Moon Clock

In this project I will show how to build a small mechanical pointer that displays the current phase of the Moon. The idea comes from one of the original indicators in Eise Eisinga’s planetarium in Franeker, where a simple hand moves around a dial to show the Moon’s shape through the month. My version follows the same principle but relies on a cheap battery-powered clock mechanism as the drive. With a few gears it turns the 12 hour motion of the clock into a functional Moon-phase pointer that you can easily recreate at home. It is a simple build and a nice way to bring a bit of astronomical history into your living room.

The gears used give a speed reduction of exactly 29.53125 or 29 days, 12 hours and 45 minutes, which is only 48.5 seconds from the true value. In other words, assuming the clock mechanism itself is perfect, after 100 years it will be only 16 hours off. But that is the average. As the moon's orbit isn't a perfect circle, the moon doesn't go around at constant speed and the exact time of any full moon might be off by +/- 14 hours.

Note this design matches my earlier instructable about a different pointer in the planetarium, the day-of-the-week pointer. You might want to have a look there, to see a bit more making off.

1. 1.5mm basswood (front plate and box at the back)
2. (oak) wood 16x52x58mm (the base)
3. 2 pieces of 2mm brass rod, 12 mm long (axle for the arrow and the big gears)
4. paint, black, two shades of teal, gold, and off-white)
5. battery operated clock mechanism (I used the 8mm version from ali, but available in many places)
6. 4 x M2 nuts and 4 x M2 16mm long bolts (the threaded part should be 16-19 mm long)
7. wood glue
8. a roll of black adhesive vinyl
9. superglue

I used a 3d printer for the gears and a laser cutter to make the rest. But in hindsight you could simply use the laser cutter to make the gears, so the 3d printer isn't really needed. I used a airbrush for the paint job.

Painting the clock face is the hardest part. The result should be as in the first color picture where the white parts are painted in gold.

Cut along the contours of the big black rectangle with the hole in it. The size of the images is exactly 97x142 mm.

In order to create an accurate result, first painted it on all sides in the light teal color, wait until it is dry then cover the front with the black adhesive vinyl. Use the b&w masks above and cut only the adhesive (using the laser cutter on very low power). Then remove the parts from the vinyl that need painting, and use the airbrush to paint it. Repeat for other masks in the appropriate colors. I didn't dare to airbrush the gold paint, so I did that using a small brush. Do the mask for the black paint ( with the text and the moon phases) last.

Finish by painting the rear enclosure in dark teal, and the pointer in black and gold

Use the laser to cut out the black shapes from the images above. Each image is sized exactly 97x142 mm.

The parts in the first black-and-white image form a small box. The open square sits halfway inside and keeps the clock movement from sliding sideways. Leave the two smallest pieces aside for now; they are used at the end of this step. Do not glue the box to the back of the clock face yet.

Print the gears on a 3D printer. The small gear should press-fit onto the shaft of the clock mechanism. Remove anything above that gear, since we only need the 12-hour drive.

The large single gear mounts on a short piece of 2 mm brass rod. Pass the rod through the base plate and glue the arrow on top with a small drop of superglue.

Glue four nuts behind the holes in the open rectangle using super glue. These nuts are used later to close the back of the box.

The other big gear, the one with a small gear on it, also gets a 2mm axle that fits through the small hole of the box.

Once the gears are in place, glue the box to the back of the clock face.

Prepare the piece of oak (or any other wood) by sanding or filing a slight angle on the shorter side of the rectangle. Glue this to the bottom of the back panel; it serves as the stand.

Finally, use the two tiny pieces you set aside earlier and glue them next to the stand to finish closing the box.

Set the arrow to the correct position for the current Moon phase. Slide the clock mechanism into place and close the back using the four bolts. That is all there is to it.

Because the arrow is glued to the axle, the only way to correct the Moon phase is to remove the clock mechanism again. In practice you should not need to do this, since the chosen gear ratio keeps the pointer accurate for a very long time.

The final photo shows this project next to the result of my other instructable.

Thanks for following along, and good luck with your build.