Q & A

q & a

Here are some of the questions which visitors to Inconstant Moon have asked. They have been divided into six sections...

	movement:	the moon's orbit and phases
	physical:	form, characteristics and geology
	phenomena:	unusual lunar events
	observation:	what can be seen... and what can't
	terminology:	naming of the moon, phases and features
	miscellaneous:	anything not covered avove

If you cannot find the answer to your question, go ahead and ask it!

halloween blue moon

q: My class has been learning about the Moon and I have used your site to introduce the phases and other interesting info. We are now trying to determine when there is actually a blue moon. Your site lists two of them when clicking on the calendar: Oct. 31 and Nov. 30; however, your chart of blue moons credits it to November. According to the info they have found, my students believe that it should technically only be Nov. 30 (though they love the idea of a blue moon on Halloween!). If you have any comments or additional info on this I'm sure my 6th graders would love to hear it. (Sharon Ball, New Jersey)
a: There will actually only be one blue moon this year, but depending on where you live it could happen on any one of three different nights. The blue moon table keeps things simple by using Universal Time (or UT, the time on the prime meridian which passes through Greenwich, London, used by astronomers everywhere). In western Europe and the eastern US there will be full moons on the first and last days of November, so the second one will be called a blue moon.

Here's where the fun starts... that second full moon, at 30 Nov 2001 20:49 UT, is very close to the end of the month. In Moscow the clocks are 3 hours ahead of UT, so it happens at 11 minutes before midnight. Any further east and it happens after midnight, on 1 Dec, and the full moon at the end of December becomes the blue moon.

And - here's the bit I suspect you're waiting for - the full moon at 01 Nov 2001 05:41 UT is very close to the beginning of the month. If you live in the Central Standard Time zone your clocks are six hours behind UT, so it is still 31 Oct 2001 23:41 CST. In the Pacific Standard Time zone it is 31 Oct 2001 21:41 PST, and so on. And since there is a full moon on 02 Oct 2001 too, that means anyone in the central and western US time zones (sorry, not New Jersey) will indeed get a Halloween blue moon!

no full moon

q: Could you please calculate when the next February without a full moon will occur. Your glossary indicates that this last happened in 1961, which was before I was born. I was hoping to have an opportunity to "not" experience this phenomenon in my lifetime. (Patrick J. Housfeld)
a: After 1999, the next February with no full moon will be in 2018. Coincidentally, I was actually born in Feb '61 so 2018 will be a kind of third anniversary for me.

apparent size

q: Why does the Moon sometimes appear to be quite large and at other times to be quite small? (Jan Karstens, Hamburg, Germany and similar question from Adrian, Berkeley)
a: There are two reasons for the Moon appearing larger or smaller. One is that, over a period of about 27.5 days, the Moon cycles between perigee and apogee, its nearest and furthest points from the Earth respectively, giving a change in apparent size of about 14%. The other is the phenomenon known as the Moon Illusion, which makes the Moon appear larger when it is near the horizon. This is purely psychological.

bright full moon

q: I heard that on 12/22/99 it will be the brightest full moon in 500 years. I also heard that it will be so bright that people who live where it snows will be able to drive without headlights. Can you give me more info on this? I live in CA and wondered if I should drive out to the desert for the full effect. Thanks for your input. (Rebecca, CA, and similar questions from Kevin Clark, Mark Takefman, Sally and others)
a: The full moon at 17:31 UT on the 22nd December will indeed be unusual as it will come within hours of

the winter solstice (at 8:00 UT, when the Sun appears at its most southerly position of the year and, coincidentally, the Earth and Moon are at their closest to it), and
lunar perigee (11:00 UT, when the Moon will be at its closest to the Earth, and this perigee will be the closest of the year).

These same events came within 24 hours of each other in 1991 and 1866, but the 10 hour time frame this December in unmatched in a century and a half. However, closer perigees occurred in 1893, 1912 and 1930. In 1912, perigee also came within 24 hours of perihelion (when the Earth is closest to the Sun). The consequence? The full moon on January 4th 1912 was some 25% brighter than average... impressive, but less than many people might have expected. The disappointing truth is that whilst tides will be particularly high and low on December 22nd, and the Moon will indeed be bigger and brighter than usual, the differences will not be that spectacular and few people will actually notice. The solstice, by the way, is a red herring. It means that the Sun is at its most southerly in the sky, and has no significant relationship with the Moon's brightness.

two blue moons

q: How many years until we have two months as close together as 1999 with "blue moons?" (Lawrence Carico, Knoxville, TN)
a: It's a long wait... we won't see it again until 2018 (January and March). February 2018 is also the next month with no full moon.

blue moons

q: Regarding the question above from Lawrence Carico... What is a Blue Moon? (Richie T, Birmingham, UK)
a: The most widely used meaning for the term is simply the second full moon in a calendar month. Contrary to popular belief this is not an old tradition, but a comparatively recent misinterpretation of an older usage. An article on the subject will appear in Inconstant Moon soon.