Electronic – Slowly turn a planisphere at the sidereal rate

motorstepper motor

My problem is to rotate a planisphere (a star chart) at the sidereal rate.

I will set the planisphere to its initial position manually, then turn on some device which rotates the planisphere on its central axis, to keep it in sync with the current sky at my location. That is, the problem is to rotate a circular, laminated bristol-board (heavy paper) disc once every 23 hours, 56 minutes. The laminated disc may be from 10 cm across to 60cm across, and is fairly lightweight. The disc may be horizontal, or vertical (hung on a wall).


  • A small, lightweight mechanism is preferred.

  • Battery power is preferred.

  • High accuracy isn't needed.

  • The disc would be rotated\updated once every 10 minutes or so.

  • I'm thinking that the mechanism would likely be placed at the edge of the disc, not the centre, and a little wheel would be firmly pressed against one face of the disc. When the little wheel spins a full rotation, it turns the disc by perhaps a few degrees.

  • I prefer simpler mechanisms over complex ones.

Can you provide suggestions for implementing this?

Here is a photo:

Best Answer

My first thought would be to take a battery-operated clock mechanism (available from many suppliers for a few dollars) and replace the minute hand with a disk that drives the edge of your planisphere disk.

For example, to drive a 60-cm planisphere, you would need a drive disk of

60 cm / 23.93447 = 2.5068 cm

Since the basic accuracy of the clock mechanism is very good, the overall accuracy would depend on how accurately the diameter of the driving disk can be controlled (relative to that of the driven disk).