Moontool for Windows

by John Walker
March 16 MIM

Moontool for Windows is a Microsoft Windows application which calculates the phase of the Moon at either the current time or at any user-specified time and displays a picture of the Moon at the correct phase, either as a system tray icon or in an open window.

Moontool Display

Moontool window When the Moontool window is open, it displays comprehensive information about the Moon. When Moontool is minimised, it becomes an icon in the system tray which shows the current phase of the Moon; double clicking its icon restores the original display. If you'd like a permanent Moon phase display in the system tray, add a shortcut to Moontool to your StartUp folder with the Run property set to Minimized.

Moontool allows the information displayed in the open window to be copied to the clipboard as a bitmap, permitting you to paste it into another document.

In addition to displaying the Moon at the current time, Moontool can calculate the appearance of the Moon at any user-specified date and time. Two dialogues permit entering date and time either as a conventional Universal time (year, month, day, hours, minutes, and seconds) or as a Julian date. These dialogues perform instantaneous conversion between Universal time and Julian date, serving thus as a Julian date calculator, handy by itself to astronomers. You can animate the display of the Moon by placing Moontool into “Fast mode”, showing the progression of phases at the rate of one day every few seconds.

Moontool's ability to display the Moon at any date in history lets you quickly answer questions such as that posed in the April 1992 issue of Sky and Telescope (page 437): did Paul Revere's midnight ride really occur under the full Moon, or did Longfellow add the Moon to his poem purely for atmosphere? Firing up Moontool and entering the time and date of Revere's ride: 05:00:00 UTC April 19th, 1775, we find that the Moon was 87% full that night, waning from the last full Moon at 21:53 UTC on April 15th, 1775. Moontool tells us that the Moon was indeed close to full that night, confirming Revere's own recollection that “the Moon shone bright”.


Moontool is a 32-bit Windows application which runs on Windows 95 and above and Windows NT 4.0 and above. A 16-bit version of Moontool which runs on any version of Windows starting with 3.0, in any mode (including 3.0 real mode, its being one of the few remaining Windows applications which retain that capability) remains available; it is completely compatible with the 32-bit version but lacks some of its user interface glitz.

The source and executable code for Moontool are in the public domain. You are free to use it in any manner you wish without permission, restriction, attribution, or compensation.

Downloading and Installation

Download Moontool: (Zipped archive, 61 Kb)

After you've downloaded the program archive, extract the files it contains with Info-ZIP or a compatible archive extract program into a new directory, then launch the Moontool.exe program from that directory.

Experienced C programmers who wish to modify this program or simply read the code to see how it works may download the source code. You're welcome to use this source code in any way you like, but absolutely no support is provided—you're entirely on your own.

When you unzip the archive, be sure to use a utility which preserves the directory structure in the archive.

Time Zone Specification

In order to calculate information about the Moon, Moontool must know the relationship between local time and Universal (or Greenwich Mean) Time. This is usually specified when Windows is installed on a computer by a dialogue which asks you to select the time zone in which the computer is located. Since the clock on Windows machines is kept in local time, it can be far from obvious if the time zone is set incorrectly. If Moontool shows incorrect Universal Time values, use the Control Panel's Date/Time item to set the correct time zone for your location, and/or reset your computer's clock to the correct local time. If daylight saving (summer) time is observed in your locale, be sure to check the box indicating so on the time zone page.

Moontool History

Moontool was originally written for the Sun Workstation under the SunView graphical user interface by John Walker in December of 1987. The program was posted to the Usenet news group comp.sources.unix in June 1988 and was subsequently widely distributed within the Sun user community. As the original posting began, “What good's a Sun without a Moon?”.

In 1988 and 1989 Ron Hitchens contributed additional features to the Sun version of the program, including the shaded Moon images which were drawn by Joe Hitchens on an Amiga computer.

In December of 1991 I implemented Moontool under the X Window system using the OpenLook toolkit. That version formed the starting point for this Microsoft Windows Moontool which was completed in March of 1992.

In March of 1999 I made this 32-bit version including a help file.

Astronomical Programming Resources

Algorithms used to calculate the positions of the Sun and Moon and the other items displayed by Moontool are given in the following books.

Meeus, Jean. Astronomical Algorithms. Richmond: Willmann-Bell, 1998. ISBN 0-943396-63-8.
A must-have; if you only buy one book, make sure it's this one. Algorithms are presented mathematically, not as computer programs, but source code implementing many of the algorithms in the book can be ordered separately from the publisher in either QuickBasic, Turbo Pascal, or C. Meeus provides many worked examples of calculations which are essential to debugging your code, and frequently presents several algorithms with different tradeoffs among accuracy, speed, complexity, and long-term (century and millennia) validity.
Duffett-Smith, Peter. Practical Astronomy With Your Calculator. 3rd ed. Cambridge: Cambridge University Press, 1989. ISBN 0-521-35699-7.
Despite the word “Calculator” in the title, this is a valuable reference if you're interested in developing software which calculates planetary positions, orbits, eclipses, and the like. More background information is given than in Meeus, which helps those not already versed in astronomy learn the often-confusing terminology. The algorithms given are simpler and less accurate than those provided by Meeus, but are suitable for most practical work.

These references can be obtained from Willmann-Bell, P.O. Box 35025, Richmond, VA 23235, USA. Phone: (804) 320-7016. In addition to their own publications, they stock most of the standard references for mathematical and positional astronomy.

Home Planet

Moontool has largely been superseded by the much more comprehensive program Home Planet, which includes a Moon and Sun panel derived from Moontool. If you enjoy Moontool, check out Home Planet; it's also free.

Other Astronomy Resources at Fourmilab

by John Walker