This data service calculates the fraction of the Moon's visible 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 Moon's phase. Simply fill in the form below and click on the "Compute Table" button.

### Notes

#### Printing

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 Page then Save As...), then use your own favorite word processor to print it.

#### How to Import the Table into a Spreadsheet

Open your favorite text editor, then copy the numerical part of the table (i.e., do not copy the table headings) from your browser and paste it into the text editor. Save the data as a text file.

In Excel for Windows, select Data on the menu bar, then From Text. Select your saved text file. Choose fixed width in the dialog box.

In Excel for Mac, select Data on the menu bar, then Get External Data, then Import from Text File. Select your saved text file. Choose fixed width in the dialog box.

#### 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 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.

#### Moon Phases

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).

#### Additional Resources

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