Northern lights and the full moon

Moonlight brightens the sky background. The aurora is a light source against that background, so a brighter sky lowers the contrast. SWPC's wording is careful: the moon changes how bright the aurora appears, not how bright it actually is. A strong display during a full moon is still a strong display.

The moon's effect depends on two things you can compute. Its phase says how much of the disc is lit. Its altitude says whether it is up at all. A full moon below the horizon adds nothing. A thin crescent high in the sky adds little.

The table takes the next new moon and the next full moon from the Meeus phase formulas. For each Glow place it counts civil night that date, and the part of it with the moon's center below the horizon.

On the new-moon night the moon rises and sets with the sun, so nearly all civil night is moon-free. On the full-moon night the moon rises near sunset and sets near sunrise, so moon-free darkness drops close to zero at most latitudes. High latitudes break that pattern, because the moon's path across the sky changes a lot through the month. Check the specific night for your town instead of assuming.

A week either side of the full moon, the moon rises or sets partway through the night. That split is often enough. A waning moon rises late, leaving the evening and the core of SWPC's 10 PM to 2 AM window dark. A waxing moon sets earlier each night, leaving the hours after it dark.

• Use the moon planner to see phase, illumination and moon-free dark hours for every night of a month.
• Use the best nights tool to rank the next 30 nights by moon-free darkness inside the 10 PM to 2 AM window.
• For a trip, check the month page for your destination, such as Tromsø in January or Fairbanks in March.

Do not skip a strong storm because the moon is full. The moon lowers contrast; it does not remove bright aurora. Glow's tools rank nights on darkness and moon only because those are the parts you can plan weeks ahead. Activity cannot be forecast that far out.

SunCalc gives the moon's altitude every 5 minutes across a night that runs from local noon to the next local noon. Moon-up minutes are those with the altitude above 0 degrees. Moon-free dark minutes are civil-night minutes, with the sun at or below -6 degrees, that fall outside the moon-up minutes.

Illumination is SunCalc's lit fraction at the midpoint of the 22:00 to 02:00 window. New and full moon dates come from Meeus, Astronomical Algorithms, chapter 49, which Glow checks against published times in its tests. The methodology page lists every threshold.

Glow does not model terrain. A moon that the numbers count as up may still sit behind a ridge, and one counted as down may light haze near the horizon. Use the counts to pick nights, then look at the real sky.