Page:Astronomy for Everybody.djvu/80

But let us try the experiment of pointing a great telescope at a star. A result which perhaps we have not thought of would be immediately presented to our sight. The star, instead of remaining in the field of view^[1] of the telescope, very soon passes out of it by the diurnal motion. This is because, as the earth revolves on its axis, the star seems to move in the opposite direction. This motion is multiplied as many times as the telescope magnifies. With a high power, the star is out of the field before we have time to examine it.

Then it must also be remembered that the field of view is also magnified in the same way, so that it is smaller than it appears, in proportion to the magnifying power. For example, if a magnification of one thousand be used, the field of view of an ordinary telescope would be about two minutes in angular measure, a patch of the sky so small that to the naked eye it would look like a mere point. It would be as if we were looking at a star through a hole one eighth of an inch in diameter in the roof of a house eighteen feet high. If we imagine ourselves looking through such a hole and trying to see a star we shall readily realise how difficult will be the problem of finding it and of following it in its motion.

This difficulty is overcome by a suitable mounting of the telescope, so as to turn on two axes, at right angles to each other. By the mounting is meant the whole system of machinery by the aid of which a telescope is pointed