County Maps US

Tools Elevation, time and sun

Moon Phase Calendar

Phases are computed, not looked up, so any month in any year is available.

How to use it

  1. Read tonight’s phase at the top: the drawing, the lit percentage, the age and the next full and new moons.
  2. Step through months with the buttons; the grid shows a small moon for every day at 9 pm local time.
  3. Days with a principal phase are outlined and named; the list under the grid gives their exact times in your zone.
  4. Switch to the southern hemisphere view if you are south of the Equator: the lit side is mirrored.

How it works

The moon’s position comes from the truncated series in chapter 47 of Jean Meeus’s Astronomical Algorithms and the sun’s from chapter 25, both computed in your browser. The lit fraction is (1 + cos i) / 2 where i is the phase angle from the true sun, moon and Earth distances. The instant of each new, first quarter, full and last quarter is found by iterating on the difference in ecliptic longitude between the moon and the sun until it reaches 0, 90, 180 or 270 degrees, which agrees with published tables to within a few minutes.

The disc is drawn from the lit fraction with the terminator as half an ellipse. Seen from the northern hemisphere a waxing moon is lit on the right; the southern view mirrors it. Between the tropics the moon appears rotated and looks different again. Times are in your browser’s time zone.

Questions

When is the next full moon?
It is shown in the box at the top, to the minute, in your local time, and every full moon is marked on the calendar grid.
How long is a lunar month?
About 29.53 days from one new moon to the next on average, which is why a full moon falls about a day earlier each calendar month and some months have two (a "blue moon").
What does the moon’s age mean?
The number of days since the last new moon. New is 0, first quarter about 7.4, full about 14.8, last quarter about 22.1.
Why does the moon look different from the drawing?
The drawing uses the northern-hemisphere orientation with north at the top. From the southern hemisphere the moon appears upside down (use the toggle), and near the Equator it appears tilted so the lit side points up or down.

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