This article includes a JavaScript utility to help you learn the phase of the Moon tonight, the day your were born, or on any historical date. You'll find instructions below.

Will moonlight interfere with nighttime observing when you plan to take vacation next summer? What was the Moon's exact phase on the day you were born? When's the next opportunity to photograph a young crescent Moon?

These and other questions are easy to answer with SkyandTelescope.com's Moon Box, a JavaScript utility that will open in a new browser window. It accurately shows the Moon's appearance on any date from 4000 BC to AD 8000, a whopping span of 120 centuries!

When you launch it, the animated Moon lets you know it is waiting for your input. Simply click the Calculate button to find the phase of the Moon right now. The animation stops at the Moon's current appearance, and the phase is described in words at the bottom.

To get the Moon's phase on any other date, use your mouse to select the month and day from their list boxes at top. If the year needs to be changed, click within its box and retype the numbers. Be sure the kind of year, AD or BC, is properly selected, and click the Calculate button again.

To check that you are entering the dates properly, practice on these examples, all of which were described in more detail in past articles from Sky & Telescope:

• Bobbing in a lifeboat in the predawn hours of April 15, 1912, Titanic survivor Lawrence Beesley described "the thinnest, palest of moons" rising over the ocean to the east. The previous evening, the Moon's absence from the sky had made it hard for lookouts to spot the iceberg that sank the ship. (S&T: October 1993, page 79)


• During World War II, Allied prisoners held at Stalag Luft III picked the "dark of the Moon" — March 24, 1944 — for their daring breakout to minimize the chance of being spotted by German guards. Their exploits are the subject of the classic 1963 movie The Great Escape. (S&T: April 1994, page 86)


• Thales of ancient Greece earned his reputation by successfully predicting the total solar eclipse of May 28, 585 BC. Our Moon Box does not show eclipses, but it does indicate that the Moon was exactly new on that date. (S&T: May 1994, page 36)


• At the start of the American Revolution, Paul Revere made his famous "midnight ride" on April 18, 1775, with bright moonlight to guide the way — a fact mentioned five times in the stirring poem of Henry Wadsworth Longfellow that begins, "Listen, my children, and you shall hear . . ." (S&T: April 1992, page 437)


• The next month to contain two full Moons is May 2007 (try the 2nd, then the 31st). Whether to call the second one a Blue Moon is a matter for debate!

If you accidentally enter an invalid date, such as February 29th in a non-leap year or September 31st in any year, an error message will appear and explain what the problem is. Note that our Moon Box uses the Gregorian calendar after 1582 and the Julian calendar before that year.

Click to open our Moon Box utility