Most clocks keep "solar time". A solar day is one in which the earth makes one rotation and the sun returns to the same place in the sky. Astronomers keep time in relation to the stars, with what is called "sidereal time". A sidereal day is one in which the earth makes one rotation and the stars return to the same place in the sky. A sidereal day is a little bit shorter than a solar day because the earth is moving around the sun and it must turn a little bit farther each day to face the sun.

This clock keeps sidereal time. It has a 24 hour dial with a seconds hand and runs on four "AA" batteries. A wheel on the back of the movement allows you to make coarse adjustments to the time displayed. A small circuit board with two push buttons allows precise synchronization of the second hand. One button causes the second hand to advance at twice normal speed. This makes it possible to advance the time if the clock is slow. The other button stops the clock until the first button is pressed. This allows you to stop the clock for a while if it's fast.

The clock is hand-tuned to run one or two parts per million slow of perfect sidereal time at room temperature. Rooms are typically cooler at night, making the clock run a little bit faster, reducing the total error over time. An error of 1.65 parts per million is an error of one second a week.

Objects in the sky are located by their celestial position. This position is commonly given as "Right Ascension" and "Declination". Every astronomical object has these coordinates. For example, The star Betelgeuse in the constellation Orion is at Right Ascension 5 hours 55 minutes and Declination 7 degrees 24 minutes. The Right Ascension is sidereal time. This allows you to know what celestial objects are "up" if you know what the local sidereal time is. Once you have a sidereal clock on your wall, if you see that it's 5:55 local sidereal time, you know that Betelgeuse and Orion are on the meridian. If it's 18:37 sidereal time, Vega and the constellation Lyra are on the meridian. If you're familiar with celestial coordinates, a sidereal clock allows you to keep in touch with what's going on in the sky even during the daytime when the stars cannot be seen.

To calculate your current local sidereal time, go to the web site http://tycho.usno.navy.mil/sidereal.html

To find your longitude, go to the web site http://www.juggling.org/bin/un.cgi/map-find

Availability
Sorry, but we are not currently making the sidereal clock.
The Sidereal Clock is housed in a black plastic body with a clear plastic cover. The cost is $125 for an 8" dial and $150 for a 12" dial.
    To place an order please use the Circuits Order Page.