!Orreryby Photodesk

Mechanical Orrery
(Astron.) A mechanical model of the solar system showing the relative motions of the planets by means of clockwork; much in vogue in the 18th century.

(Named after Charles Boyle, Earl of Orrery)

Spacetech. The name itself conjures up images of planets hurtling around the solar system, whilst we earthlings throw our very best technology at them, propelled by fire-excreting rockets into the depths of space. Spacetech is now called Photodesk.

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I came across this program whilst searching for educational software for Physics. The advert on their web site for Orrery is a concise introduction to a highly sophisticated package.

Orrery arrives on two DD floppy discs, cunningly cased in an ordinary CD box, with its interior re-designed to house them. The cover of the box is a fairly basic piece of artwork printed using a standard inkjet machine. Ideally this should have been done professionally, or should be re-done using the rather superior quality print attainable from today's modern printers. However, despite its rather ordinary appearances, the best is yet to come.

Webster Icon The first disk houses the HTML manual, and a copy of Andrew Pullan's Freeware browser, Webster. Considerable effort has been put into writing a comprehensive manual. The only fault I have found with it is that all the HTML files and GIF images lie within the same directory. It should not have been hard to put the images tidily away in a folder, and leave the INDEX/HTML file on its own. Confusion could easily arise between the INDX/HTM file and INDEX/HTML. This point should be addressed by Spacetech, as not everyone using the manual will, necessarily, be familiar with HTML hierarchies.

Once you do find your way to the correct index page, the friendly layout immediately invites you to explore the contents of the software. The starry background used (a negative image) is particularly effective at setting the scene.

Users unfamiliar with HTML are given a good introduction on how to use the manual - once they have made it to the front page. My previous point therefore stands, although the introduction is again very well thought out.

The introduction and installation instructions are very clear, even including help on setting up your machine, whether you have an A3000 or a StrongARM RiscPC. Users even vaguely familiar with RISC OS would have no problems here.

The Orrery

Once run, the rather neat Orrery icon appears on the icon bar in the normal way. Clicking menu brings up three options, in addition to Info and Quit:

The latter two add two more useful features to the main program.


Ephemeris is a compilation, published at regular intervals, in which the daily positions of the sun, moon, planets and certain stars are tabulated, with other data necessary for the navigator and observational astronomer.

The Ephemerides option in Orrery is unique in that it presents the ephemeris data for the planets and the sun in a graphical form, lasting an entire year (that you can select). This can then be exported as a Draw file for use.

Ephemerides for 2002
Ephemerides for 2002

At first glance, this option could be easily overlooked. In fact, it does indeed provide a useful tool for all astronomers, the graph being useful every day for an entire year, saving time churning through tables of data.

In fact, because of the Draw file format, individual planetary positions can be extracted at will. This could easily become a very useful teaching aid, especially with planets closer to the sun (inferior planets) whose orbits are noticeably sinusoidal in shape.

Solar System Live can also calculate ephemerides for you, but without the unique graphical version by Orrery.


This option will prove less useful to enthusiasts, but will prove useful to teachers. It has always been an easy task to draw the Sun and planets on a blackboard. What is more difficult is getting the relative distances involved across to pupils.

Jupiter and Saturn at correct separation
A snapshot of Jupiter & Saturn

This feature displays the planets running horizontally outwards from the Sun to scale both in size and separation (although the two scales are, necessarily, different!). The great size of the Sun is immediately apparent, a fact more noticeable because at the default scale Mercury lies within it (as a tiny dot). The only planets visible are Mercury, Venus, Earth and Mars. Even on a 1024x768 display, Jupiter can only be seen by scrolling horizontally.

While studying the outer planets, it is useful to increase their size to maximum (8) as the images used are so beautiful.

The Orrery

Orrery MenuThe main Orrery contains a wealth of useful features. At this early stage, it is probably useful to confirm your current position on the Earth! This is done through Misc-Observer. A rather neat map of the Earth appears, with your current latitude and longitude marked. The "Home" position defaults to Portland, where the software is developed.

To change the current position, either click on the map, or enter new values under "Location". Clicking on "Home" resets to Portland. There is no option, however, for changing the default from the program itself. In order to do that, a quick glance at the manual tells you that you must edit the !Run file. Thankfully, the file itself contains detailed notes on which variables to change. This is easy to do, and clearly prevents pupils setting up the software to the "wrong" location. Should they wish to work on a model from a different location on the Earth, thankfully this can be saved using the next option.

The save option will prove very useful. In addition to saving the "model", i.e. time, view, location on earth, step rate, etc., you can also save a Sprite file of the current display, and from the Earth-centred (Geocentric) view, usefully a Draw file can be saved out.

The ability to save a model does, of course, mean you can return to it later. This gives a teacher the ability to set up a particular view of the Solar System, and have all pupils viewing it at the same time. Alternatively, it enables you to demonstrate a particular event (e.g. Pluto's orbit crossing Neptune's). Helpfully, Spacetech provide 9 setup files in the Glossary. These give an idea of the diverse number of astronomical events that can be observed.

Some models I have devised...

I discuss eclipses separately later.

Back to the Orrery. The Viewpoint option switches between the two possible views, Heliocentric (Sun-centred) and Geocentric (Earth-centred).

Sunset with the evening stars
The Sun setting with the Evening stars

Eclipses of the Moon

The Moon can occasionally pass into the shadow of the Earth. When this happens (during the Full Moon phase), the Moon turns a deep coppery red. The reason for this is that light from the Sun travels around the Earth in its atmosphere - hence the redness (dust and atmosphere scatter blue light).

Unfortunately, version 2.02 does not account for Lunar eclipses. Perhaps we could see this implemented in a future version of the software?

Solar Eclipses

I decided to see whether they have accounted for the Total Eclipse of the Sun on August 11th, 1999. Entering the date in the Geocentric view, the New Moon is clearly visible approaching the Sun. Unfortunately, as you play time forward, the eclipse takes place, but the overlap is never complete. In addition, the sky does not go dark! However, moving from Penzance, Cornwall to Bucharest in Romania (where the maximum of the eclipse will occur) the Sun completely disappears. The accuracy of the software is clearly impressive.

Eclipsing Sun
The Moon begins to devour the Sun (8:46 a.m., Wed, 11 Aug 1999)

It would, however, be nice to see the sky go dark during solar eclipses in future revisions of the software.


This software is highly advanced, both in technical aspects and, importantly, in the highly-effective user-interface. As it stands, it is probably the best astronomical software for use in schools - and it can only get better. Any teacher who wishes to rapidly show the difficult concepts of distance in the local universe, together with the complex way that the planets move, could not go wrong by using this.

For the casually interested enthusiast, this software will provide hours of enjoyment, eventually becoming a regular tool in an astronomer's kit. It is highly recommended.

Andrew P. Harmsworth MA(oxon.) PGCE FRAS - 14/11/02 - The Leys School, Cambridge
    Praise for Orrery   "... an engrossing journey through space. Orrery deserves a place in the library of any astronomer."
    Times Educational Supplement
    "... educational software at its best."
    Acorn User
    This software will provide hours of enjoyment... it is highly recommended.
    Andrew P. Harmsworth

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