A MINIATURE SOLAR SYSTEM

Wanganui Chronicle, 6 May 1947, Page 7

A MINIATURE SOLAR SYSTEM

By “Sky Pilot.” The Planet Saturn, which for sometime has been such a glorious sight both in tiie sky to the observer and in the telescope, is fast moving towards the western sky. On the other hand the Planet Jupiter, the largest of the planets, a little over half as far out as Saturn, is coining into view and will be a great sight for weeks to come. This is a most fascinating celestial object for small and large telescopes. Even tvith a 2” Refractor the Moon system of this planet can be seen presented with the planet as “a miniature solar system.” Night after night shows changes in the positions of the moons and adds to the attractiveness of the view. Large telescopes will reveal much more showing the bands of the planet some of which appear quite dark. Jupiter is 483,000,000, miles distant. That is how far through space our eyes gaze when we see this planet. Travelling at the rate of 8 miles per second in its orbit it takes this planet about 12 years to make •its revolution. The distance is has to travel is of course much greater lhan that of the earth for we are only 93,000 000, miles from the Sun. Then again the earth is travelling at the rate of 18a miles per second round its orbit, which is over twice as fast as Jupiter’s speed. It might be well to mention that the further a planet is out from the Sun the slower it travels. Pluto, the furthest out travels at the rate of 3 miles per second. Saturn s speed is 6 miles per second. Jupiter has 11 satellites or moons, quite a little family to carry around as its makes its journey .round the sun. Four of the moons are bright enough to be seen with the naked eye as faint stars if they were not so close to the planet as they are. For this reason they may easily be picked up with the smallest telescope. They have been given names but they usually go by numbers, in order of their distance from the planet. The first ano second are about the size of the moon; the third and fourth about fifty per cent larger. The largest, is bigger than Mercury. Their orbits are nearly circular and in the plane of the planet’s equator. The density of the first three is comparable with ordinary rocks, that of the fourth with that of water. Their light upon the planet does not equal more than 30 per cent, of the light of the full moon, on the earth. Part of the fascination of observing Jupiter is due to the eclipses and transits of the satellites. “As the satellites revolve around Jupiter, they appear to us to move back and forth almost on the same straight line; the forward swing takes them behind the planet and through its shadow (the fourth satellite often clears it), the backward swing in front of the planet, when their own shadows are cast upon it. For observation with a telescope it is such phe:K/iena as transits, occultations, shadow-transits, and eclipses that arouse the interest. Observers may readily observe these phenomena as their frequent occurrences are predicted in astronomical almanacs and Whittaker’s almanacs. By observing the eclipses of Jupiter’s satellites Roemer fir t demonstrated the finite speed of light. DIFFICULT TO OBSERVE The other seven satellites are difficult to observe oven with the very largest telescopes as they have diameters of iess than 100 miles in most 1 cases. The fifth satellite revolves in its orbit in about 12 hours and is therefore very close to the Primary. It revolves at the rate of one thousand miles per minute. The six outer satellites urc quite different from the

(i• hers in that their orbits have greater eccentricity and they are not in the same plane with the others. Their distance from the planet range from cast to west. All this for the observer with the small telescope of course counts for nothing as he cannot see them. Still the knowledge of them adds to the wonder of the planet and its system of moons. Some astronomers think that the outer satellites are asteroids captured by Jupiter and. are only temporary. It is interestrng to note that from Jupiter the outermost -moons would be invisible to the unaided eye and the sixth would appear as a faint The study of the moons of Jupiter has -.cry great interest from another standpoint. Our earth has one satellite 240,000 miles from it which plays a very important part in the life story of the earth and is very important so far as the question of habitation of the earth is concerned. Jupiter, the largest of all planets, eleven times the diameter of the earth has eleven moons, four of them very prominent indeed. Does a factor like (his after all not naturally arouse the curiosity and wonder of the ob■er.er? It is just one of the many things in the Universe that has pulled us up with a jolt and left us dazed With curiosity.