# Slowly turn a planisphere at the sidereal rate

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).

Constraints:

• 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:

• Can you show us a photo of this planisphere? – Rocketmagnet May 17 '13 at 13:21
• I can do so later in the day. It's a DIY item, not commercial. – John O May 17 '13 at 13:26

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).

• That's an elegant and simple idea. – John O May 17 '13 at 13:52
• Even simpler, use the hour hand shaft. Put a hole twice the shaft's diameter in the center of the planisphere, and hang the planisphere by the center hole over he hour hand shaft. – Bobbi Bennett May 17 '13 at 14:40
• @BobbiBennett: Twice the shaft's diameter, minus 0.27%. Probably tricky to get accurate enough to be useful. If the hour hand shaft is 2mm, then the hole needs to be 3.9891 mm, with a precision of better than 1 micron. – Dave Tweed May 17 '13 at 14:51

At the risk of oversimplifying the problem, here is a suggested sequence of steps that may serve the purpose, perhaps with some empirically determined tweaks:

• Buy an inexpensive electronic wall clock movement, such as those sold for DIY clocks - look for one using older technology, i.e. discrete crystal and possibly (though unlikely) through-hole parts, plus an electromagnetically driven rotation actuator.
• By replacing the crystal on the board with ones of higher and lower frequency, determine what range of possible tick rates the clock movement can work with.
• Calculate convenient gear reduction ratios for a tick rate within this range, as required for a gear or pair of gears to reduce the hours (or minutes) actual rotation rate to the desired one
• At the simplest, a 1 minute tick rate (no crystal changes) and a 1:10 gear reduction should provide the required rotation.
• Get your friendly neighborhood laser cutting shop to cut you the required gears, in a fairly large size (since precision is not essential, and bigger gear teeth can tolerate laser cutting imprecision better).
• Attach these gears to (a) the hours or minutes shaft, and (b) a shaft on the planisphere disc, to mesh without too much strain.

While this isn't actually an electronic design solution, hence perhaps not ideal for this site, it addresses the problem through minimal engineering and readily accessible tools.

In order to bring this into the realm of electronics, I offer a solution involving a microprocessor:

First, I like the idea of using an existing clock as the timebase. The hour hand on a standard clock makes 1 rotation every 12 hours, or every 720 minutes. You want your planisphere to make 1 rotation every 1436 minutes. That works out to 50.139% as fast as the regular clock. Here's where the microprocessor comes in. Wire up an output of the processor to switch power to the clock, probably using a transistor or FET. Now program the processor to switch on for 300.834 seconds out of every 600 seconds (10 minutes).

Attach your planisphere to the hour hand shaft of the clock, and Bob's Your Uncle.

• They make 24 hour movements too where the hour hand makes one revolution every 86400 ticks rather than every 43200 ticks. The advantage is that the minute hand is still useful, although I do get that for this application it's not required. – nsayer Jul 17 '15 at 16:32

For what it's worth, I sell ready-to-use 24 hour sidereal clock movements on Tindie. I take ordinary movements and replace the controller board with one of my own design that includes a microcontroller so that I can make the clock tick however I want. One of the firmware options is for sidereal time.