An orrery is a model of the solar system, designed to predict the relative positions of the planets. It models their relative orbital motions through a system of gearing. It doesn’t reproduce their relative distances, and it usually only approximates their relative sizes. There have been mechanical models like this for almost as long as there have been mechanical clocks, but the namesake orrery belonged to Charles Boyle, 4th Earl of Orrery.* It was a (relatively) simple structure, built by instrument maker John Rowley, depicting the relative motions of Earth and Moon around the Sun.

This modern equivalent features eight planets, three dwarf planets, and the Moon. It’s produced by Eaglemoss, generally known for superhero figurines and science fiction models. It was originally distributed in twelve monthly instalments along with a part-work publication, and the current construction manual to some extent reflects that origin—there’s a lot of superfluous assembling and disassembling that only makes sense in that context.

It’s certainly one of the larger kit boxes I’ve received:

And the first thing you see when you open it is a dire warning:

Unpack and work on only ONE stage at a time.

Yeah. Like any model-maker is going to pay the slightest attention to that. So out they all came, and I kept track of the five separate trays using a subtle technique:

Each layer, as you see above, has a parts map sitting on top. And under the map are all the parts, in sections labelled with the assembly order.

And each assembly stage in the parts trays has a corresponding section in the construction manual, clearly set out with pretty 3D images:

You’d actually need to be pretty careless to go astray. And one of the advantages of burrowing down into the deeper layers is this discovery at the bottom of the box:

Each tray provides only the precise number of tiny screws required for the assembly of that section, so this is good to know about.

There’s also a toolkit:

(Though very few of these tools are actually necessary during assembly.)

---

The first thing I did was put on a pair of cotton gloves before I handled all that lovely shiny brass. Here’s the Sun:

It sits on top of a drive shaft that comes up through a central column from a motor in the base. Here it is with the planetary arm that supports Mercury, and the very first gear in the train that will drive the revolution of all the other arms:

And here are the Venus and Earth arms, and the start of the gear train:

The big brass plate is a toothed disc representing the ecliptic, the plane of the Earth’s orbit:

There are 136 teeth on this disc, and as the Earth moves around the ecliptic those teeth engage with an 11-tooth gear at the base of the Earth arm, which drives the revolution of the Moon around the Earth. That provides a ratio between the Earth’s sidereal period and the Moon’s synodic period that is within 0.05% of the correct value.

The plate is marked up with ecliptic longitude, and also with a couple of sets of planetary positions—for 1 January 2000 and for 1 October 7BC. The first is the origin of the current J2000 astronomical epoch. The second, I think, is the date of a so-called “Great Conjunction” of Jupiter and Saturn that has been invoked to “explain” the Star of Bethlehem.

Below this plate, it becomes a matter of just stacking up the gear train and planetary arms, making sure everything’s tightly secured but freely rotating on the nylon bearings supplied:

Each planetary arm sits within a gear sandwich:

These sandwiches alternate with, and mesh with, offset gear arms:

So as we descend the gear stack, there are two stages of step-down between planets, with each planet arm rotating more slowly than the one above.

After all the gearing is assembled, it’s time to put together the base. The upper surface of the base is engraved:

This is marked up with the months and days of the year. The month names are in Latin, which is a clever way of catering to an international market without having to produce multiple translations. Within that ring of markings, the zodiacal “houses” are marked—the familiar “horoscope signs”. And there’s also another set of planetary positions, corresponding to the date engraved in the centre of the base plate: 24 August 2006, which I think probably relates to the original release date of the orrery.

Around this base plate fits a moveable ring:

The ring is rotatable to allow for the precession of the equinoxes.

The divisions here mark out the true angular span of the zodiacal constellations along the ecliptic plane—notice that a thirteenth, non-zodiacal, constellation, Ophiuchus, takes up quite a lot of room. Here it is on a star map, intruding across the ecliptic plane between Scorpius and Sagittarius:

If I have a quibble about this, I’d have preferred the zodiacal houses on the base plate to have been marked with their zodiacal symbols, and the constellations on the ring to have been given their Latin names. But I can see why that didn’t happen—there’s not much room for text within the true span of Scorpius, just to the left of Ophiuchus, above.

The underside of the base is where the power gets in, and where the controls are. There’s an on-off switch that also toggles between forward and reverse, and a speed control knob.

The motor mounts on the underside of the base plate, and there’s a shaped piece of foam sound insulation that slots into the interior space:

It’s not clear how the foam fits into the base, or how the motor needs to be orientated to avoid fouling the other gizmos when everything’s closed up. Here’s the correct orientation:

It is all held together with three long screws from underside to top, which have to be positioned almost blind—the hardest part of the whole assembly.

Then the whole column of gears and planet arms simply drops into the central hole in the base plate, and secures with a screw. There’s a little bit of fiddling to get the drive rod to engage with the motor—you have to rotate the Sun and top gear assembly until it drops into place.

And here it is:

I’ve aligned all the planet arms so that they could be tweaked to the “same vertical” and fully locked in place. (The locating holes for the planet arms are oversized, and allow a bit of sagging if the grub screw isn’t properly tightened. You can see that’s happened with Jupiter, above, which needed a bit of final adjustment.)

The planets supplied with the kit are rather garishly painted. Here’s Jupiter:

So I splashed out some more cash on a set of planets made from more-or-less suitable shades of semi-precious stone. (The set also models Saturn’s rings rather better, suspending them clear of the planet, rather than attaching them to the surface.)

Here they all are, in order of distance from the Sun:

I’m not convinced by the yellow-jade Mercury, and am disappointed that the Tiger’s Eye for Saturn wasn’t more dominated by orange, but I think that overall they do a reasonable job. All the moons apart from Earth’s are purely decorative—there’s no attempt to make them revolve around their parent planet. And, having suggested the steep inclination of Uranus’s moon system, it’s a shame the designers didn’t do the same for Pluto.

The end product was so pretty I couldn’t bear to see it getting dusty, so I ordered up a display case from LasAcryl (whom I’ve previously used for my model of the Discovery spacecraft from 2001: A Space Odyssey).

This necessitated smuggling the power supply into the case somehow. To keep the hole in the base as small as possible, I decided to cut and rejoin the wires between the transformer and the power jack. And, since I was doing that, I popped in a rocker switch at the join.

At first I imagined cutting a groove in the underside of the display base to accommodate the wire, but instead I just slipped a few pieces of folded felt under the base, raising it by a millimetre or so off the table surface—ample room for the wiring. Here’s the wire making its unobtrusive entrance.

But really, you need to see it in action, don’t you?

(You can also see, reflected in the brasswork, the Boon Companion and me fidgeting around behind the camera.)

Here it is running inside its display case:

And here’s a slightly jerky hand-held view of the gear train in action. If you wait until the end of the video, you’ll also get a nice view of the Earth going past, with a small basal gear meshing with the ecliptic plate, driving the revolution of the Moon.

---

* Charles Boyle was the grandson of Roger Boyle, an Anglo-Irish landowner who was created Earl of Orrery in 1660, having successfully enforce anti-Catholic British policy in Ireland under both Cromwell and the newly restored Charles II. His title comes from Orbraige, one of tribute kingdoms of Munster, a name that mutated to Orrery after the British Plantations of Ireland.