Telleriums are models of the Earth, Moon, and Sun; they are typically designed to show the seasons, eclipses, phases of the Moon, etc.
I've also included some other models that don't strictly fall into either category, but may be of interest.
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.
- 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.
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. A planetarium sold as the "University of Oxford Young Science Series Solar System" is identical, but does not include the telescope. Hamleys also appear to sell it under their own name, again without the telescope, but I have not had hands-on access; it's possible that changes have been made.
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 separate 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.
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 one star globe and all accessories.
Moon in my Room (Uncle Milton)
This is a simple wall-mounted model showing the phases of the visible hemisphere of the Moon (somewhat flattened) by means of a series of lights inside the moulded plastic. There's a remote control and it can be set to cycle through the phases automatically or manually, and shut down after a set period. The full set should include the model, remote control, and a CD containing a 15-minute talk on the science. List price is £29.99 but it's available considerably cheaper on Amazon and other sites.
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 was around £140, the current model uses more plastics.
Owikit Solar System
This solar-powered model kit is an interesting example of how to get things nearly right - the inner planets move faster than the outer, but they do so in pairs, not individually, with each pair always in opposition. So Venus and Mercury are always on opposite sides of the Sun, Earth is always opposite Mars, and so forth. Pluto is omitted. If you can live with its limitations it isn't bad at showing the movement of the planets (although the gear ratios are wrong for a realistic simulation of relative motion), and solar power means you'll theoretically never need batteries (but see below). This is a kit, and assembly is needed, plus simple tools including a sharp blade and something to pry the pieces apart if you get things wrong. Paints are provided. US retail price is $19.95.
One thing to be aware of is that getting this to work properly requires a lot of patience; the moving parts tend to bind, and the motor has to overcome a lot of friction. It doesn't work with LED or fluorescent lights, the solar cell needs bright daylight or old-style light bulbs, which may cause problems for indoor use. It could possibly work better with a battery instead of a solar cell, but I haven't yet tried that.
Science Museum UK Glow Solar System Mobile
This is exactly what it looks and sounds like - a simple mobile designed to hang over a child's bed. What may not be clear from the picture are that it's actually a fairly small mobile - the four support arms are basically straws about a foot long - and that the planets are coloured cardboard disks, not spheres.What you get is the planet disks, some cardboard stencil masks and luminous paint which can be used to make the planets glow in the dark, nylon cord, and a wall chart. Retail price from the Science Museum is £10 with some of the proceeds going to museum funds.