No, I’m not taking up drinking under the full moon (although that sounds like a nice idea!). I’ve got a new backyard astronomy project. Fresh off my foolhardy attempt to capture every day’s stage of the Winter moon (and having missed only two!), I’ve decided that now I’ll shoot for all the full moons of 2010, with the first full moon from December 2009 thrown in for good measure.
Above are the first three, the Blue Moon from 2009, and the Winter Moon and the Trapper’s Moon. For simplicity’s sake, I’m using the names from my iPod Touch app, MoonPhase; it’s not always right—it called December’s second full moon a Blue Moon, when in fact it was the Christmas Moon; it called the full moon from December 2 the Christmas Moon! For those who follow these things, the first day of Christmas is December 25; it doesn’t end until Epiphany in January. So the December 30 moon would have been the real Christmas moon, and the 3rd moon of the season, on December 2, should have been the blue one…
One of my goals in undertaking this project is to be able to see for myself how visible librations in latitude and longitude are throughout the year. For 2010’s moons so far, it looks like the Winter moon (Jan 30) has both the eastern and northern limbs tilted a little further away from us (Mare Crisium is a little closer to the limb) than the Trapper’s moon. (The “roll” in the two images is just due to incorrect alignment in Photoshop; I try to align the moon with the axes “true” but I invariably miss by a little bit. But that doesn’t affect the features near the limbs that are visible; only the actual difference in which exact face the moon is showing us affects that.)
Compared to the Blue Moon from December 1, 2009, the western limbs are very unfavorable. Look how much farther from the limb the Mare Humorum is in that first shot compared to either January or February.
For librations in longitude, then, the Mare Humorum in the west and Mare Crisium in the east form convenient signposts. For libration in latitude, the going is a little trickier, because both the northern and southern poles are hard to get your bearings on. But let’s not make things too difficult: just look at the most prominent crater at full moon: Tycho. That giant crater with its enormous ray system is a lot farther from the southern limb of the moon in December’s shot than in either January or February’s images. So December’s moon is showing us its southern side (the libration in latitude) and its western side (libration in longitude)
For the curious, here is a little table with the librations of the three moons. I took all the data from this website; the data on another useful iPhone app, Moon Atlas, appears to use a different convention to indicate direction of longitudinal libration, and I need to keep it simple, sister. Plus the app only displays libration in graphical form, so you have to interpolate the fractional values, which makes it a lot harder:
[table “” not found /]So negative values of latitudinal libration mean that the western limb is displayed (we see farther “around” to the west) and the eastern limb is “hidden”; negative values of longitude mean the northern limb is displayed, while positive values mean the southern limb appears favorably.
And, in case the pictures weren’t worth 1000 words, I’ll “translate” them for you: December’s blue moon had negative libration in latitude, so the western limb displayed to advantage, but positive libration in longitude, so the southern limb was also well viewed, while the northern limb was shy.
And so on for the Winter moon and Trapper’s moon.
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