Debtor Day Calculations

Anyone looking at the Moon can immediately tell if it is a Full Moon or a New Moon. However, the other phases are less obvious. Who can tell the difference between a gibbous Moon and a waning gibbous Moon?

How can someone tell the difference between a waxing crescent and a waning crescent? Or even the first quarter and the third quarter? The solution for the Perl programmer is very simple. They just need to load the correct module.