The circle determined by the intersection of the heavens with a
→ cone whose → vertex
is the → eye, and
whose elements are tangent to lines of the Earth's surface.
Same as → visible horizon. Assuming that there is no
→ atmospheric refraction, apparent horizon coincides
with → geometric horizon. See also
→ sea horizon.

A measure of a star's observed brightness (opposed to
→ absolute magnitude); symbol m.
It depends on the star's → intrinsic brightness, its distance
from the observer, and the amount of → interstellar absorption.
The brightest star → Sirius has an apparent magnitude of
-1.46, while the weakest stars visible with the naked eye in the most
favorable observation conditions have magnitudes of about +6.5. The
stars of magnitudes less than +23 are measured by professional
observatories, whereas those of magnitudes less than +30 by a telescope such as
the → Hubble Space Telescope (M.S.: SDE).

1) The position on the celestial sphere at which a heavenly body
would be seen from the Earth at a particular time.
2) A position given by the coordinates calculated for a star, if it were
seen from the Earth's centre, relative to the → real equator
and the → real equinox, at a certain date. It includes the
displacements from one heliocentric direction, given in a stellar catalogue, due to
→ precession, → nutation,
→ aberration, → proper motions,
→ annual parallax, and
light gravitational deviation (M.S.: SDE).

The instant of time when the object is in the East and the geometric
→ zenith distance
is equal to 90° plus the → horizontal refraction
plus the semidiameter minus the → parallax.

The instant of time when the object is in the West and the geometric
→ zenith distance is equal to 90° plus the
→ horizontal refraction plus
the semidiameter minus the → parallax.

The → mean sidereal time
corrected for the → nutation
and shift in the obliquity of
the ecliptic that occurs as a result of the Moon's gravitational effect.
Apparent sidereal time differs from mean sidereal time in that the
→ true vernal equinox point is used.

The duration of one rotation of the Earth on its axis
(→ Earth's rotation), with respect to
the → apparent Sun.
It is measured by successive transits of the apparent
Sun over the lower branch of a → meridian.

The → true Sun as seen by an observer on Earth.
The term "apparent Sun" is used in contrast to → mean Sun,
which refers to an average of the Sun's position). See also:
→ apparent solar time and
→ mean solar time.