Illuminated Fraction of the Moon

Further detail (about the Java Applet on the preceding page):

• The applet is actually updating once per few seconds, but the moon changes slowly, so there is not a lot of activity.
• A "+" suggests a waxing moon (getting brighter), while a "-" suggests a waning moon (getting darker).
• The applet does not take lunar eclipses into account, so it can be extremely wrong at certain interesting moments!
• Your computer's local clock must be set accurately, or you'll be looking into the past or future (accordingly).
• The applet seems to have trouble under certain browsers (like IE for the Macintosh and certain flavors of Mozilla); I'm trying to see what is going on in this regard.

A fun book to read about these subjects is Jean Meeus's Astronomical Formulae for Calculators. Sadly, he doesn't talk much about how he does things, but at least he provides interesting results.

You are welcome to take the tiny Java programs that I wrote, Moon.java and JulianDay.java, as well as ShowMoon.java, the simple applet that drives them.

The United States Naval Observatory has cool pages on this subject. Finaly, note that a Google search under "illuminated fraction of moon" provides interesting results.

Enjoy our moon, and please be sure to report any bugs, bad coding practices, or whatever else you think is pertinent. :)

Kleanthes Koniaris, email.