Moon numbers

This page contains a JavaScript program that will generate a monthly table of numbers of use to lunar observers. To make a table, just type in the year and month in the form below, and click the 'make table' button. After some seconds (or a bit longer on MSIE 4 and 5), a new Web browser window will appear showing a page of output. Save or print the page, then just close the new window to return here. Save this web page to your hard drive if you want to make more calculations.

Enter year and month as YYYYMM:

The position routines for the Moon ephemeris are approximate, and I have left out the nutation, physical libration and light time corrections. The ephemeris here is geocentric, and duplicates the moon section of the NASA Twelve Year Planetary Ephemeris. The next version of this page will produce a topocentric ephemeris calculated for your location.

Key to table

The following quantities are listed in the monthly table.
Date: The UT date in the given Month. All quantities below are calculated at 0h UT on the UT date.
GST: Greenwich Siderial Time at 0h UT on the date. Divide your longitude (W taken as negative) by 15 and add to GST to get your Local Siderial Time. Compare with the RA of the Moon to see how far Moon is from your Meridian at 0h UT.
Ra: The geocentric right ascension of the Moon rounded to 0.01 of an hour.
Dec: The geocentric declination of the Moon rounded to 0.1 of a degree.
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.
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.
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.
PA rot: Position angle of the Moon's axis of rotation.

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/moontab.html
Last modified: 19 March 2000