Inputs
This calculator uses a common synodic-month approximation (good for planning and rough comparisons). For telescope scheduling, rise/set times, or sub-degree precision, you’d use an ephemeris service.
Results
FAQ
What does “illumination” actually mean?
Illumination is the percentage of the Moon’s visible disc that is lit by the Sun from Earth’s viewpoint. New Moon is ~0% (dark), Full Moon is ~100% (fully lit). The same illumination can occur twice per month (once waxing, once waning).
What’s the difference between waxing vs waning for visibility?
Waxing phases (after New Moon) tend to be more visible in the afternoon/evening sky. Waning phases (after Full Moon) tend to be more visible late at night into the morning. This tool gives a simple “best time of day” guide rather than exact rise/set times.
Is this “accurate enough”?
For planning, comparing dates, and general stargazing decisions: yes. It’s an approximation based on the synodic month length. If you need precise phase moments, limb orientation, libration, or rise/set times for your exact location, you’d use an ephemeris (e.g. an almanac).
Why can the same illumination happen twice?
Because the Moon brightens on the way to Full Moon (waxing), then dims afterwards (waning). For example, 60% illumination happens once before Full Moon and once after it. Waxing/waning tells you which side of the cycle you’re on.
How is “Dark-sky impact” calculated?
It’s a simple rule-of-thumb based purely on illumination: lower illumination = less moonlight = better for faint objects. It does not account for altitude, local light pollution (Bortle), haze, clouds, or seeing.