Lego Orrery

Click on the image for a movie (14 MB, needs Flash)

My son Benjamin and I built this orrery on Thanksgiving weekend in 1994. It was featured in the May-June 1995 issue of Lego Mania magazine.

Orreries are mechanical models of (at least part of) the solar system, illustrating the motions of the planets. The first modern orrery was built around 1704 by George Graham. The device gets its name from Charles Boyle, who was the fourth Earl of Orrery and who commissioned the instrument maker John Rowley to make one from Graham's design for the Earl's son, John.

This orrery, made entirely of Lego, shows the Sun, Earth, and Moon. Originally it was powered by a classic 4.5V Lego motor, which was discontinued at about the time this model was created. A Lego 9V motor, also discontinued, is shown in these photos. The current Lego 9V motors have internal gearboxes that reduce the rotation speed and greatly increase the torque; it would be unnecessary to gear down one of these motors as much as has been done in this model.

We exhibited this orrery at Brickworld 2007, and for the occasion, my son Jeremy replaced its original Sun with the elegant one shown in the movie, based on Bram Lambrecht's Lowell sphere generator. The new Sun is nearly four times as massive as the original (visible in the Lego Mania article), which contributes to the wobble accompanying its rotation; replacing the current belt drive (the yellow band joining the pulleys near the top of the left-hand photo) with a chain drive might reduce the shimmy.

Sharp-eyed observers might notice a few other elements that have been replaced since the Lego Mania article was published; these changes are cosmetic rather than functional.

How it compares with the real thing

At Brickworld, one of the first questions many people asked was "Is it to scale?" Well ... no. If everything else were made to the scale of the Sun, none of it would fit in the same room, and we wouldn't be able to see most of it. Space is pretty empty.

But the time scale is reasonably consistent:

How it works

There is a small pulley (a bushing) on the motor axle, that drives the large pulley to the left, and a worm gear (not visible in the photos) on the same axle as the large pulley. In the time scale of the model, one complete rotation of the large pulley is equivalent to one hour in the real solar system. Two separate gear trains are driven by this worm gear.

In the left-hand photo, a 24-tooth ("24t") gear is driven by the worm gear, and transmits its rotation via an axle that passes through the large grey 56t gear to the pair of 24t gears above the long yellow beams. Since the worm gear behaves like a one-tooth gear, the 24t gears turn once per "day" (24 revolutions of the worm gear). The 56t gear (actually, one piece of a large turntable) has a large center hole and is not driven by the axle that passes through it. One 24t gear is connected by a chain to the earth-moon gear train at the end of the long beam; the other 24t gear meshes with a 27:1 gear train (using three 24t/8t steps of 3:1 reduction) to drive the 27-day solar rotation. Note that there is a turntable between the two yellow cylinder bricks, so the pulley above the turntable turns at a slower rate than the gears below; the leftmost pulley is driven by the 24t gear on top, and is not connected to the one below.

The second gear train driven by the worm gear is to its right, and is visible in the center photo. Again, the worm drives a 24t gear to turn once per day, and then a 360:1 reduction follows: 24t/8t (3:1), 24t/8t (3:1), 40t/8t (5:1), 16t/8t (2:1), a follower to reverse direction (24t/24t), and finally the 56t wheel is chain-driven by a 14t gear (4:1). Since the long beam that bears the Earth and Moon is attached to the 54t wheel, the model Earth makes one complete orbit of the Sun in 360 "days", not exactly a year, but fairly close!

At the end of the long beam (see the right-hand photo) the 24t gear, driven by the chain, turns once per "day", as does the Earth, driven by the 1:1 belt drive beneath the long beam. The large white pulley on the Earth's axle/axis (above the beam) is actually a wheel hub with a round center hole, and it is not driven by the axle; rather, it rotates approximately once per 27.3 "days" (via two 24t/8t steps of 3:1 reduction, and a final reduction of about 3.1:1 in the belt drive from the small pulley to the wheel hub. The Moon, at the end of the black arm attached to the wheel hub, thus orbits the Earth in about 27.3 days. Note that the Moon's orbital period is shorter than the interval from (for example) full moon to full moon (one lunar month, or 29.5 days), because the Earth's motion around the Sun requires the Moon to travel more than a full orbit before returning to the same phase. (Naturally this feature is also exhibited by the orrery.)

Components

The entire orrery is constructed from unmodified Lego components, without glue or lubricants. Most are common Lego Technic elements.

None of the components are really unusual, although it should be noted that the 2x6 red bricks on the short end of the beam are Lego weighted bricks (which were formerly available in a package of 4, set number 9863, and have also been included in many Lego Dacta sets). These bricks enclose a metal slug; they weigh 50g each, and are useful for balancing crane arms, bridge spans, and small planets (as in this model). The 56t gear, as noted, is the upper half of a Lego Technic large turntable, which can be separated from the lower half without damaging either element. The various belts are also Lego elements; the originals were all black rubber (which is less durable than the blue and yellow neoprene replacements in the photos).

We built the Earth at a time when green bricks were unavailable (although it includes a few transparent green plates and bricks). We have occasionally thought of making a rounder Earth, but (just as in the real solar system) we have only one Earth and it seems worthwhile to take care of the one we have, bumps and all. The chain links, which look and work like micro-scale bicycle chain links, have been available on and off for many years; they are currently included in the Technic motorcycle (an expensive way to acquire them!) and are available in bulk packs as well.

Don't miss ...

There are other Lego orreries on the web. See Don Rogerson's site for an especially accurate one, and for links to several more.

George Moody