Orreries (named for the Earl of Orrery) are mechanical models of the solar system originally intended as educational tools, and also often used as ornaments. They are often called Planetariums. Generally speaking they aren't made to scale - it would be impractical to show orbital distances at anything like true scale, and the giant planets are so large that the small inner planets would be virtually invisible at any practical scale. Antique Orreries can be extremely expensive, but there are many fakes and replicas.

Telleriums are models of the Earth, Moon, and Sun; they are typically designed to show the seasons, eclipses, phases of the Moon, etc.

It should be made clear from the outset that unless you spend thousands of pounds on something made with watchmaker precision, all of these devices are inherently less accurate than software equivalents. The best of them can only show planetary positions accurately over a few orbits, while software can plot positions over millions of years. If you want to go the software route I particularly recommend the free programs Celestia and Stellarium, both available for PC and Mac, and the commercial program Red Shift for the PC.

Where models score is in putting across astronomical concepts such as retrograde motion, eclipses, etc. - these things seem to be more convincing if you can actually see the geometry in three dimensions, not just on a flat screen. There are dozens of different models, with various levels of complexity and accuracy. Generally speaking, they seem to be made either as teaching tools or as ornaments and toys with a little "science" thrown in to justify their existence.

For an Orrery, the main things to consider are

• The number of planets shown?
• Is our moon shown?
• Does the Sun light up?
• Is it motorized or rotated by hand?
• If it's motorized or has gears, does the speed of movement vary correctly? The outer planets should move slowly, the inner planets very quickly
• Robustness etc.

For a Tellerium, the main things to consider are

• Does the Sun light up, and if so how brightly?
• Is it motorized or rotated by hand?
• Is the movement of the Moon properly geared to the movement of the Earth around the Sun?
• Does the movement of the Moon include some vertical variation, and variation in distance from the Earth? If not, it won't be very good for studying eclipses.
• Robustness etc.

I've had hands-on experience with several models, but my interest is primarily in their use as teaching tools, not ornaments or antiques. I hope to add to the list that follows if I can get a good look at more designs:

Discovery World Planetarium

This battery-powered motorized model is comparatively large and looks very pretty, but it's a pretty toy rather than anything like a serious model. The outer planets rotate faster than the inner ones, and after a while the arms carrying the planets sag and get in each other's way. It talks, with a button for a brief description of the sun and each planet. The planets are big and aren't to any sort of consistent scale. There should be an accompanying CD-ROM with more information on the planets. When originally sold these models were packaged with a telescope, and the list price for the pair is under £20, which is a good indication of their quality.

Orbit Orrery

A very basic manually-moved model of the planets out to Pluto, without any gearing. The sun lights up and the Earth's moon is shown. It's well made but it's small and the specification is minimal - despite this the list price is surprising high, at around £20 from scientific instrument vendors. There are cheaper alternatives that give better value.

4M Planetarium Model

A simple model kit that makes a basic Orrery. No gears, lights, or motors, but it's reasonably sized - about a foot across when built - and should be reasonably attractive if painted well. The planets vary in size, they aren't to scale but the order of size seems about right. There's no model for the moon, but in other respects it seems a good basic kit. List price is around £10

Smithsonian 3-in-1 Planetarium

This is another non-motorized Orrery kit which also offers some of the features of a tellerium and a projecting star globe. The sun is big and lights up, with either an orange dome for normal use or a clear dome showing constellations. A nice touch is a second dome for the Southern hemisphere. All nine planets are shown, with fairly long arms giving a total span of up to four feet. The planets come as unpainted white spheres, the kit includes paints and a teeny brush. One drawback is that the rings of Saturn are represented by a disc of solid plastic that looks remarkably unconvincing when painted; I ended up cutting it off and replacing it with coloured rings printed on a disc of transparent acetate sheet. The planets are sized to give an impression that Jupiter is the biggest, then Saturn, etc., but there are obvious limits to how accurate this can be at any given scale - for example, Pluto is nearly half the diameter of Earth. The Earth doesn't have a moon, but the kit includes a separat and much larger model of the Earth and Moon, which can be put onto one of the arms to show basic Earth / Moon / Sun astronomy. There's also a reasonably comprehensive manual, though the presentation isn't wonderful, and a colour guide for painting the planets.  There are some nice details, such as a protective shield around the bulb which stops little fingers from damaging it while changing the domes. This version of the kit is no longer made - the current incarnation looks very similar but I'm not sure if it is identical. When available it sold for around £15-£20. I'd say it's probably the best of these kits I've seen so far in terms of value for money.

Educational Insights Geosafari Motorized Solar System

This looks very similar to the Smithsonian 3-in-1 planetarium but it's a little smaller and the planets come ready painted, although not to a particularly high standard. The "motorized" part of the model is a bit of a let-down; there's no gearing at all, so all of the planets rotate at the same rate (or can be moved separately by hand). One nice feature is a 360-degree angle finder that makes it easy to set planetary positions for any given date; the instruction book gives positions from 2002 to 2005. The sun lights up fairly brightly, and a spare bulb is included, and there's a northern hemisphere star dome that can be put on in place of the yellow sun. But that's really about it for features; there's nothing like the extra Tellurium parts of the Smithsonian model, and the accompanying leaflet is only eight pages, with activities that seem to be aimed mostly at junior school level. It's sturdily built, and if the design included gearing it would be a very nice little model. As it is the movement of the planets is misleading, a minus rather than a plus. This is still available new at around $20 in the USA, £20 in the UK.

Helios Planetarium

This is an excellent electrically-driven model of the classic planets out to Saturn, with relative orbit speeds reasonably accurate, aimed very much at the educational sector, secondary schools and colleges. It's typically supplied with a star dome (usually for the Northern hemisphere although a Southern dome is also available), a tool for setting the planet positions accurately and a manual of experiments and activities. Depending on what was originally sold, it may also include a larger globe of the Earth and components for using it as a Tellerium, plotting orbits of spacecraft, and other activities. It's a little on the noisy side, but hard to beat as an all-in-one solution for astronomical education.  The list price is £300 (without star globe) to £400 with all accessories.

Orbit Tellerium

This hand-driven Tellerium is made mainly of plywood and is very much a teaching aid, not an ornament. It's fairly large and heavy, about 60cm long and weighing several kilos. It includes a lamp for the Sun, Earth globe, and Moon, with accurate gearing for the Moon's rotation around the Earth and the Earth around the Sun. It shows the seasons very clearly, and (with a small globe replacing the large one) does an excellent job of showing the different types of eclipse. It needs a 6V power supply for the lamp which should usually be supplied as part of the kit. List is around £140.