as that of milestones
on the great celestial highway traversed by the planets, as well as on
the byways of space occasionally pursued by comets. Not that curiosity
as to their nature, and even conjecture as to their origin, were at any
period absent. Both were from time to time powerfully stimulated by the
appearance of startling novelties in a region described by philosophers
as "incorruptible," or exempt from change. The catalogue of Hipparchus
probably, and certainly that of Tycho Brahe, some seventeen centuries
later, owed each its origin to the temporary blaze of a new star. The
general aspect of the skies was thus (however imperfectly) recorded from
age to age, and with improved appliances the enumeration was rendered
more and more accurate and complete; but the secrets of the stellar
sphere remained inviolate.

In a qualified though very real sense, Sir William Herschel may be
called the Founder of Sidereal Astronomy. Before his time some curious
facts had been noted, and some ingenious speculations hazarded,
regarding the condition of the stars, but not even the rudiments of
systematic knowledge had been acquired. The facts ascertained can be
summed up in a very few sentences.

Giordano Bruno was the first to set the suns of space in motion; but in
imagination only. His daring surmise was, however, confirmed in 1718,
when Halley announced[3] that Sirius, Aldebaran, Betelgeux, and Arcturus
had unmistakably shifted their quarters in the sky since Ptolemy
assigned their places in his catalogue. A similar conclusion was reached
by J. Cassini in 1738, from a comparison of his own observations with
those made at Cayenne by Richer in 1672; and Tobias Mayer drew up in
1756 a list showing the direction and amount of about fifty-seven proper
motions,[4] founded on star-places determined by Olaus Roemer fifty years
previously. Thus the stars were no longer regarded as "fixed," but the
question remained whether the movements perceived were real or only
apparent; and this it was not yet found possible to answer. Already, in
the previous century, the ingenious Robert Hooke had suggested an
"alteration of the very system of the sun,"[5] to account for certain
suspected changes in stellar positions; Bradley in 1748, and Lambert in
1761, pointed out that such apparent displacements (by that time well
ascertained) were in all probability a combined effect of motions both
of sun and stars; and Mayer actually attempted the analysi