its diameter would bo about two millions of miles, supposing

the stars to have the aamedsnatty as the Sun. The ratio of the diameters of the sphere and globe Is fifty millions to one. If the sphere be extended Until its diameter is 200 billions of miles, it will then embrace Sirius, Vega, 61 Cygni, and three or four other small stars; the volume of Sirius is more than fifty times that of all the others together, the Sun and A Centauri included. The diameter ol S.rius is at least ten, millions of miles, and the others _ added would not sensibly affect it This_ gives a ratio of sphere to globe, twenty millions to one. This is about as far as w< can go with any degree of certainty at present. The probability is, however, that on further extensioo of the sphere the ratio would again rise towards the former figure; for it is not likely that another giant like Sirins exists within a moderate range. If we take 40 millions to 1 as tke average ratio, we shall,; perhaps, be near the truth. The cube of 40 millions is 64,000 trillions, which is the ratio of the volumes of the sphere to the material globe. Now the density of the Sun is nearly 1,200 times the density of ordinary air. And 61,000 trillions divided by this number gives, in round numbers, 50 trillions as the number of times that the density of common air exceeds the mean density of the Universe, as near as we can judge.

Day of Mend. Now Zealand Moan Time. Oc Mend. Eises. fceta. month. Alt. h. m. s. h. m. li. m. 5 27° 10m. 0 18 37 7 26 5 1 15 80 ' 5 0 12 8 7 12 5 12 25 S3 28 0 9 47 6 55 5 25

Day. Mend, Alt. App. Biam. On Meridian a.m. De*. Min. Secs. H. M. 6 27 55 46 10 6 13 27 36 41 9 43 20 27 20 87 9 28 27 27 10 33 9 17