Lunar ephemeris with topocentric librations

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This page contains a JavaScript program that will generate a monthly lunar ephemeris for your location. The libration figures and the position angle of the Moon's rotation axis have been corrected for your position on the earth, so they are topocentric values. To calculate a monthly ephemeris, just

Enter year and month as YYYYMM:
Enter your longitude as ąddmm (west negative)
Enter your latitude as ąddmm (north positive)
Enter your time zone in hours difference from Greenwich (West negative)
Enter the name of your location (used for table)

The position routines for the Moon ephemeris are approximate, and I have left out the nutation, physical libration and light time corrections. Save this web page to your hard drive if you want to make more calculations.

Key to table

The following quantities are listed in the monthly table.
Date: The date in the given Month. All quantities below are calculated at 0h zone time on the date.
LST: Local Siderial Time at 0h zone time on the date. Compare with the RA of the Moon to see how far Moon is from your Meridian at 0h zone time.
Ra: The geocentric right ascension of the Moon at 0h zone time on the current day.
Dec: The geocentric declination of the Moon at 0h zone time on the current day.
L: Selenographic longitude of the sub-Earth point. This number tells you how far round the Moon's face is turned by the optical libration. This figure is corrected for your position on the Earth's surface.
B: Selenographic longitude of the sub-Earth point. This figure tells you how much the pole of the Moon appears tilted towards or away from us by the optical libration. This figure is corrected for your position on the Earth's surface.
PA rot: Position angle of the Moon's axis of rotation. This figure is corrected for your position on the Earth's surface.
Co: Colongitude of the Sun. Listed in ephemerides.
Bs: Selenographic latitude of Sub-Solar point. Tells you the latitude of the 'pole' of the bright hemisphere of the Moon.
Ls: Selenographic longitude of Sub-Solar point. Tells you the longitude of the 'pole' of the bright hemisphere of the Moon.
L term: Selenographic longitude of terminator. I personally find this a bit easier to interpret than the colongitude. Following a suggstion from Ralph Sadler, the letter 'r' indicates that the sun is rising at the given longitude (waxing Moon), and 's' indicates that the sun is setting (waning Moon).
% Ill Illuminated percentage of the Moon's disc
PA bl: Position angle of the bright limb. Imagine a line drawn from the centre of the Moon's disc to the part of the bright limb midway between the cusps or horns of the crescent. The PA Bright Limb is the angle between this line and the Celestial North Pole.

Use offline

Save this page to your hard drive for use off-line - it will still work. Explore the program by loading the page into a text editor. My excursion into Javascript was prompted by an article called 'Scripting: A programming alternative' by David Benn in the March 2000 issue of Sky and Telescope. A look at Peter Hayes' code helped me produce the table as a Web page.


Keith Burnett
Original location: http://www.xylem.demon.co.uk/kepler/topomoon.htm
Last modified: 25 April 2000