a crystal sphere,
and that they could not alter their relative positions; and indeed until
the last century or two, instruments were not delicate enough to measure
the small relative shift that stars make. It is within the last seventy
years that we have been able to measure the "annual parallax" of certain
stars,--that is, the difference in the position of a star when viewed by
the earth from the opposite ends of a diameter of the earth's orbit
round the sun. Besides their yearly shift due to "annual parallax," most
stars have a "proper" or "peculiar motion" of their own, which is in
most cases a very small amount indeed, but can be determined more easily
than "annual parallax" because its effect accumulates year after year.
If, therefore, we are able to observe a star over a period of fifty, or
a hundred or more years, it may seem to have moved quite an appreciable
amount when examined by the powerful and delicate instruments that we
have now at our disposal. Observations of the exact positions of stars
have been made ever since the founding of Greenwich Observatory, so that
now we have catalogues giving the "proper motions" of several hundreds
of stars. When these are examined it is seen that some groups of stars
move in fellowship together through space, having the same direction,
and moving at the same rate, and of these companies the most striking
are the stars of the Plough, that is _`Ayish_ and his sons. Not all the
stars move together; out of the seven, the first and the last have a
different direction, but the other five show a striking similarity in
their paths. And not only are their directions of movement, and the
amounts of it, the same for the five stars, but spectroscopic
observations of their motion in the line of sight show that they are all
approaching us with a speed of about eighteen miles a second, that is to
say with much the same speed as the earth moves in her orbit round the
sun. Another indication of their "family likeness" is that all their
spectra are similar. A German astronomer, Dr. Hoeffler, has found for
this system a distance from us so great that it would take light 192
years to travel from them to us. Yet so vast is this company of five
stars that it would take light seventy years, travelling at the rate of
186,000 miles in every second of time to go from the leading star,
_Merak_--Beta of the Bear--to _Mizar_--Zeta of the Bear--the final
brilliant of the five. So bright and great are t