﻿Terra Method
Computes the position and velocity vectors of a terrestrial observer with respect to the center of the Earth.

Namespace: ASCOM.Astrometry.NOVAS
Assembly: ASCOM.Astrometry (in ASCOM.Astrometry.dll) Version: 6.0.0.0 (6.1.1.2619)

# Syntax

C#
public void Terra(
OnSurface Location,
double St,
ref double[] Pos,
ref double[] Vel
)
Visual Basic
Public Sub Terra ( _
Location As OnSurface, _
St As Double, _
ByRef Pos As Double(), _
ByRef Vel As Double() _
)
Visual C++
public:
virtual void Terra(
OnSurface Location,
double St,
array<double>^% Pos,
array<double>^% Vel
) sealed

#### Parameters

Location
Type: ASCOM.Astrometry..::..OnSurface
Structure containing observer's location
St
Type: System..::..Double
Local apparent sidereal time at reference meridian in hours.
Pos
Type: array<System..::..Double>[]()[][]%
Position vector of observer with respect to center of Earth, equatorial rectangular coordinates, referred to true equator and equinox of date, components in AU.
Vel
Type: array<System..::..Double>[]()[][]%
Velocity vector of observer with respect to center of Earth, equatorial rectangular coordinates, referred to true equator and equinox of date, components in AU/day.

# Remarks

If reference meridian is Greenwich and st=0, 'pos' is effectively referred to equator and Greenwich.

This function ignores polar motion, unless the observer's longitude and latitude have been corrected for it, and variation in the length of day (angular velocity of earth).

The true equator and equinox of date do not form an inertial system. Therefore, with respect to an inertial system, the very small velocity component (several meters/day) due to the precession and nutation of the Earth's axis is not accounted for here.