Moon
From chi and h
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Phases
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The Earth's Moon is the brightest object in our night sky. Yet, there are moonless nights and nights when the Moon shines all night. This is because the Moon moves around the sky once every month (once every "synodic month" to be precise). When it is close to the Sun we do not notice it in the daylight sky. Two weeks later it is opposite the Sun, rises at sunset and sets at sunrise. Going hand in hand with this movement goes a change in the illumination of the Moon by the Sun. Opposite the Sun we see exactly the sunlit half of the Moon and it appears "full". A week earlier or later, the sunlight coms from one side so that half the visible Moon is sunlit and half is in darkness. You might call this Half Moon, the former is called First Quarter and the latter Last Quarter. When the Moon is close to the Sun we cannot see it at all. This is called New Moon, as the Moon will reappear as a crescent in the evening sky two or three days later.
The montage shows the change of phases from one day to the next. The synodic month is 29.5 days long and the 30th image slot is left blank. The first and 29th slot would show the New Moon. As this is not visible, I use a picture of an annular and a total solar eclipse. Solar eclipses occur at New Moon, but only about twice a year.
The Gregorian calendar months have different length that do not match the lunar phases. Months are a secondary feature of that calendar, the main units are the year and the day. The Islamic calendar, on the other hand, is a pure lunar calendar and adjusted according to observations of the lunar crescent after New Moon. The image here was taken just after the end of the 30th day of Muharram and the start of the first day of Safar of the Islamic year 1426 AH.
Moonscapes
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The Moon is extremely rich in surface detail. The large, dark, often circular, and almost craterless areas are maria (singular mare, Latin for "sea"). Although they seem almost as flat as the Earth oceans, the surface is solid. The maria are huge impact basins that filled up with lava from the lunar interior. In contrast, the bright areas riddled with craters are call highlands. These areas are indeed higher than the maria; the surface is older and has suffered more meteorite impacts after their formation.
Lunar craters are very varied. They are not volcanoes, but result from the impact of meteorites. The largest are 250 km in diameter, while the smallest you can image are as small as your equipment allows. Old craters have craters within them, large craters have central mountains and terraced rims; dome of them have huge "rays" of ejecta.
The rims of maria create mountain ranges. There are steps of perhaps 100 m height in maria, which can be seen when close to the terminator (the boundary between light and shadow). In fact, all features are better observed near or not far from the terminator, as it is mostly the play of light and shadow that allows us to see these features.
- The Moon deserves the use of high resolution, i.e. a camera behind a telescope, in prime focus or using eyepiece projection for extra magnification. I find it difficult to focus a dSLR in this situation; it would need a zoomed live-view feature to help focussing. A webcam has live-view "built in", actually on the laptop screen. While this helps getting a good image, the field of view is very small. Atmospheric seeing should limit resolution to one to three arc seconds, corresponding to between 1.5 and 5 km on the Moon. By selecting better than average frames and stacking a number of these, images can usually be unsharp masked and may then show detail at the 1 or 2 km level.
- Ideally, the telescope should be tracking while you take images. This makes it easier to compose the images, and it may also help a little if the exposures are a bit long. When taking stacks of images, tracking should be at lunar speed to avoid a significant drift of the field of view across the Moon during the sequence of frames.
Lunar eclipses
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A lunar eclipse occurs when the Full Moon moves through the shadow of the Earth. In the first image, two thirds of the Moon have moved into the penumbra - a zone of semi-shadow where the amount of sunlight is reduced only slightly. In the second image, the Moon has moved deeper into the shadow and 30% of it is now in the umbra - the dark core where in theory no sunlight reaches. Indeed, the umbra appears to be completely dark at this short exposure.
In the third image, the Moon is 70% inside the umbra. This longer exposure reveals that some - mostly red - light does enter the umbra. This is because the Earth's atmosphere acts as a lens and refracts some sunlight into its shadow. Also note the two stars: On the left is 59 Leonis (5.0 mag), on the right 56 Leonis (5.8 mag). Use the stars as reference points, and the final image shows how the Moon has moved by more than its radius and is now entirely deep into the umbra. The image is overall fainter, because no direct sunlight hits the Moon, which would in turn spoil our sky background as it does in the third image. Still, the umbra is not evenly dark, the top of the Moon is brighter, because it is less deep into the umbra than the rest of it.
- Unlike moonscapes, photographing a lunar eclipse really requires to have the whole moon in the field of view. Deep into the eclipse, other objects nearby may be interesting to capture as well. A field of view of 2° or more seems advisable. At the same time, the Moon itself should be recorded with at least, say, 500 pixel diameter.
- Observe that the umbral exposures here are 1000 times longer than exposures of the penumbral Moon, which is almost at Full Moon brightness. If you go for longer focal length, even longer exposures are needed. Tracking (at lunar rate) is necessary.
Conjunctions and occultations
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The Moon on its monthly round of the sky passes all the planets. Most of these encounters are no closer than a few degrees, but sometimes the encounter is more dramatic. In the first image, while for Europe the Moon passes below the planet (some hours after this image during the day), the United States observe an occultation, with the Moon passing in front of the planet. In the second image, Europe was lucky and the Moon occulted the ring planet Saturn. The image shows the planet just after it emerges from behind the Moon.
- The imaging strategy is dictated by how close the planet and Moon get. The field of view has to match this. The first image is essentially similar to the atlas of lunar phases. The second image is equivalent to imaging the planet or lunar detail, and so a webcam was used. Luckily, the Moon was only a crescent and not too bright compared to Saturn. An occultation of a planet is a very photogenic and dynamic event. You should consider taking a time-lapse or real-time movie.
