| 1 | /*
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| 2 | *+
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| 3 | * Name:
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| 4 | * palRverot
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| 5 |
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| 6 | * Purpose:
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| 7 | * Velocity component in a given direction due to Earth rotation
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| 8 |
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| 9 | * Language:
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| 10 | * Starlink ANSI C
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| 11 |
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| 12 | * Type of Module:
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| 13 | * Library routine
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| 14 |
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| 15 | * Invocation:
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| 16 | * double palRverot ( double phi, double ra, double da, double st );
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| 17 |
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| 18 | * Arguments:
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| 19 | * phi = double (Given)
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| 20 | * latitude of observing station (geodetic) (radians)
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| 21 | * ra = double (Given)
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| 22 | * apparent RA (radians)
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| 23 | * da = double (Given)
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| 24 | * apparent Dec (radians)
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| 25 | * st = double (Given)
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| 26 | * Local apparent sidereal time.
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| 27 |
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| 28 | * Returned Value:
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| 29 | * palRverot = double
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| 30 | * Component of Earth rotation in direction RA,DA (km/s).
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| 31 | * The result is +ve when the observatory is receding from the
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| 32 | * given point on the sky.
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| 33 |
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| 34 | * Description:
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| 35 | * Calculate the velocity component in a given direction due to Earth
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| 36 | * rotation.
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| 37 | *
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| 38 | * The simple algorithm used assumes a spherical Earth, of
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| 39 | * a radius chosen to give results accurate to about 0.0005 km/s
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| 40 | * for observing stations at typical latitudes and heights. For
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| 41 | * applications requiring greater precision, use the routine
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| 42 | * palPvobs.
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| 43 |
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| 44 | * Authors:
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| 45 | * PTW: Patrick T. Wallace
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| 46 | * TIMJ: Tim Jenness (JAC, Hawaii)
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| 47 | * {enter_new_authors_here}
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| 48 |
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| 49 | * History:
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| 50 | * 2012-03-02 (TIMJ):
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| 51 | * Initial version
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| 52 | * Adapted with permission from the Fortran SLALIB library.
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| 53 | * {enter_further_changes_here}
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| 54 |
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| 55 | * Copyright:
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| 56 | * Copyright (C) 1995 Rutherford Appleton Laboratory
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| 57 | * Copyright (C) 2012 Science and Technology Facilities Council.
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| 58 | * All Rights Reserved.
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| 59 |
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| 60 | * Licence:
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| 61 | * This program is free software; you can redistribute it and/or
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| 62 | * modify it under the terms of the GNU General Public License as
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| 63 | * published by the Free Software Foundation; either version 3 of
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| 64 | * the License, or (at your option) any later version.
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| 65 | *
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| 66 | * This program is distributed in the hope that it will be
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| 67 | * useful, but WITHOUT ANY WARRANTY; without even the implied
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| 68 | * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
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| 69 | * PURPOSE. See the GNU General Public License for more details.
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| 70 | *
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| 71 | * You should have received a copy of the GNU General Public License
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| 72 | * along with this program; if not, write to the Free Software
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| 73 | * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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| 74 | * MA 02110-1301, USA.
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| 75 |
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| 76 | * Bugs:
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| 77 | * {note_any_bugs_here}
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| 78 | *-
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| 79 | */
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| 80 |
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| 81 | #include "pal.h"
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| 82 |
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| 83 | #include <math.h>
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| 84 |
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| 85 | double palRverot ( double phi, double ra, double da, double st ) {
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| 86 |
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| 87 | /* Nominal mean sidereal speed of Earth equator in km/s (the actual
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| 88 | * value is about 0.4651) */
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| 89 | const double espeed = 0.4655;
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| 90 | return espeed * cos(phi) * sin(st-ra) * cos(da);
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| 91 | }
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