By observing the eclipses of _Jupiter_’s satellites, it was first discovered that light is not propogated instantaneously, though it moves with an incredible swiftness: For if light came to us in an instant, an observer in T will see an eclipse of one of the satellites, at the same time that another in K would. But it has been found by observations, that when the Earth is in K, at her nearest distance from _Jupiter_, these eclipses happen much sooner than when she is in T. Now having the difference of time betwixt these appearances in K and T, we may find the length of time the light takes in passing from K to T, which space is equal to the diameter of the Earth’s annual orb. By these kinds of observations it has been found, that light reaches from the Sun to us in the space of eleven minutes of time, which is at least at the rate of 100,000 miles in a second. _FINIS._ [Illustration] AN INDEX OF THE ASTRONOMICAL TERMS Made Use of in this BOOK. _Acronical_ Rising and Setting of the Stars Page 96 _Almacanthers_ 63 _Altitudes_ ib. ——— _Meridian Altitude_ 63 _Amplitude_ 62 _Amphiscians_ 91 _Annual Motion_ 7 _Antœci_ 92 _Antarctic Circle_ 53 ——— _Pole_ ib. _Antipodes_ 93 _Arctic Circle_ 52 _Arctic Pole_ 53 _Ascension_ 68 ——— _Right_ ib. ——— _Oblique_ 69 _Ascensional Difference_ ib. _Ascians_ 91 ——— _Heteroscians_ ib. _Asterisms_ 36 _Atmosphere_ 81 _Axis_ 43 ——— _of the World_ 49 _Azimuth_ 61 Babylonish _Hours_ 71 _Bissextile_ 78