Spherical astronomy or
positional
astronomy is the branch of
astronomy that is used to determine the location
of objects on the
celestial sphere,
as seen at a particular date, time, and location on the
Earth. It relies on the mathematical methods of
spherical geometry and the
measurements of
astrometry.
This is the oldest branch of astronomy and dates back to
antiquity. Observations of celestial objects
have and continue to be, important for religious and
astrological purposes, as well as for
timekeeping and
navigation. The science of actually measuring
positions of celestial objects in the sky is known as
astrometry.
The primary elements of spherical astronomy are coordinate systems
and time. The coordinates of objects on the sky are listed using
the
equatorial coordinate
system, which are based on the projection of the
Earth's
equator onto the
celestial sphere. The position of an object in this system is given
in terms of
right ascension (α) and
declination (δ). The latitude and local
time can then be used to derive the position of the object in the
horizontal coordinate
system, consisting of the
altitude and
azimuth.
The coordinates of celestial objects such as stars and galaxies are
tabulated in a
star catalog, which
gives the position for a particular year. However the combined
effects of
precession and
nutation will cause the coordinates to change
slightly over time. The effect of these changes in the movement of
the Earth are compensated by the periodic publication of revised
catalogs.
To determine the position of the
Sun and
planets, an astronomical
ephemeris (a
table of values that gives the positions of astronomical objects in
the sky at a given time) is used, which can then be converted into
suitable real-world coordinates.
The unaided human eye can detect about 6000
stars, of which about half are below the horizon at any
one time. On modern star charts, the
celestial sphere is divided into 88
constellations. Every star lies within
a constellation.
Constellations are
useful for
navigation.
Polaris lies close to due north to an observer in
the northern hemisphere.
This star is always at a position nearly over
the north
pole.
Positional phenomena
Ancient structures associated with positional astronomy
include
External links
Course Notes and Tutorials
Software
NOVAS, an
integrated package of subroutines for the computation of a wide
variety of common astrometric quantities and transformations, in
Fortran and C, from the U.S. Naval Observatory.
References
- Robin M. Green, Spherical Astronomy, 1985, Cambridge University Press, ISBN
0-521-29180-1.
- William M. Smart, edited by Robin M. Green, Textbook on
Spherical Astronomy, 1977,
Cambridge University Press, ISBN 0-521-29180-1. (This classic text
has been re-issued).
See also