{ Practical astronomy | Astronomy | The Moon }

The Moon

The most prominent feature of the Moon are its phases. These are simply a result of the geometry of the illuminating Sun, the Earth as basis for our observation, and the Moon.

When observed with binoculars or a telescope, the Moon proves extremely rich in surface detail. The large, dark, often circular, and almost crater-less areas are maria (singular mare, Latin for "sea"). Although they seem almost as flat as the Earth's oceans, the surface is solid. The maria are huge impact basins that filled up with lava from the lunar interior some time after the impacts. In contrast, the bright areas riddled with craters are called 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; some 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 not far from the terminator, as it is mostly the play of light and shadow that allows us to see these features.

The Moon on 2001-10-27.

Physical parameters:

• Average distance: 384,000 km
• Radius: 1740 km
• Mass: 1/81 M_Earth
• Surface gravity: 1/6 g_Earth

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.

Image parameters:

• Camera: Logitech QuickCam VC
• Detector: 2.9 × 2.2 mm
• Focal length: 2000 mm
• Field of view: 5 × 4'
• Aperture: 200 mm
• Location: Earlyburn
• Processing: Mosaic of 116 images, some regions were missed
  in taking the mosaic, unsharp mask.

It is thought the Moon formed in the collision of a Mars-sized planet with the Earth only about 50 million years after the Solar System itself formed. The smaller planet was destroyed and much of the Earth was also pulverised. These ejecta formed a doughnut-shaped synestia around what remained of the Earth. Within a few decades the cloud of debris would have divided into what rained back to Earth and what formed the Moon in close orbit. (Lock and Stewart 2019a)

Tidal forces between Earth and Moon act to transfer angular momentum from the fast spin of the two bodies to their orbital motion around each other. This leads to an increase in the distance between Earth and Moon from an initial 20000 km to the current 400000 km. This is still increasing by 38 mm/yr. In return, the bodies' spins slow down until they show each other the same face at all times. The Moon has already reached this stage, while the Earth has slowed down from a 6-hour day to a 24-hour day.