Phase of the Moon

(See below for notes on usage and the calculation method.)

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Usage

The user interface in the applet works as follows:

Time display box: Shows the date and time at which the phase will be calculated. This can be edited directly, or adjusted using the buttons. When the desired value is shown, press the "OK" button.
"<<" and ">>" buttons: Set the time at which the phase will be calculated, respectively back or forward a day from that currently displayed.
"Now" button: Set the time at which the phase will be calculated to be the current clock time.
"OK" button: Do the calculation for the time displayed.

A note on GMT times

This applet uses the standard Java facilities to convert time to GMT. Most current browsers implement this correctly; however a bug has been introduced in the JDK1.1 and HotJava 1.0 implementations such that in fact BST is displayed in the summer, not GMT! If you are running under one of these implementations, please read "GMT" as "UK local time". I hope this problem will be corrected before it becomes too widespread!

Accuracy of calculation

The applet calculates the phase of the Moon, using methods taken from the book Practical Astronomy with your Calculator by Peter Duffett-Smith (ISBN 0-521-35699-7). These methods typically give timing accurate to around 10 or 20 minutes, For comparison, accurate times for the present decade are available at the US Naval Observatory.

The phase is that seen at the centre of the Earth. For any other location there would be a correction for parallax which would be of similar magnitude to the error inherent in the calculation.

If you're interested, the source code for this applet is available here (under the GNU GPL).