|14th-century astrolabe at the Whipple Museum of the History of Science|
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 (Wh.1264), |
showing the calendar for January/Aquarius,
with various saints' days in the inner circles.
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
|Smaller than an astrolabe,|
but less likely to work after
being dropped in the bath
|A 20th-century astrologer?|
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|
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.