Reading the moon in Lightpath
- Today › Light timeline: the dotted line is the moon's altitude. The MOON · 4% readout is the illuminated fraction.
- Planner: the dotted map line is the moon's azimuth for the slider time; the MOON card shows altitude and fraction. The line under the cards names the phase (New moon, Waxing gibbous and so on).
- Shot Score: at night the Moon bar drops as the illuminated fraction grows while the moon is up. During the day it is ignored.
- Coming up: Moonrise and Moonset are listed as events with times.
The moon as the problem
Moonlight is sunlight, and even a half moon is bright enough to lift the sky background by several stops. For Milky Way work you want the moon below the horizon or under about 20% illuminated. That happens for roughly ten nights around new moon — and, usefully, for the hours after moonset on any waning night, or before moonrise on any waxing night. The timeline shows exactly when those gaps fall, which is how you shoot the core during a gibbous week.
Faint subjects like the zodiacal light and the Perseids' fainter members need an even darker sky; the AMS calendar rates each shower's peak by moon phase for exactly this reason.
The moon as the subject
A big, glowing moon on the horizon with a landmark in front of it needs three things to line up:
- Phase. Only within a day of full does the moon rise at sunset, so that both the landscape and the moon are lit. The night before full is often best: the moon rises shortly before sunset, so the foreground still has light.
- Azimuth. Use the Planner exactly as for a sun alignment (guide 3): drag the slider to moonrise, see where the dotted line crosses your landmark, and change the date until it lines up. Because the moon's rise point drifts about 12° a day along the horizon, the window for a given landmark is narrow.
- Focal length. The moon is 0.5° across. Use Tools › Field of view to see what fraction of the frame that fills; below about 200 mm on full frame it stays small.
Exposure is the classic trap. The full moon is a sunlit rock — the "looney 11" rule (f/11 at 1/ISO) is a fair start for the disc alone. To hold detail in both the moon and a twilight foreground, shoot the moment in blue hour when their brightness is closest, or bracket and blend.
Field tips
- The moon's rise time slips about 50 minutes later each day. If you missed tonight, tomorrow's is later and further along the horizon.
- The moon appears about half a degree higher than geometry says when on the horizon, because of refraction. It "rises" a couple of minutes early.
- Save a good moon alignment as a recipe; the Return alert will hunt forward up to 420 days for the next match of phase-independent azimuth, and you can check the phase on the Planner when the date approaches.
References
Primary
Supporting
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