Friday, March 2, 2012

Comparing Lunar Librations

With an unexpectedly clear sky last night, I found myself observing the Moon (well before the occultation of Zeta Tauri). I would have guessed the waxing gibbous Moon was about 8 days old at the time. I was caught up in observing and imaging what I could see, not what I might have expected to see.

Later that night, while looking through the images I'd taken, I realized that normally on an eight day old (it was actually nine days old, although I didn't know that yet) Moon, Rupes Recta- the Straight Wall- can be seen. I hadn't noticed it while observing, but I didn't specifically look for it. Later, while looking through my images, I saw that Rupes Recta was in full daylight, and therefore not visible. This led me to believe that libration had turned the visual western limb away from us.

I remembered that in early October 2011 I had noticed libration tilting the other direction, allowing me to see the "far side" along the visual western limb. I checked an image I took October 4 that is a similar lunar phase. Sure enough, the images from these two nights show a starkly different lunar landscape.


October 4, 2011- 7:50 PM CDST.


March 1, 2012- 8:12 PM CST.

Wanting to know more about these two nights, I looked up some information from the Virtual Moon Atlas:

October 4, 2011- 7:50 PM CDST

Distance- 383908km
Apparent Diameter- 31.13’
Lunation- 7.53 days
Illumination- 59.3%
Sub-solar latitude- -1.4°
Libration in latitude- -2° 57’
Libration in longitude- +7° 14’          

March 1, 2012- 8:13 PM CST

Distance- 391694km
Apparent Diameter- 30.51’
Lunation- 9.15 days
Illumination- 60%
Sub-solar latitude- +1.5°
Libration in latitude- +1° 53’
Libration in longitude- -6° 0’

Credit: Virtual Moon Atlas.

Although the phase looks very similar, VMA gives a difference in lunation of over 36 hours. The difference in illumination, though, is only 0.7%, close enough that even very skilled observers would notice little difference, in my opinion. The difference in libration in longitude, however, is over 13 degrees. Luna's visual western limb was indeed turned away from us last night.

For further comparison, I’ve found an image from October 5, and details from Virtual Moon Atlas.


October 5, 2011- 9:00 PM CDST

Distance- 388667km
Apparent Diameter- 30.74’
Lunation-  8.58 days
Illumination- 69.6%
Sub-solar latitude- -1.4°
Libration in latitude- 4° 09 ’
Libration in longitude- +6° 39’

Credit: Virtual Moon Atlas

While the lunation of 8.58 days on October 5 is still over half a day short of last night's 9.15 days, the Moon shows a much fuller disk, and was over 69% illuminated. It's a neat comparison, but I think October 4 better illustrates the difference in libration.

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