SATILLITZ11 of Jupiter, are four little moons, or secondary planets, performing their revolutions about Jupiter, as that planet does about the Sun.
Simon Marius, mathematician of the Elector of Brandenburgh, about the end of November 1609, observed three little stars moving round Jupiter's body, and proceeding along with him ; and in Janu ary, 1610, he found a fourth. In January, 1610, Gallileo also observed the same in Italy, and in the same year, published his observations. These satellites were also observed in the same month of January, 1610, by'Thomas Ilarriot, the author of a work upon algebra, and who made con stant observations on these satellites,' from that time till the 26th of February, 1612.
When Jupiter comes into a line be tween any of his satellites and the Sun, the satellite disappears, being then eclips ed, or involved in his shadow. When the satellite goes behind the body of Ju piter, with respect to an observer on the Earth, it is then said to be occulted, be ing hidden from our sight by his body, whether in his shadow or not. And when the satellite comes into a position be tween Jupiter and the Sun, it casts a sha dow upon the face of that planet, which we see as an obscure round spot. And lastly, when the satellite comes into a line between Jupiter and us, it is said to tran sit the disk of the planet, upon which it appears as a round black spot. The pe riods or revolutions of Jupiter's satellites are found out from their conjunctions with that planet, after the same manner as those of the primary planets are dis covered from their opposition to the Sun. And their distances from the body of Jupiter are measured by a micrometer, and estimated in semi-diameters of that planet, and thence in miles. The peri odical times and distances of these satel lites, and the angles under which their orbits are seen from the Earth, at its mean distance, from Jupiter, are as be low.
The eclipses of the satellites, especial ly of those of Jupiter, are of very peat use in astronomy. First, in determining pretty exactly the distance of Jupiter from the earth. A second advantage, still more considerable, which is drawn from these eclipses, is the proof which they give of the progressive motion of light. It is demonstrated by these eclipses, that light does not come to us in an instant, although its motion is extremely rapid.
For if the motion of light were infinite, or came to us in an instant, it is evident that we should see the commencement of an eclipse of a satellite at the same moment, at whatever distance we might be from it; but, on the contrary, if light move progressively, then, it is as evident, that the further we are from a planet, the la ter we shall be in seeing the moment of its eclipse, because the light will take up a in arriving at ub ; and so it is found in fact to happen, the eclipses of these satellites appearing always later and later than the true computed times, as the Earth removes further and further from the planet. When Jupiter and the Earth are at their nearest distance, being in conjunction both on the same side of the Sun, then the eclipses are seen to happen the soonest ; and when the Sun is directly between Jupiter and the Earth they are at their greatest distance asun der, the distance being more than before by the whole diameter of the Earth's an nual orbit, or by double the Earth's dis tance from the Sun ; then the eclipses are seen to happen the latest of any, and later than before by about a quarter of an hour. Hence, therefore, it follows, that light takes up a quarter of an hour in travel ling across the orbit of the Earth, or nearly eight minutes in passing from the Sun to the Earth ; which gives us about 12 millions of miles per minute, or 200,000 miles per second, for the veloci ty of light. A discovery that was first made by Mr. Roemer.
The third and greatest advantage de rived from the eclipses of the satellites, is the knowledge of the longitudes of pla ces on the Earth. Suppose two observ ers of an eclipse, the one, for example, at London, the other at the Canaries; it is certain that the eclipse will appear at the same moment to both observers; but as they are situated under different meri dians, they count different hours, being perhaps 9 o'clock to the one, when it is only 8 to the other ; by which observa tions of the true time of the eclipse, on communication, they find the difference of their longitudes to be one hour in time, which answers to 15 degrees of longitude.