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  • Writer's pictureSeb Falk

What’s the difference between an astrolabe and an equatorium?

Updated: May 13, 2020

This is a question I’ve been asked several times, and it’s a reasonable one given that this blog is called “Astrolabes and Stuff”, yet I’ve mainly been posting about equatoria.  So I’m going to answer it in this post.

First, it’s important to stress the one thing astrolabes and equatoria have in common.  They are both tools, calculating devices.  As such, they simplify (and approximate) tasks that could have been done by geometrical calculation – but the astrolabe and equatorium get them done much faster.

So, what are they calculating?  Here’s the key difference.  An astrolabe (from the Greek astro-, meaning “star“, -labos, meaning “instrument”, “measurer”, “meter”) tracks the stars.  On the other hand the word “equatorium” comes from the Latin æquatio, which generally means “correction” rather than “equation”, and refers to the correction that needs to be made to convert a planet’s mean position to its actual position.  So this is an instrument squarely focused on the planets.

What about the Sun and Moon?  Are they stars or planets?  The answer is of course neither – but also both, in some ways.  This is where we come to the more fundamental difference between astrolabes and equatoria: the timescales on which they operate.

TECHNICAL BIT (feel free to skip this paragraph).  Ancient astronomers noted two kinds of regularity in the planets’ motions: tropical and synodic periods.  The tropical period of a planet is the average amount of time it takes to go all the way around the ecliptic (i.e. to travel round all the stars).  The synodic period is the amount of time between periods of retrograde motion.  (For a quick explanation of the significance of retrograde motion, see this post of mine.)

Of all the planets known since antiquity, Saturn takes the longest to go all the way around the ecliptic – to pass all the constellations and end up back where it started.  The 4th-century-BCE astronomer Eudoxus estimated that this tropical period took 30 years; we now say 29.42 years, so he wasn’t too far off.  An equatorium has no problem showing this.  Of course there are many ways in which you could say that we require even longer periods of time to be shown – for example, the Greeks adopted the Babylonian idea of great cycles – the amount of time it took before a planet was behaving in the same way at the same place on the ecliptic.  Jupiter’s great cycle is 83 years.

Whereas an equatorium tracks planets over years, most of the functions of an astrolabe are best appreciated within a single day, and the information provided by a normal astrolabe repeats after one year.  Here are some of the things it can tell us: 1.  Where a star, or the Sun, will rise on the horizon 2.  How long a star, or the Sun, will stay above the horizon (i.e. the time between rising and setting) 3.  The longitude of the Sun (i.e. its position on the ecliptic) on a given day

Detail of the same astrolabe as above

4.  The height of the Sun (or any star) at noon 5.  How long it will take after sunset to get dark (or to get light before sunrise) 6.  The time of day, for a given date 7.  The date, if you observe the Sun at noon, sunrise or sunset 8.  The height of a building, if we know how far away from it we are.

There may be many other functions too.  For example, an astrolabe may operate in unequal hours (where there are always 12 hours between sunrise and sunset) as well as equal hours; it may show us the astrological Great Houses, and the calendar on the back may feature a range of saints’ days as well as the months and zodiac signs.

Not as waterproof as an astrolabe

This dazzling array of functions means that astrolabes are sometimes compared to Swiss Army knives.  But these days I’m more struck by the comparison to a smartphone.  They are both up-to-date technological items, but more importantly they are both amalgamations of quite a lot of previously existing technology in one easily portable and user-friendly unit.  I suspect they also have in common the fact that despite their enormous range of functions, it tends to be the more basic functions that get/got used most often.

A 20th-century astrologer?

Which brings us to another crucial point: smartphones and astrolabes are both must-have status symbols, valued as much for their visually (and tangibly) attractive design as for their practical functions.  I suppose in a sense smartphones are now too common for the analogy to work perfectly – astrolabes were perhaps more like the earliest mobile phones for the 1980s yuppies – a sign of success, with a faint but perhaps false suggestion of technological know-how.

An equatorium, on the other hand, is a decidedly specialist item.  Its lack of functions – as we’ve seen in previous posts, it can tell us the longitudes of the Sun, Moon and planets, the latitude of the Moon, but not much else – raises the question of who would want one, and why.

These are very important and hotly debated questions.  I’ll return to them in future posts, but for now it’s probably fair to say that whereas astrolabes had wide-ranging appeal, equatoria were mainly for people with narrow astrological interests.  Not only did they not tell you as much, but they were harder to use and the theory underpinning them was more complex.

Less attractive equatorium

As a result, they never developed into status symbols in the same way that astrolabes did.  Equatoria could be made shiny and attractive – check out this one at the Museum of the History of Science (MHS) in Oxford – but that was much rarer.  (It’s possible that such shiny equatoria were as much demonstrations of the maker’s knowledge and skill as items that customers actually wanted.)

Such shininess may account for one key difference between astrolabes and equatoria today: there are way more astrolabes!  There are 182 in the MHS collection, but only a handful of equatoria survive in the whole world.  It’s a handy reminder that the survival of objects, and their display in museums today, depends on more than just their significance (however you’d define that).

I’ll come back to some of these issues in future posts.  If you are burning to find out more about astrolabes right now, I recommend this excellent site.

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