Astronomical Notes.

Evening Post, Volume LXI, Issue 30, 3 August 1895, Page 2 (Supplement)

Astronomical Notes.

■^>tSpeoially written for the Evbnino Post.)

It is not difficult to understand how the distances of the heavenly bodies from the earth have been found. When surveyors wish to know how far off a distant object is, they carefully measure a "base-line," and then the slant, or angle, of the object with this line from both ends. All this is then mapped down, so that the results from this map give the required distance with all exactness. The "base-line" for moondistance is the distance between the Observatory of Greenwich and that of the Cape, which is well known. At each Observatory the angles between the moon and this line are observed at fixed dates, and so, as before, tho moon's distance is gained, and the first step taken to form a scale for onr map. But this scale is too small for so important a matter, and the sun's distance must be found by more direct measurement. When the moon is at her first or third quarter, a line from the sun to the moon, from it to the earth, and from it to the sun, will form a right-angled triangle S.M.E., where S. is the sun, M. thd moon, 13. the earth. The line M.E. is the known distance of the moon from the eatth. The angle at M. is a right angle, that at E. is now measured, and thus in this triangle two angles and one side are known, and all else can be found by trigonometry, and side E.S. the sun's distance./ But another method was discovered by Br. Halley, from Venus' transits. Place an observer at the centre of the earth, and one at the N. and one at the S. Pole. The central observer fpund that Venus took, say, 10 hours to cross the sun. Each of the others found that their time was 9 hours. If a circle be drawn with diameter 10 inches, this line will be the observed path of Venus at the central station, and lines 9 incheß long placed in this circle parallel to the central line will give the paths from both N. and S. stations. But the distance between these N. and S. lines will be exactly 2 J times the well-known distance between the N. and S. observers. From this distance is found the value of the diameter, or 850,000 miles; and now the sun, being so. large, must be just so fax off (92,000,000 miles) to look so small. Ths apparent sizes of the sun and moon are smaller than that of a three-penny piece, as csn be easily seen by looking at them through dark glass. " Bnt it is a great difficulty to get the exact times of these Venus transits, 'and, another plan was proposed by Gill, the Cape astronomer. A.n observer takes the distance of Mars from small stars, when Mars is opposite the sun, in the evening and in the morning. These distances will differ, from the movement of the earth in its daily rotation between the observations, causing an apparent shift in Mars from the stars more or less as Mars is nearer or further away from the earth. As the length of the base-line, the distance of the stations from the earth's movement, is known, the observed change of the planet's position from the small stars gives its distance. Two small asteroids are to be used in this work, as the brightness of Mars is a disadvantage. This work involves no excitement as in Venus transits, which only ooour twice in 100 years. It can be repeated again and again, until an almost absolute oertainty is gained. Mr. Petrie discovered that thiß sun-distance was known to the builders of the Great Pyramid. He showed that if 'a line from one corner of the base to the opposite corner were divided into 10 lengths, 9 of these lengths would give the Pyramid's height; and that, if this height were multiplied by 10 raised to the 9th power, the result would be the distance of the sun. Thus there is now gained a scale for our map, in the sun's distance, and all else, as in an ordinary map, con be exactiy measured. A. Stock.