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over a distance equal to that of the earth from the sun in about eight minutes twenty seconds, or five hundred seconds. This determination has been more accurately made in another way by the aberration of the stars. This is a slight change in their position due to the combined motion of the earth and the ray of light by which we see the star. By accurate observations on the aberration, it is found that light travels from the earth to the sun in almost exactly 499.6 seconds. It follows that if we can find how far light will travel in one second, we can determine the distance of the sun by multiplying the result by 499.6. The measurement of the velocity of light is one of the most difficult problems in physics, as it requires the measurement of intervals of time only a few millionths of a second in duration. Those who are interested in the subject will see the method of doing this explained in special treatises; at present it is sufficient to say that light is found to travel 299,860 kilometres, or 186,300 miles in a second. Multiply this by 499.6 and we have 93,075,480 miles for the distance of the sun from the earth.

A third method rests on the measures of the sun's gravitation upon the moon. One effect of this is that, as the moon performs its monthly revolution round the earth, it is at its first quarter a little more than two minutes behind its average position, to which it catches up at full moon, and passes; so that at last quarter it is two minutes ahead of the mean position. Toward new