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Fraction of the Moon Illuminated

This data service calculates the fraction of the visible lunar disk illuminated by the Sun for every day in a selected year between 1700 and 2100. The computed fraction is a quantitative way of describing the lunar phase. Choose a reference time for the calculations, the year of interest, and the time zone of the location of interest. Then click the "Get Data" button at the end of the form.

Year For example: 1986

Reference time  

Time Zone Example: 5 hours West of Greenwich (UTC-5) is Eastern Standard Time
Need a time zone?  Try the time zone map.

Notes

Fraction of the Moon Illuminated

The fraction illuminated is geocentric, that is, it is computed for a fictitious observer located at the center of the Earth.

The fraction applies both to

  • the illuminated area of the apparent lunar disk divided by the total area of the disk; and
  • the illuminated portion of the apparent lunar angular diameter divided by the total diameter, for the diameter that would intersect the Sun if extended.

Lunar Phases

The lunar phases are not technically defined in terms of fraction illuminated. However, the phase of the Moon can be identified from the fraction illuminated. To the accuracy given in the table, the fraction illuminated at New Moon is 0.00, at First and Last Quarter is 0.50, and at Full Moon is 1.00. First and Last Quarter can be distinguished by noting whether the fraction illuminated is increasing or decreasing. First Quarter occurs when the fraction illuminated is increasing (Moon waxing; in evening sky) and Last Quarter occurs when the fraction illuminated is decreasing (Moon waning; in morning sky).

Time Zones

When using the "Need a US Time Zone" button, the service will fill the Time Zone hours field and set the Time Zone direction button according to the standard US time zone entered.

For locations that require it, you may enter the time zone in decimal hours. For example, for locations in India, you may enter 5.5 hours east of Greenwich for the time zone. The time zone field can accommodate up to five characters.

Only standard time zones are available; for Daylight Saving Time, you will have to adjust the time manually.

Additional Resources

For more information, see