Volume 5

Popular science / edited by Arthur Mee.

Date:
1912-1913
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Popular science / edited by Arthur Mee. Source: Wellcome Collection.

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    Jupiter has no fewer than eight moons, which circle around him. Four of them are of very recent discovery, but the four larger satellites were the first objects to be dis covered after the invention of the telescope. It was on January 7, 1610, that Galileo turned his new instrument upon the giant planet, and saw that he was attended by four little stars which moved to one side of Jupiter and then to the other side, and passed before and behind him, giving the impression of a solar system in miniature. Galileo named them the “ Medicean stars,” after his noble patron, Cosmo de Medici. There was not a little annoyance and in credulity among scholastic philosophers when Galileo thus added to the number of the heavenly bodies, and upset theories which had satisfied everyone for a long time. The Moons and Recently Found Moonlets of the Giant Planet Until 1892 these four moons which Galileo discovered were regarded as the only satellites of Jupiter. They were named, in order of their nearness to the planet, Io, Europa, Ganymede, and Callisto. In size the two inner ones, Io and Europa, are not very different from our own moon, but the diameters of Ganymede and Callisto are nearly twice her diameter. Their distances from Jupiter are respec tively 260,000 miles, 414,000 miles, 661,000 miles, and 1,162,000 miles. Taking them in the same order, the periods of their revolutions, reckoned in days of Jupiter, are about four and a quarter days, eight and a half days, seventeen and a quarter days, and forty and a half days. Although these four greater moons would be of considerable size from the terrestrial point of view, they are very minute as compared with the planet to which they belong. Ganymede, for instance, has nearly twice the volume of the planet Mercury, and about two-thirds of the volume of the planet Mars, yet the volume of the four major satellites put together is little more than one-eight-thousandth of the volume of Jupiter. The Orbits and Occultations of Jupiter's Little Retinue of Moons The orbits in which the satellites move are in the plane of the equator of Jupiter, which is also the plane of the ecliptic or orbit of the earth. They appear, therefore, as tiny stars lying along a horizontal line which passes through the centre of the planet ; and the system of Jupiter and the four moons of Galileo cover a space in the sky about equal to two-thirds of the apparent diameter of our moon. These moons may be seen in transit over the face of Jupiter, and disappear, or are occulted, when they pass behind his disc. They are eclipsed when they pass into the huge shadow which extends outward from the planet away from the sun ; and from the fact that they become invisible when eclipsed it is unlikely that Jupiter, as some have supposed, throws out any light of his own. Further, they eclipse Jupiter himself, and may be seen to do so. Not that their tiny forms can throw the giant planet into darkness, but their shadows can be seen travelling over his face as minute black spots. From time to time it happens that all four of the Medicean stars become invisible simultaneously, some being eclipsed, and others being occulted or in transit over the planet’s face. These moons, like Jupiter himself, receive, area for area, only one-twenty-seventh of the sunlight which is received on earth, and therefore cannot have much importance as luminaries. Otherwise, they appear to be more efficient reflectors than our moon, with the exception of Callisto, which is curiously dark. The Puzzling Variation in the Light cf Jupiter’s Moons The four moons not only differ con siderably among themselves in respect of brightness, but, as Galileo himself dis covered, they vary individually from time to time in the amount of light which they reflect. There are regular and also irregular variations. Callisto appears to change in reflective power according to the position which it holds in its orbit, and is therefore believed to turn as our moon does, keeping always one face to its planet, and so turning surfaces of different degrees of whiteness and blackness in regular alternation towards the sun. The very puzzling irregular variations in lumin osity have been noticed by many astrono mers since Galileo, and admit of little doubt, although some of the most skilful observers with photometric instruments have failed to obtain reliable evidence of irregular changes in brilliancy. Definite markings have been made out, by telescopes of high power, upon the four greater satellites of Jupiter. Those upon the surface of Ganymede are specially interesting, because they appear to re semble the canal markings upon the planet Mars. Ice-caps, recognised by their great brilliancy, have been distinguished upon Callisto and Ganymede. Io, the innermost