I’ve mentioned, many times, the curious wobble of the moon known as libration, and of course the different features and details you can see when photographing anything other than a completely full moon. Now, courtesy of Astronomy Picture of the Day comes a wicked animation of it, with lots of additional details.
The video was created by NASA’s Scientific Visualization Studio using images from the Lunar Reconnaisance Orbiter, and includes dates, times, the moon’s position around the Earth, and even an axial tilt and libration graph, which is the part I found the most surprising; I’d been under the impression that the axial variation was somewhere around 10-15°, and instead it’s in the realm of 50. Part of what makes this hard to realize is that, when viewed from Earth, the moon is almost always cocked at an angle, or at least, we’re viewing it at such. Since it describes an arc through the sky, it changes angle as it travels – in relation to our viewing position (we’re really the ones that are changing due to the rotation of the earth and having to ‘turn our heads’ to look at it as we rotate.) If, for instance, I had the camera mounted to a properly-aligned tracking platform, the rotational angle would be cancelled out and then I’d be able to plot the libration more accurately. This is possible, but extremely fussy, and not something I’m planning to tackle just for moon shots. For long exposures of fainter nebulae and such, well, we’ll see about that…
But it’s also worth watching the terminator, the edge of the shadow, as it reveals craters and mountains by their shadows as it tracks across the face of the moon, the reason why I don’t bother shooting a full moon anymore (aside from, you know, having a few dozen frames of it already.) Bear in mind that, the less of the face that’s illuminated, the more the moon is visible during daylight hours, something you can actually see with the polar perspective at top left. For very thin crescents – less than 2% illuminated – they’re generally only visible during twilight hours, being below the horizon during the night hours and overwhelmed by the atmospheric scatter during the day.
If you’re feeling adventurous, tomorrow morning the moon will rise ahead of the sun by only about 20 minutes, and will be a crescent a mere 0.6% illuminated; if you have very clear skies and a great view of the eastern horizon, you might be able to catch it as it breaks the horizon, solely from the Earthshine it will display before the sky lightens too far. The weather here is looking far from promising, but if it clears, I may make the attempt myself. This is what Stellarium shows for my prime viewing location:
That blurry bit at the bottom is the rendered ‘horizon’ in the program, showing how low it is and how bright it’s getting, more or less – your location will almost certainly differ to a degree, so check with your own copy of Stellarium (because if you haven’t downloaded it by now, seriously, what are we gonna do with you?) And believe it or not, there really is an illuminated crescent there, on the lower left side – you can see that it would be overwhelmed almost immediately by sunlight. However, it’s easier now than at other times of the year, largely because the colder weather reduces the atmospheric humidity. Don’t make the mistake that, since the nights are longer, this gives you more time to see the moon – that’s a result of the Earth’s axial tilt, and it means the moon is reduced in visible times as much as the sun is.
So if you try, good luck! But enjoy the video anyway.