Resources / Celestial timing
Moon Phase Tracker
Phase, illumination and age for any moment you choose, the Moon drawn as your hemisphere sees it, and the exact quarters of the month.
Preparing the instrument for your time and place
The tradition
The Moon's month was the first calendar. From one new Moon to the next is 29.53 days on average, the synodic month, and lunar calendars from Babylon to the Hebrew and Islamic years still count by it.
In the electional and magical literature the Moon's phase is a condition of the work: the medieval authors want her increasing in light for things meant to grow, and judge her before any other body because she carries the beginning.
Reading the instrument
- Elongation is the Moon's distance east of the Sun: 0° at new Moon, 90° at first quarter, 180° at full and 270° at last quarter. The ring of the dial is drawn in it.
- The four exact phases are instants. The eight names each hold the 45° about one of them, so a quarter lasts about three and two-thirds days by name.
- The lit fraction is a true share of the disc, from the angle between Sun and Moon. The Moon is drawn as seen from the hemisphere of the place: south of the equator the lit side is reversed.
Where it descends from
- Meeus, Astronomical Algorithms (1998), chapters 47 and 48
- The lunar series and the illuminated fraction the instrument computes; the phases are found from them by search.
- Sahl ibn Bishr, On Elections
- One of the medieval electional texts that judge the Moon's condition first.
What this does not claim
The Moon is computed to within about 15″ of a reference ephemeris and the exact phases to within about half a minute. The first sighting of the crescent, which lunar calendars wait for, depends on the sky and the observer and is not predicted here.