TELLABRE. The same as _astrolabe_.

ASTERIA. _See_ SEA-STAR.

ASTERISM. Synonymous with _constellation_, a group of stars.

ASTERN. Any distance behind a vessel; in the after-part of the ship; in
the direction of the stern, and therefore the opposite of _ahead_.--_To
drop astern_, is to be left behind,--when abaft a right angle to the
keel at the main-mast, she drops astern.

ASTEROIDS. The name by which the minor planets between the orbits of
Jupiter and Mars were proposed to be distinguished by Sir W. Herschel.
They are very small bodies, which have all been discovered since the
commencement of the present century; yet their present number is over
eighty.

ASTRAGAL. A moulding formerly round a cannon, at a little distance from
its breech, the _cascabel_, and another near the muzzle. It is a half
round on a flat moulding.

ASTRAL. Sidereal, relating to the stars.

ASTROLABE. An armillary sphere.--_Sea-astrolabe_, a useful graduated
brass ring, with a movable index, for taking the altitude of stars and
planets: it derived its name from the armillary sphere of Hipparchus, at
Alexandria.

ASTROMETRY. The numerical expression of the apparent magnitudes of the
so-called fixed stars.

ASTRONOMICAL CLOCK. A capital bit of horology, the pendulum of which is
usually compensated to sidereal time, for astronomical purposes. (_See_
SIDEREAL TIME.)

ASTRONOMICAL HOURS. Those which are reckoned from noon or midnight of
one natural day, to noon or midnight of another.

ASTRONOMICAL OBSERVATIONS. There have been occasional slight records of
celestial phenomena from the remotest times, but the most useful ones
are those collected and preserved by Ptolemy. Since 1672, science has
been enriched with a continued series of astronomical observations of
accuracy and value never dreamed of by the ancients.

ASTRONOMICAL PLACE OF A STAR OR PLANET. Its longitude or place in the
ecliptic, reckoned from the first point of Aries, according to the
natural order of the signs.

ASTRONOMICAL TABLES. Tables for facilitating the calculation of the
apparent places of the sun, moon, and planets.

ASTRONOMICALS. The sexagesimal fractions.

ASTRONOMY. The splendid department of the mixed sciences which teaches
the laws and phenomena of the universal system. It is _practical_ when
it treats of the magnitudes, periods, and distances of the heavenly
bodies; and _physical_ when it investigates the causes. In the first
division the more usefu