. The most important of these are
Ursa Major and Minor, Andromeda, Cassiopeia, Cepheus, Cygnus, Lyra,
Aquila, Auriga, Draco, Booetes, Hercules, Pegasus, and Corona Borealis.

To an observer of the nocturnal sky the stars appear to be very
unequally distributed over the celestial sphere. In some regions they
are few in number and of small magnitude, whilst in other parts of the
heavens, and especially in the vicinity of the Milky Way, they are
present in great numbers and form groups and aggregations of striking
appearance and conspicuous brilliancy. On taking a casual glance at the
midnight sky on a clear moonless night, one is struck with the apparent
countless multitude of the stars; yet this impression of their vast
number is deceptive, for not more than two thousand stars are usually
visible at one time.

Much, however, depends upon the keenness of vision of the observer, and
the transparency of the atmosphere. Argelander counted at Bonn more than
3,000 stars, and Hozeau, near the equator, where all the stars of the
sphere successively appear in view, enumerated 6,000 stars. This number
may be regarded as including all the stars in the heavens that are
visible to the naked eye. With the aid of an opera glass thousands of
stars can be seen that are imperceptible to ordinary vision.
Argelander, with a small telescope of 2-1/2 inches aperture, was able to
count 234,000 stars in the Northern Hemisphere. Large telescopes reveal
multitudes of stars utterly beyond the power of enumeration, nor do they
appear to diminish in number as depth after depth of space is penetrated
by powerful instruments. The star-population of the heavens has been
reckoned at 100,000,000, but this estimate is merely an assumption;
recent discoveries made by means of stellar photography indicate that
the stars exist in myriads. It is reasonable to believe that there is a
limit to the sidereal universe, but it is impossible to assign its
bounds or comprehend the apparently infinite extent of its dimensions.

Scintillation or twinkling of the stars is a property which
distinguishes them from the planets. It is due to a disturbed condition
of the atmosphere and is most apparent when a star is near the horizon;
at the zenith it almost entirely vanishes. Humboldt states that in the
clear air of Cumana, in South America, the stars do not twinkle after
they reach an elevation of 15 deg. above the horizon. The presence of
moisture in the atmosphere inte