Keith's astrolabes: a Java program displaying four types of astrolabe
- planispheric, universal, equinoctial, spherical.
Use them here or construct paper versions.
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Keith's Astrolabes
(This page.)
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Keith's Astrolabes

The java applet provided with this page displays modern versions of four types of medieval astrolabe. You can explore the many features of these on your computer where the menu, buttons and keys allow you to select different features. You can also construct the astrolabes after printing out the components on paper and film.

The Java applet was originally written in 1999 to 2002, after which version v74 was displayed here. It was updated in February 2012 to correct errors concerning the planets, particularly the moon, when the version was adjusted to v74xx.

If you had a java-enabled browser, you could see:

my representations of four types of astrolabe.

You need a Java-enabled browser to see my astrolabe program. After the many Java files have slowly downloaded, it appears in a separate window.

Please note that some versions of advert blockers may stop the display of the java window which shows the astrolabe, erroneously assuming that it is an advertisement.

Please also note that if you click on the above box to reopen the Astrolabe window, its window may appear behind this one.

Information files

If you want to read my astrolabe information files, start by looking at the: information file index.

The information files explain how to use the different astrolabes supported by this program, and provide information on various aspects of astrolabes. They also explain how to print and construct 'paper' versions.

Keith's Astrolabe

In medieval times, astrolabes were used to measure the position of the Sun or a star and thence to find the time. They were also used to find precisely where the Sun and certain stars were located at any date and time throughout the year. However, a skilled operator could use an astrolabe to discover a great deal more.

My original purpose in writing the program which displays the java applet was to use it to explore the many facilities which medieval astrolabes provided. I hope you will now want to use it to do the same.

Before you start to experiment freely with the buttons, menu and keyboard keys, I recommend that you increase the size of the astrolabe window to make it as large as possible. Note that the 'Reset' button on the button panel. allows you to reset all of the settings to their initial values.

The program shows freely adapted versions of four types of medieval astrolabe:

a conventional astrolabe
an equinoctial astrolabe
a spherical astrolabe
a universal astrolabe

Many of the features of these astrolabes can be varied. For instance, the conventional and equinoctial astrolabes can be set to suit almost any latitude in the Northern and Southern hemispheres.

As the centuries are changed by holding down the year +++ or --- buttons, you will see the stars precess, or the position of the offset calendar scale on the back of the astrolabe move as it is affected by the slow rotation of the earth's perigee. (Only the Gregorian calendar is currently supported.)

The Astrolabe Web Site

In my opinion, the best site on the internet concerned with astrolabes is James E Morrison's web site at

As well as details of an astrolabe you can purchase and a PC DOS program you can download, you will find pages covering many aspects of astrolabes, including:

The Stereographic Projection,
The Parts of an Astrolabe,
Uses of the Astrolabe,
The Astrolabe Quadrant,
and the less common types of astrolabe.

Links and References are at

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Difficulties with old PC and Macintosh Browsers

(Note: this was written in 2002.)

This astrolabe program appears to work as intended on most Java-enabled browsers including all later versions. Unfortunately, if you are displaying my astrolabe with some versions of Netscape Navigator issue 4.5 or with some versions of Macintosh browsers you may experience problems.

First, check that my astrolabe program isn't displayed in two windows, one directly over the other. (Move the window and see if another window is displayed underneath.) Should this occur, close both of these windows and click on the box at the top of this page which says, "Click here to reopen the astrolabe window." The astrolabe window which then opens should work correctly.

Second, if the menu headings don't appear or don't look correct, close the astrolabe window and click on the box at the top of this page which says, "Click here to reopen the astrolabe window." The astrolabe window which then opens should work correctly.

Third, if the background colour of the astrolabe window is not initially white, click on the Settings/background/yellow menu item when you will have a yellow background (or select a grey). The white background doesn't work on some browsers. The astrolabe window should then work correctly except that the area above the horizon line, which should be pink, will now be the background colour.

You can contact me at

(Sorry, you can't click on this link.)