Object note
The Moon
The first thing anyone points a telescope at, and the object most often observed at the wrong time.
- Distance
- About 384,400 km average
- Cycle
- 29.5 days, new moon to new moon
- Best phase
- Any crescent or quarter
- Worst phase
- Full
Why the full moon is the worst time to look
Detail on the Moon comes from shadow. At full moon the Sun is almost directly behind the observer, so the surface is lit face-on and every crater and mountain range flattens into a bright, nearly featureless disc.
The opposite is true along the terminator — the line between lunar day and night. There the Sun is low, shadows are long, and crater walls, central peaks and rilles stand out in strong relief. Three or four days after new moon is the best time all month.
Following the terminator
The terminator moves about 12 degrees of lunar longitude per day, so the features on view change noticeably from one night to the next. Over a single week you can watch sunrise sweep across a whole quadrant.
This is also why the Moon repays nightly observation more than almost any other target: it is never quite the same twice.
Libration
The Moon keeps the same face toward Earth, but not perfectly. Its orbit is elliptical and tilted, so it appears to rock gently back and forth over a month. The effect is called libration, and it brings an extra 18 percent or so of the surface into view at one time or another.
A few features near the limb are only well placed during favourable libration. Mare Orientale is the classic example — mostly hidden, and only glimpsed at the edge when the geometry is right.
Observing notes
- Filter. Even a crescent moon is bright enough to be uncomfortable at high magnification; a neutral density filter helps.
- Magnification. The Moon takes magnification better than any other target. Atmospheric steadiness, not optics, sets the limit.
- Timing. Observing in twilight, before the sky is fully dark, actually reduces glare and often shows more detail.
- Safety. Never observe the Sun without a full-aperture solar filter. No exceptions, and no eyepiece filters.