Getting the Phases of the Moon Right

Monday, February 6th, 2006 at 05:29 PM by Administrator

Now for one of those interesting dilemmas in science, how much accuracy are you willing to work toward. The “Current lunar Phase” sidebar panel was one of those projects that I spent a small amount of time putting together. The image it displays is from the U.S. Naval Observatory and is extremely accurate. However, the list of dates and times underneath it is somewhat of a “poor man’s approximation”.

You see, the moon’s orbit is NOT a perfect circle, and therefore it speeds up and slows down as it gets closer and farther from the Earth during its orbit. This means that computing the precise times for various phases of the moon is actually a bit tricky because one can NOT just take the length of the average lunar month (29.530588853 days) and divide by four to get the number of days between sucessive major phases (new moon, first quarter, full moon, and third quarter). However, for most people, getting the phase within a day is accurate enough, so when computing the dates of future “major phases”, I just use a simple algorithm that computes the number of lunar months elapsed since a previous new moon in January 2000, and then estimate the phases from there. Sometimes, you have to be really careful doing things like this.

This afternoon, Dave Williams noticed that image of the moon displayed in our “Current Lunar Phase” Sidebar panel was not matching the statement below it. Just over 50% of the moon’s face appears lit, which means it is very close to “First Quarter.” However, the panel was titling the image a “Waxing Crescent,” implying a moon with considerably less than 50% of its visible face lit. Turns out the algorithm I was using to compute lunar months since that new moon in January of 2000 was miscomputing the current date by about 1 and a half days. This has now been fixed and the image is correctly labeled a “First Quarter” moon.

Note that my dates for the phases of the moon are no more accurate, but at least now what I say is the current phase of the moon should match up reasonably well to the actual current phase of the moon.   By the way, those of you demanding accuracy should using the Local Astronomical Info page, which computes the locations of astronomical objects (but not the phases of the moon) extremely accurately.

Posted in Astronomy News

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