Keith's astrolabes: precession as demonstrated with my Java applet
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Precession can be shown on the traditional astrolabe and on the equinoctial astrolabe displayed by this program. There are no facilities for changing the date on the universal astrolabe or the spherical astrolabe.
Precession is the name given to the effect whereby the stars appear to rotate around the ecliptic pole by one degree in about 71.66 years. This value, and the dates given below, are only approximate so please don't quote them.

On an astrolabe, precession is only significant over the centuries. To advance the centuries quickly, it is most convenient to click and hold down the mouse button when the pointer is over the +++ or --- button in the date section of the button panel, which causes the century to change by up to three times per second with a reasonably fast compuer. To observe the effect of precession on the traditional astrolabe, if you select the latitude of 66.6 degrees North or South, you will see that the stars move along the almucantars as you change the centuries.

If you hold down the --- button, you will see that the star called Polaris, which is very close to the North Pole around the year 2000 AD, moves away from it and the star called Thuban moves close to the North Pole roughly in the year 2600 BC.

Going the other way, holding down the +++ button, Vega becomes the North Star when the year is about 14000 AD.

Conversely, you will see that there is no bright star close to the South Pole at 2000 AD, but if you hold down the --- button you will see that the star which is known as beta-Hydrus was over the South Pole roughly around the year 300 BC.

On the equinoctial astrolabe, the North and South Poles are both shown in the centre of the astrolabe. There is no point in selecting a latitude of 66.6 degrees because it only shows horizons with their azimuths, and doesn't show almucantars. Also, it doesn't allow separate hemisphere settings of North and South of the Equator, because both these projections are shown on top of each other. Instead, click on the ws button (Winter Solstice button) at the bottom of the time and date button panel.

From 2000 AD, hold down the --- button and watch the filled circle indicating Polaris move away from the North Pole. The hollow circle showing beta-Hydrus will then slowly move over the South Pole when the date setting indicates about 300 BC and then, as beta-Hydrus moves away, the filled circle showing Thuban moves over the North Pole when the date setting indicates about 2600 BC.

Precession has sometimes been considered in an endeavour to date - or to confirm the date of - medieval astrolabes. However, the position of the stars on the rete can only give a rough indication of the epoch of the star chart used when the star pointers were initially set - or last adjusted. On the back of the astrolabe, the relationship of the date and the First Point of Aries can usually provide a better indication with astrolabes manufactured before the introduction of the Gregorian Calendar.

Incidentally, the program doesn't allow for the 'real' motion of the stars.

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Copyright Keith Powell 1999-2002