Oddities of the Full Moon

By Joe Rao

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 go unnoticed, misinterpreted or just cause general confusion.

The next Full Moon on Tuesday, May 4, will offer an eclipse to sky watchers 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 months eclipse will occur on the eighth and my calendar says that Full Moon falls on the ninth. 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. Thats 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.

If you live in North America and want to check to see if the calendar now hanging on your wall has made the proper time conversions for the lunar phases, flip to October and see what day it says that the Full Moon will occur. If it says Oct. 28, 2004 then the manufacturer probably didnt bother to convert from UT.

For most of North America (except those who live in the Canadian Maritime Provinces), Full Moon officially occurs on the evening of Oct. 27, when, incidentally, another total lunar eclipse will take place.

June is the month for proms and weddings and many no doubt have already consulted their calendars to time special events such as these to coincide with that month's Full Moon.

Most Americans will have the Full Moon occurring on the evening of Wednesday, June 2, 2004. But for those who live in the Eastern time zone, Full Moon officially occurs at 12:20 a.m. Eastern Daylight Time on Thursday, June 3.

Since the majority of people who venture outside do so during convenient evening hours, those who gaze skyward on Wednesday night, June 2, will be looking at a Moon that will indeed appear very much "full."

Most Easterners who consult calendars and local newspapers, however, will see June 3 listed as the night of Full Moon. But when they look skyward that Thursday evening, they will be looking not at a full Moon, but at a waning gibbous Moon!

The advertised "Full Moon" will have long since passed and the Moon for that Thursday night will actually be nearly a full day past the time it turned full and as such, should look noticeably out of shape.