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Thursday, March 17, 2011

 

When is a Full Moon Not a Full Moon?

Over at the NASA Science blog is a post about the upcoming "Supermoon" on Sunday March 20 (March 19 UT). Whereas as astrologer Richard Nolle uses a very broad definition of "supermoon" in his claims, the NASA site seems to use a very narrow definition of "Full Moon", when they claim that the close perigee full Moon of 2008 was a "near miss".

Strictly speaking Astronomical Full Moon occurs when the Moon is directly opposite the Sun (and in this sense the NASA site is completely correct). For this full Moon Astronomical Full Moon is at 18:10 UT on 19 March, which for east coast Australians is 5:10 am on the morning of the 20th with perigee (closest approach) of 356575 km occurring pretty much at the same time.

However, in general people think of "full Moon" as the entire time from Moonrise to Moonset on the designated Full Moon night, not just the hour around astronomical Full Moon. You don't look up at the big round thing in the sky and think "that's not the full Moon, the full Moon was two hours ago" after all, do you.

In the case of the 2008 full Moon the perigee of 356565 was 4 hours out from
Astronomical Full Moon (4:00 am on 3 December as seen from Adelaide, with perigee tat 8 am). However at Astronomical Full Moon it was only 20Km more distant than this years perigee full Moon, and from 2 hours to 6 hours post Astronomical Full Moon the 2008 Moon was closer than this years Moon at exact perigee. In the case of looking up at the big round thing in the sky (for those parts of the world where the Moon didn't set at perigee), on the night of the full Moon the 2008 Full Moon was bigger for longer. So for most people the "full Moon" they were seeing was indeed closer in
2008 than 2011 (and in this sense the NASA article is misleading).

Again, as I have explained before, the human eye, for those of us with good eyesight, can only reliably detect differences of 1 arc minute in diameter. This full Moon (March 19/20) will be 33.7 arc minutes in diameter, last months full Moon was 33.4 arc minutes in diameter, so if you remember that one, you won’t notice any difference. If you wait for next months full Moon, that will be 33.5 arc minutes in diameter, so you won’t notice any difference either.

You may notice a difference if you can remember what the August 2010 apogee full Moon looked like, when the Moon was 14% smaller. However, remember that to the unaided eye, the Moon is around half the width of a finger held at arms length, so detecting a 14% difference between a perigee and apogee Moon will be difficult. Although the difference is within our visual acuity, comparing the Moon you are seeing now with your memory of the Moon 5 months ago is non trivial for most of us.

Any perceived difference in full Moon size this weekend will be almost certainly psychological for most of us (especially when the Moon is close to the horizon, when the Full Moon Illusion makes the Moon look abnormally large.

However, if you have a decent zoom lens on your camera, or can attach your camera to a small telescope, take a picture this Sunday, then using the same settings to image the upcoming October 12 apogee (furthest distance) full Moon. THEN you will see a difference in the Moons' diameter.

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