How to use it
- Choose a place, a date and a time.
- Read the azimuth, the altitude, the phase and the distance, and moonrise and moonset.
- The sky chart shows the moon's path for the day; the map shows its direction.
5 practical uses
- Plan a moonrise photo over a landmark.
- Know when the moon is up for a night walk or a night sail.
- Check the moon's height for a telescope session.
- Time a night shoot for a film.
- Plan around moonlight for a camping trip.
The maths behind it
The moon's ecliptic longitude, latitude and distance are found from its mean longitude, mean anomaly and mean distance with the main periodic terms, then converted to right ascension and declination. The altitude and azimuth follow from the hour angle as for the sun, with a parallax correction for the moon's nearness. The illuminated fraction is (1 + cos i) / 2, where i is the phase angle between the sun and the Earth as seen from the moon.
The numbers
Azimuth and altitude are as for the sun: compass direction, and height above the horizon. The distance varies from about 356,000 km at perigee to 406,000 km at apogee, which is the difference between a "supermoon" and an ordinary one. The illuminated fraction is the share of the disc that is lit as seen from Earth; the phase name follows it. For a calendar of phases, see the moon phase tool.
Positions come from the SunCalc library, accurate to a fraction of a degree, which is enough to plan a moonrise photo but not an occultation.