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moment of time. The actual determination of the parallax requires a combination of observations too complex to be set forth in the present book, but the fundamental principle is that just explained.

In order to get the dimensions of the whole solar system, it is only necessary to know the distance of any one planet from us at any given moment. The orbits and motions of all the planets are mapped down with the greatest possible exactness, but with the map before us we are in the position that one would be who had a very exact map of a country, only there was no scale of miles upon it. So he would be unable to measure the distance from one point to another on his map until he knew the scale. It is the scale of our map of the solar system which the astronomer stands in need of and which he has not, even with the most refined instruments, yet been able to determine as accurately as he could wish.

The fundamental unit aimed at is that already described—the mean distance of the earth from the sun. Measures of parallax are by no means the only method of determining this distance. Within the last fifty years other methods have been developed, some of which are fully as accurate as the best measures of parallax, perhaps even more so.

One of the most simple and striking of these methods makes use of the velocity of light. By observations of Jupiter's satellites, made when the earth was at different points of its orbit, it has been found that light passes