OUR CELESTIAL NEIGHBOURS.
Modern astronomy has completely revolutionised our conceptions of the earth's place in the universe. Instead of regarding this globe as the centre of creation, we know that it is an infinitesimal speck amongst countless other worlds, some far transcending even our sun in magnitude. Hence the motions, sizes, and distances of these companion worlds greatly interest ns, and, as Mr Gore has done so much in calculating the orbits of the binary stars, there is a peculiar fitness in his giving an account which be appropriately entitles " The Worlds of Space." One of the first questions that forces itßelf on the mind after realising the multitude of existing werlds is, Are they inhabited 7 This does not admit of solution, bat the farther question — Are they habitable? — can be answered with a considerable degree of probability so far as concerns our nearest neighbours — the planets that belorc- 'o the solar system. The giant planets- . j-.ter, Saturn, Uranus, and Neptune — are so far from the sun that its light and heat will not be sufficient to develop life anything analogous to that on the earth. There is considerable evidence, too, that their physical state is very different from tha earth's and
*"Tlie Worlds of Space." By J. E. Gore, F.R.A.S., &c
I that possibly they are eti)l at a red heat, Mercury, on theother hand, is so near the son that .the temperature must be far too high tor life. The moon presents every indication of beinc a dead world. There remain, then, Venus and Mars as probably habitable, and of the two, Venus will represent the early dfiya of terrestrial life, while Mars would give us a picture, could we only see it, of the old age which is the destiny of our planet. Leaving the solar system, it is no longer possible to disown any bat self-luminous bodies, and we can only infer on general principles that these suns will be light centres to planets invisible to us. The distances of the stars are so enormous that they are beyond comprehension, and have to be reckoned in terms of the velocity of light. Although light travels at the speed of 186.000 mile* per second, it would take it above forir years to' 1 come' from our nearest neighbour, Alpha Oentanri. This star is indeed unusually near,' at no other can' be found at leas than seven years' distance in the same measurement. It may be" asked, How is it possible to ascertain this ? The actual process is extremely delicate, but its principle i» very simple. If we look at a distant spire and, after walking 100 yds along- the road, again notice its position, we shall find it has slightly changed its " bearing." if these angles ar« accurately measured, it is then a very simple question of trigonometry to determine the distance of the spire from the road. Just in the same way, when the earth ii at opposite extremities of its orbit, -the nearest stars show an angular displacement or parallax, from which their distance is calculated.
Stellar parallax is so excessively minute that in most casea it cannot be measured. Mo>ri< than the mere ascertainment of their distance ba-», however, been accomplished in many instances. There is a surprisingly large number of binary stars, consisting of two components revolving about their common centre of gravity. As' soor as the period of revolution is known by observation, it becomes possible to calculate tha mass of the comnined system io terms of that of tbe sun, and the dimensions of tbeir orbit Wonderful as this is. it- has been surpassed by the discovery, by means of the spectroscope, of binary systems whose components are so close tbat no telescope ia powerful enough to separata them, and yet it is possible to calculate their orbits and velocities. Mlzar, the secdnd star in the tail ol the Groat-Bear, is an excellent illustration. Looked at on a clear night it is fcolerablj e«sy to see close to it a small star, popularly known as " Jack by the horse's head." Miz-r itself is resolved by the telescope into a double star, and the principal component ia itself a Bpeotrobcopic double, T?ith a period of 104 days, an orbit about the size of tbat oi Mars, and a mass 40 times that of the sun.
Any of onr readers interested in astronomy will thoroughly enjoy this book, and we will only add than it is beautifully illustrated by photographs of the nebula aad starcluatera.