Timing & Alignment: Oddities of the Full Moon
Fri Apr 30,10:13 PM ET

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By Joe Rao
SPACE.com's Night Sky Columnist, SPACE.com

In February this column discussed how long a Full Moon lasts and whether the Moon is ever really full (it isn't). These are not the only things about Earth's only natural satellite that often go unnoticed, misinterpreted or just cause general confusion.



The next Full Moon on Tuesday, May 4, will offer an eclipse to skywatchers in much of Asia and parts of Europe and Africa. Not all Full Moons are accompanied by an eclipse, of course. That's because the orbit of the Moon around Earth is tilted slightly with respect to the orbit of Earth around the Sun. So only when all three bodies align perfectly can there be an eclipse.

There are also interesting questions of timing. Last November I received several e-mails asking this question:

"You say that a lunar eclipse can only occur at Full Moon. But this month's eclipse will occur on the 8th and my calendar says that Full Moon falls on the 9th. Why is this so?"

Just about all calendar manufacturers and newspapers base the dates of the lunar phases on the calculations of the U.S. Naval Observatory. Accurate Moon phase data is in fact, available from their Astronomical Applications Department covering the years from 1700 to 2035.

There is one thing, however, that some publishers overlook. All the dates and times provided by the Naval Observatory are given in "Universal Time" (abbreviated UT) which is sometimes referred to, now colloquially, as "Greenwich Mean Time" (abbreviated GMT). The two terms are often used loosely to refer to time kept on the Greenwich meridian (longitude zero). If the times are not converted to your local time zone, you can sometimes end up being one day off on the date of a particular phase.

In the case of last November's lunar eclipse, the date and time of Full Moon according to the Naval Observatory was Nov. 9 at 1:13 UT. That's why some calendars and newspapers said the Full Moon would occur on Nov. 9. But they weren't careful to make the proper conversion for North American time zones. In this case, for Philadelphia, as an example, Full Moon occurred at 8:13 p.m. EST on Nov. 8, since Greenwich time runs five hours ahead of Eastern Standard Time. So in Philadelphia, the Full Moon (and the eclipse) occurred before midnight of the previous day.