Astrolabe
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"What Time Is It?"
- The most repeated question in the Continuum RPG

Basic Information

Most fantasy games are set in an era before digital watches, even before cuckoo clocks (which date to the early 1600s), so how do your characters know what time it is? Hourglasses and water clocks are impractical for travel. sundials are useless after the sun has set, so how do you know when the second shift is supposed to take over for the first watch? Most campaigns just sort of hand wave it, but you don't have to. Luckily, there's a 2,000-year-old invention your characters can carry in their haversacks that not only tells the time, it will also teach them a few things about astronomy. This tool is called the astrolabe.

Description

The astrolabe is a small circular device, usually made of wood or brass (but modern versions can be made of heavy paper), ranging from about four or five inches to more than a foot in diameter. It has moving parts you slide around to take readings of various sorts. There are 3 or 4 main parts to an astrolabe.

  • The mater is the backplate, the circular frame that holds the rest of it together. The outer rim of the mater is marked with hours, degrees of arc, or both. The back of the mater usually has engraved scales or charts to allow for the various secondary applications of that particular astrolabe, these might vary significantly, depending upon the purposes intended by that specific astrolabe's creator.
  • The next layer up from that is called a tympan or climate. Tympans are engraved plates with circular grids, made for a specific latitude. Some astrolabes would have multiple climate plates, which could be swapped out for different latitudes. If you don't have a tympan for roughly your current latitude, many of your numbers and measurements will be inaccurate.
  • Over that goes the rete, a narrow plate of mostly cut-out empty space, like an ornate and delicate lattice. Most of the artistry in creating an astrolabe happens on the rete, it's where the designer has the most freedom for embellishment and variations of form. The point of the rete is that it marks out the most visible stars in the sky, so it usually consists of circular framework with various narrow arms pointing inward. You rotate the rete over the tympan to find the positions of various celestial bodies.
  • The last layer is optional - some won't have anything above the rete, others will have a flat bar called the rule or label. This bar is used as a sighting line, you look along it to spot an object (such as a star, or the top of a building), then compare the rule's position on the frame of the mater to know what angle that object is above the earth.

Uses

Using an astrolabe takes a few moments, it's not as fast as glancing at your wristwatch. However, since it can do so much more than just tell the time, it's probably a fair trade off. Texts from the middle ages listed 1,000 uses for an astrolabe. Here's a few highlights of that list:

  • Tell the time
  • Tell the date
  • Surveying of property and land
  • Tell an object's altitude in the sky
  • Tell the height of a building
  • Determine your current latitude and longitude
  • Navigate over land
  • Calculate distances
  • Determine what direction you're facing
  • Identify individual stars in the sky
  • Predict eclipses
  • Predict the movement of the planets
  • Calculate sines and other mathematical formulas
  • Determine the Qibla (what direction Mecca is in), and when to pray to Allah (provided you have a muslim-built astrolabe)
  • Draw up a horoscope
  • Denote times and dates of occult significance

Exactly which of those tasks a given astrolabe can perform depends largely on the intent and interests of the artisan who created it. All astrolabes can tell the time at latitudes for which they have a tympan (see above), or determine the latitude if the current time is known. Many of their functions can only be done at night or twilight. The more stars and planets you can see, the more calculations your astrolabe can be used for. Most of the surveying functions, however, can be done by day.

Related Instruments

An astrolabe did for the ancients many of the same functions that a calculator, watch or computer might do for us. Here's a list of other related instruments and inventions that can be used for similar tasks:

History

It is said the first astrolabe was invented by Hipparchus circa 200 BC, and were used in ancient greece. Books specifically about Astrolabes date back as far as the 6th Century. Books from England and India on the astrolabe date to the 14th Century.

Astrolabes became common during the Islamic Golden Age, being useful for traveling muslims to keep up with their prayer requirements. In the 10th Century an islamic author penned a treatise on 1,000 uses for an astrolabe.

The first metal astrolabes made in Europe date to the 15th Century. Metal astrolabes are more accurate and less fragile than their wooden counterparts. They don't wear down as much or break as easily, and can be made consistently with very precise measurements.

Today, the best collections of astrolabes can be found at the Oxford Museum of the History of Science, Oxford, UK, and at the Adler Planetarium and Astronomy Museum in Chicago, IL.

Sources

Game and Story Use

  • If, like me, you've been wincing every time the PCs in your D&D game talk of splitting the nights up into precise shifts of guard duty, knowing about the astrolabe will reaffirm your willing suspension of disbelief. They actually can tell 2 am from 3 am, afterall!
  • Astrolabes are a great way to insert a little bit of flavor. They're a perfect trapping for a caravan leader, alchemist, astronomer, architect, occultist, explorer or sage to carry around. The night watchman might carry one as a tool, or a badge of honor.
  • In games with a lot of modifiers for having the right tool for the job, an astrolabe could add bonuses to rolls of occult, astronomy, architecture, math, orienteering, navigation, or other knowledge skills.
  • A magic astrolabe might push the occult angle further, having unusual powers, or giving bonuses to spellcasting. All those star charts, sliding rules, and conversion equations might reveal the laws of magic.
  • Some astrolabes are very intricate and decorated, and could be classified as treasure in and of themselves.
    • Especially those that are hundreds of years old, which are priceless to the proper collector or museum. As the guy says in the TED video (see bibliography), some of the older astrolabes are worth more than a whole block of houses today.
  • An astrolabe can be used as a bit of characterization, the (antique?) tool that a particularly geeky (in the modern world) or well-traveled (in the middle ages) character is constantly tinkering with.
  • Once you've introduced an astrolabe into your game, you can later on use it to drop a clue about a cosmic horror or sky-related plotline. You engineer a situation where the PCs are lost, disoriented, or concerned about the time remaining until dawn. Naturally, the players say "we'll use that astrolabe thing that the GM was geeking out about last week". When they line it up with the heavens to get their bearings, they can't help but notice that several of the stars aren't where they're supposed to be! (Cue dramatic music)
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