NOTICE TO USERS
Commander, Naval Meteorology and Oceanography Command is consolidating the command's web presence in accordance with Department of Defense (DoD) and Navy guidance. The U.S. Naval Oceanography portal will be the single access point for all public facing Meteorology and Oceanography products and services. This publicly-accessible portal is currently online at http://www.usno.navy.mil and is being populated. In the near future, non-DoD users will be redirected to this portal.
DoD customers can access all operational data, products and services via the NIPRnet Naval Oceanography Portal site at https://oceanography.navy.mil (CAC required).
This page provides a way for you to obtain a table giving, for every day in a year, the fraction of the Moon's visible disk illuminated by the Sun. This is a quantitative way of describing the Moon's phase. Simply fill in the form below and click on the "Compute Table" button.
Notes on the Data:
The table is 111 characters wide, so to print it, you will have to use landscape orientation. You may have to save the table to a file on your computer (for example, in Internet Explorer, click on File then Save As...), then use your own favorite word processor to print it.
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 Moon's apparent disk divided by the total area of the disk; and
- the illuminated portion of the Moon's apparent angular diameter divided by the total diameter, for the diameter that would intersect the Sun if extended
The Moon's 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 it is 0.50, and at Full Moon it 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).
For more information, see
- Phases of the Moon and Percent of the Moon Illuminated in FAQ (descriptions and definitions)
- Phases of the Moon in Data Services (dates of the four major phases)
- Complete Sun and Moon Data for One Day in Data Services
