f position which
objects undergo as a result of our own change of place, is to observe
the landscape from the window of a moving railway carriage. As we are
borne rapidly along we notice that the telegraph posts which are set
close to the line appear to fly past us in the contrary direction; the
trees, houses, and other things beyond go by too, but not so fast;
objects a good way off displace slowly; while some spire, or tall
landmark, in the far distance appears to remain unmoved during a
comparatively long time.

Actual change of position on our own part is found indeed to be
invariably accompanied by an apparent displacement of the objects about
us, such apparent displacement as a result of our own change of position
being known as "parallax." The dependence between the two is so
mathematically exact, that if we know the amount of our own change of
place, and if we observe the amount of the consequent displacement of
any object, we are enabled to calculate its precise distance from us.
Thus it comes to pass that distances can be measured without the
necessity of moving over them; and the breadth of a river, for instance,
or the distance from us of a ship at sea, can be found merely by such
means.

It is by the application of this principle to the wider field of the sky
that we are able to ascertain the distance of celestial bodies. We have
noted that it requires a goodly change of place on our own part to shift
the position in which some object in the far distance is seen by us. To
two persons separated by, say, a few hundred yards, a ship upon the
horizon will appear pretty much in the same direction. They would
require, in fact, to be much farther apart in order to displace it
sufficiently for the purpose of estimating their distance from it. It
is the same with regard to the moon. Two observers, standing upon our
earth, will require to be some thousands of miles apart in order to see
the position of our satellite sufficiently altered with regard to the
starry background, to give the necessary data upon which to ground their
calculations.

The change of position thus offered by one side of the earth's surface
at a time is, however, not sufficient to displace any but the nearest
celestial bodies. When we have occasion to go farther afield we have to
seek a greater change of place. This we can get as a consequence of the
earth's movement around the sun. Observations, taken several days apart,
will show the effect