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View Full Version : Lunar libration - how does it work?



geddeth
2004-Nov-07, 07:48 PM
Forgive me for not using the proper terms and phrases here, I really am a newbie when it comes to astronomy.

I recently got into a heated argument with my GF about whether or not the moon can be seen to 'tilt' during waxing and waning. I.e. if it's possible with the naked eye to note the variations in what I've learned is called libration.

I got into this discussion because she thought to be able to tell that the moon's 'smile' had rotated slightly - the northern and southernmost visible points not being aligned on a line perpendicular to the Earth's horizon. She thus implied that this scenario changed during the year, and that she could remember when the moon in this state had been rotated different.

I generally disagreed with her, but I am now curious whether there is something to this. I have learned that libration may be the phenomenon I'm looking for here, but information on this seems to be hard to find.

Can anyone explain (in layman's terms, preferably) if libration causes the moon's 'smile' to rotate during the year, and whether this may cause our view of the moon to differ during the year?

Thanks.
G.

PS. I won't be losing more than a bit of pride with her, so don' feel sorry for me if I'm wrong ;-)

GOURDHEAD
2004-Nov-07, 08:21 PM
Look here (http://en.wikipedia.org/wiki/Libration). If your GF is a much more careful observer than I, maybe she did notice a change in the moon's smile. But then maybe there's more libation than libration involved here.

geddeth
2004-Nov-07, 08:43 PM
The following image (supposedly) shows the moon's phases for every day of the year.

http://img8.imgspot.com/u/04/311/14/moon200472dpi3.jpg

Comparing, say, November 7th with February 15th shows the Moon in approximately the same phase, but the inclination differs. Not sure which libration type might be responsible here, though.

I am most interested in knowing whether this difference can actually be observed with the naked eye.

Also, does the observer's geographical location influent on this phenomenon?

Thanks,
G.

gavwvin
2004-Nov-07, 11:05 PM
http://antwrp.gsfc.nasa.gov/apod/image/991...unation_ajc.gif (http://antwrp.gsfc.nasa.gov/apod/image/9911/lunation_ajc.gif)

This animated gif made up of photographs over the course of a month illustrates this well

geddeth
2004-Nov-07, 11:16 PM
@gavwin: Thanks, I came across this nice animation in my searches. GOURDHEAD's link from Wikipedia contains a similar image, and it's a very interesting aspect of the moon's phases that I wasn't aware of.

It doesn't really answer my question about whether the perceived angle of the moon's rotational axis changes throughout the year though, since it only conveys the images from one month.

G.

geddeth
2004-Nov-07, 11:19 PM
Originally posted by GOURDHEAD@Nov 7 2004, 08:21 PM
But then maybe there's more libation than libration involved here.
BTW: Yes, I believe libation was an issue ;) But only on my part, which is why want a deeper understanding of this instead of attempting to calculate the behaviour in my head (as I believe I did at the time). :lol:

G.