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View Full Version : Did the moon really appear 14% bigger?



AdamLyon
2007-Oct-29, 09:39 PM
I read over at SpaceWeather (http://spaceweather.com/archive.php?month=10&day=24&year=2007&view=view) that the full moon on the 25-26 appeared 14% larger and 30% brighter than other full moons. I can't find this information anywhere else reliable. (I can't tell if SpaceWeather is reliable or not, I saw a newspaper article that said it was a NASA site but I don't see that connection anywhere.) The distance part is true, the moon was closer than normal around then (source: http://aa.usno.navy.mil/data/docs/geocentric.php), but 14% bigger seems like a stretch.

Anyone have any hard data on this?

antoniseb
2007-Oct-29, 09:53 PM
Well, the full Moon can vary in size by a noticeable amount based on whether it is full at perigee, apogee, or in between. The recent full Moon was close to perigee. It is plausible that the total solid angle could be 14% larger than the smallest case.

grant hutchison
2007-Oct-29, 10:50 PM
Seems like a slightly garbled version of something that is true.
The moon's distance from the Earth can vary from 356400km to 406700km: the larger figure is 1.14 times the smaller, making the moon appear 1.14 times larger at closest approach than at farthest excursion. For a given phase, that'll make it 1.142 = 1.30 times brighter. That's presumably where the 14% and 30% come from. The correct comparison for these figures is therefore between the recent full moon and the smallest possible full moon, rather than with any other full moon.

Grant Hutchison

Edit: Yes, I see that the SpaceWeather report is actually compatible with the above, since they say "... as much as 14% bigger and 30% brighter ..."; so not necessarily that much bigger for any given full moon.

Tobin Dax
2007-Oct-29, 10:57 PM
Antoniseb and grant covered the science, so I'll add this: Last night, while driving home, I saw a big, bright, and orange gibbous moon rising in the east. Now, I'm not usually out driving in a flattish area at moonrise, but this was quite a moon. One of the most beautiful astronomical sights I've seen for a while.

Nereid
2007-Oct-30, 12:40 AM
Check out this recent APOD (http://antwrp.gsfc.nasa.gov/apod/ap071025.html) ... not quite 14%

Swift
2007-Oct-30, 12:16 PM
I can't tell if SpaceWeather is reliable or not, I saw a newspaper article that said it was a NASA site but I don't see that connection anywhere.
And lastly, spaceweather.com is a great site. I believe the connection to NASA is that the webmaster for the site works for NASA, but it is not officially a NASA site. An excellent site, particuarly for aurora and near-Earth object information, as well as what is new and exciting in the sky.