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`astron(`

`jds`, `objects` [, `object0`,
observer=`observer`, equinox=`equinox`,
elements=`elements`] [, /xyz, /equatorial, /ecliptical,
/horizontal, /elongation, /date, /tdt, /lighttime, /aberration,
/nutation, /apparent, /qelements, /fk5, /conjspread, /planetocentric,
/keepdimensions, /vocal, /vsop87a, /vsop87c])

Returns the positions of a set of heavenly bodies at a specific set of
times. Unless the user specifies otherwise, the times are taken to be
in UT, light-time, nutation, and aberration effects are not included,
the observer is assumed to be at the center of the Earth, and
ecliptical coordinates (longitude and latitude in degrees, distance in
AU) for equinox J2000.0 are returned, referred to the VSOP dynamical
ecliptic and equinox. The data for the major planets and the Sun are
derived from the VSOP87C or (by default) VSOP87A models of Bretagnon
and Francou (*Astronomy and Astrophysics*, Vol. 202, pp. 309-315,
1988), optionally truncated as by Meeus (*Astronomical
Algorithms*, Willmann-Bell Inc., 1991, ISBN 0-943396-35-2).

`jds`Julian dates for which results are desired. (calendar)

`objects`scalar or array of numbers of objects for which info is to be returned. Currently supported: 0 for the Sun, 1 for Mercury, 2 for Venus, 3 for the Earth, 4 for Mars, 5 for Jupiter, 6 for Saturn, 7 for Uranus, 8 for Neptune, 10 for the Moon. If object numbers not listed here are specified, then orbital data is sought in an auxilliary file, as discussed below.

`object0`number of the object relative to which the other coordinates are requested, if any.

`observer`[North latitude/degrees, West longitude/degrees, height/m] of the observer. If none is specified, then the observer is assumed to reside at the center of the planet.

`equinox`The year of the equinox relative to which the results are returned. If a scalar, then the unit is Julian years, with (J) 2000.0 equivalent to JDE 2451545.0. If a string, then the first character must be either a j or J (indicating Julian years), or a b or B (indicating Besselian years). The equinox of the date can be requested through the /date keyword.

`elements`A nine-element array containing orbital elements for the target object. The elements are, if

`/qelements`

is also specified: the equinox, the epoch, the perihelion distance, the eccentricity, the inclination, the longitude of the ascending node, the argument of the perihelion, the time of the perihelion, and the absolute magnitude. If`/qelements`

is not specified, then the length of the semimajor axis replaces the perihelion distance and the mean anomaly at the epoch replaces the time of perihelion. The times are in JDE, the angles in degrees, the distances in AU.

`/date`

selects the equinox of the date.

`/xyz`

selects return of x y z coordinates in AU. If

`/xyz`

is not specified, then polar coordinates are returned (right ascension/longitude in degrees, declination/latitude in degrees, and distance in AU).`/equatorial`

return equatorial coordinates (based on the celestial equator).

`/ecliptical`

return ecliptical coordinates (based on the ecliptic).

`/horizontal`

return azimuth (West from South) and height (in degrees), and distance (in AU).

`/elongation`

return elongation (angular distance to Sun) in degrees, phase angle (angular distance of observer to Sun as seen from target object) in degrees, and estimated visual magnitude (which may be wildly off for geometries unlike those seen from Earth).

`/tdt`

selects dates in TDT (terrestrial dynamical time) rather than UT. The difference between TDT (a time based on planetary motion) and UT (a time based on the Earth’s rotation) varies slowly with time. LUX uses a quadratic fit to past estimates of the difference to calculate an estimated difference for the times you specify. All the planetary calculations are performed in TDT.

`/lighttime`

selects inclusion of light-time effects: the returned coordinates say where the target object was at the time when light left it to reach the observer at time

.`jds``/aberration`

selects inclusion of aberration effects.

`/nutation`

selects inclusion of nutation effects.

`/apparent`

selects return of apparent positions, including the effects of light-time, aberration, and nutation.

`/qelements`

indicates that

is specified in terms of the perihelion (i.e., comet-style) rather than the semimajor axis (i.e., planet-style).`elements``/fk5`

returns coordinates relative to the FK5 system. If

`/nofk5`

is specified, then the coordinates are referred to the mean dynamical ecliptic defined by the VSOP system.`/conjspread`

return the average position (ecliptical longitude and latitude) and the the conjunction spread of all indicated objects, measured in degrees. The conjunction spread is a measure for how close together all indicated objects are in the sky. For two close objects, the conjunction spread is practically equal to their mutual distance. For many objects on a great circle (e.g., planets on the ecliptic), the conjunction spread is equal to twice the standard deviation of their positions along the great circle.

`/keepdimensions`

returns dimensions for the number of objects and number of dates even if these are equal to 1.

`/vocal`

makes the routine print intermediate results.

`/vsop87a`

selects the VSOP87A model (cartesian heliocentric coordinates for J2000.0) to base the results on.

`/vsop87c`

selects the VSOP87C model (cartesian heliocentric coordinates for the equinox of the date) to base the results on.

If objects not listed above are indicated in

and/or
`objects`

, then data for such objects are sought in
auxilliary files. Such files can be read using the `object0``readorbits`

routine. If no such files have yet been read when needed, then file
`$LUXDIR/orbits` is attempted.

If the requested time falls outside of the time period spanned by the data lines for the indicated object, then the orbital parameters closest in time to the desired time are used. Otherwise, linear interpolation between the two orbits that are closest in time on either side to the desired time is performed.

See also: calendar, readorbits, Astronomy and Calendars

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