HARDWICKE'S SCIENCE-GOSSIP. 



265 



ASTRONOMICAL OBSERVATORIES AND WHAT IS 



DONE THERE. 



By F. W. LEVANDER, F.R.A.S., etc. 

 Part II. 



^I^S^^pHE more we see, the 

 more we want to 

 see, and at various 

 times our friend 

 presents to our 

 astonished gaze sun, 

 moon, planets, 



stars, and those 

 wondrous whisp- 

 like things we call 

 nebulas. If we wish 

 to feast our eyes 

 with views of the 

 planets, one, or, 

 perhaps, all of the 

 most interesting of 

 that class — Mars, 

 Jupiter, Saturn — 

 may be above the 

 horizon. In Mars 

 we see, as has been happily remarked, the counter- 

 part of our own earth. Up to recent years it was 

 thought to be unattended by any satellite, but in 

 August 1S77, two tiny moons were discovered. That 

 the planet itself has an atmosphere is evident from 

 the fact that when, owing to the inclination of the 

 plane of its orbit to that of the ecliptic, the sun 

 shines more directly on either of its poles, the winter 

 snows which have accumulated there gradually dis- 

 appear, to be again renewed in due course. Other 

 markings there are that are permanent, correspond- 

 ing to our land and water ; all these have received 

 the names of former or living men of science. By 

 observations of these streaks and spots, the time of 

 the planet's rotation on its axis has been found to be 

 a little more than 24 hrs. 37 min. Under certain 

 circumstances the existence of these markings can be 

 observed with very small optical power — the writer 

 No. 312.— December 1890. 



has seen that there are such spots with an object- 

 glass of only one and a half inches in aperture ; with 

 even a six-inch glass the polar snows appear intensely 

 bright. The planet's diameter is about half that of 

 the earth and its mean distance from the sun about 

 140,000,000 miles. 



The appearance of Jupiter is very different from 

 that of Mars. On his surface we see grey (and some- 

 times other coloured) streaks or belts, which vary in 

 size, shape and position, but are always more or less 

 parallel to the equator of the planet. His four moons, 

 appearing sometimes on one side of him, sometimes 

 on the other, now passing over, and again hidden, by 

 his disc, owing to their motion round their primary, 

 are always objects of interest to a young astronomer. 

 To observations of these comparatively small bodies, 

 and to Romer's investigations of them in 1675, * s due 

 our knowledge that light is not instantaneous in its 

 movement. We now know that its speed is not far 

 short of 190,000 miles per second. The mean or 

 average distance between the sun and the earth is 

 about 92,000,000 miles, that between the sun and 

 Jupiter about 483,000,000. As both planets are con- 

 tinually moving round the sun, their relative distances 

 are of necessity constantly varying. The light which 

 emanates from the sun and illuminates Jupiter's 

 satellites, and is by them reflected to the earth has 

 accordingly to travel over distances ranging, roughly 

 speaking, from 400,000,000 to 580,000,000 miles. 

 The fact that the observed times of their motions 

 across and behind the disc of their primary— their 

 transits and eclipses — were not in accordance with 

 those obtained by calculation, led to the above- 

 mentioned discovery. 



To see Saturn at his best such a concatenation of 

 favourable circumstances is required as seldom occurs 

 in our climate. But if we are fortunate enough to 

 meet with a really good night, what a glorious, all- 



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