Spherical astronomy
Spherical astronomy, or positional astronomy, is a branch of observational astronomy used to locate astronomical objects on the celestial sphere, as seen at a particular date, time, and location on Earth. It relies on the mathematical methods of spherical trigonometry and the measurements of astrometry.
This is the oldest branch of astronomy and dates back to
The primary elements of spherical astronomy are
The coordinates of celestial objects such as
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 perceive about 6,000 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 nearly due north to an observer in the Northern Hemisphere. This pole star is always at a position nearly directly above the North Pole.
Positional phenomena
- Planets which are in conjunction form a line which passes through the center of the Solar System.
- The ecliptic is the plane which contains the orbit of a planet, usually in reference to Earth.
- Elongation refers to the angle formed by a planet, with respect to the system's center and a viewing point.
- A quadrature occurs when the position of a body (moon or planet) is such that its elongation is 90° or 270°; i.e. the body-earth-sun angle is 90°
- Superior planets have a larger orbit than Earth's, while the inferior planets (Mercury and Venus) orbit the Sun inside Earth's orbit.
- A transitmay occur when an inferior planet passes through a point of conjunction.
Ancient structures associated with positional astronomy include
See also
- Astrological aspects
- Astrogeodesy
- Astrometry
- Celestial coordinate system
- Celestial mechanics
- Celestial navigation
- Diurnal motion
- Eclipse
- Ecliptic
- Elongation
- Epoch
- Equinox
- Halley, Edmond
- History of Astronomy
- Jyotish
- Kepler's laws of planetary motion
- Occultation
- Parallax
- Retrograde and prograde motion
- Sidereal time
- Solstice
References
- Robin M. Green, Spherical Astronomy, ISBN 0-521-31779-7
- William M. Smart, edited by Robin M. Green, Textbook on Spherical Astronomy, ISBN 0-521-29180-1. (This classic text has been re-issued)
External links
- Software
- NOVAS is 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.
- jNOVAS is a java wrapper for library developed and distributed by The United States Naval Meteorology and Oceanography Command (NMOC) with included JPL planetary and lunar ephemeris DE421 binary file published by the Jet Propulsion Laboratory.
- Course notes and tutorials