The Astrolabe Using the Stars for Navigation and Timekeeping

Want to know where you are on Earth? Check out Google Maps or Google Earth. Want to know what time it is? Your watch or iPhone can tell you that in a flash. Want to know what stars are up in the sky? Digital planetarium apps and software give you that information as soon as you tap them on. We live in  a remarkable age when you have such information at your fingertips. For most of history, this wasnt the case. While today we might use  star charts to locate objects in the sky, back in the days before electricity, GPS systems, and telescopes, people had to figure out that same information using only what they had handy: the daytime and nighttime sky, the Sun, Moon, planets, stars, and constellations. The Sun rose in the East, set in the West, so that gave them their directions. The North Star in the night-time sky gave them the idea of where North was. However, it wasnt long before they invented instruments to help them determine their positions more accurately. Mind you, this was in the centuries before the invention of the telescope (which happened in the 1600s and is credited variously to Galileo Galilei or Hans Lippershey). People had to rely on naked-eye observations before that. Introducing the Astrolabe One of those instruments was the astrolabe. Its name literally means star taker. It was in use well into the Middle Ages and Renaissance and is still in limited use today. Most people think of astrolabes as being used by navigators and scientists of old. The technical term for astrolabe is inclinometer—which describes perfectly what it does: it allows the user to measure the inclined position of something in the sky (the Sun, Moon, planets, or stars) and use the information to determine your latitude, the time at your location, and other data. An astrolabe usually has a map of the sky etched onto metal (or can be drawn onto wood or cardboard). A couple of thousand years ago, these instruments put the high in high tech and were the hot new thing for navigation and timekeeping. Even though astrolabes are extremely ancient technology, theyre still in use today and people still learn to make them as part of learning astronomy. Some science teachers have their students create an astrolabe in class. Hikers sometimes use them when theyre going to be out of reach of GPS or cellular service. You can learn to make one yourself by following this handy guide on the NOAA website. Because astrolabes measure things that move in the sky, they  have both fixed and moving parts. The fixed pieces have time scales etched (or drawn) on them, and the rotation pieces simulate the daily motion we see in the sky. The user lines up one of the moving parts with a celestial object to learn more about its height in the sky (azimuth). If this instrument seems very much like a clock, thats not a coincidence. Our system of timekeeping is based on sky motions—recall that one apparent trip of the Sun through the sky is considered a day. So, the first mechanical astronomical clocks were based on astrolabes. Other instruments that you may have seen, including planetariums, armillary spheres, sextants, and planispheres, are based on the same ideas and design as the astrolabe. Whats in an Astrolabe? The astrolabe may look complex, but its based on a simple design. The main part is a disk called the mater (Latin for mother). It can contain one or more flat plates that are called tympans (some scholars call them climates). The mater holds the tympans in place, and the main tympan contains information about a specific latitude on the planet. The mater has the hours and minutes, or degrees of arc engraved (or drawn) on its edge. It also has other information drawn or engraved on its back. The mater and tympans rotate. Theres also a rete, which contains a chart of the brightest stars in the sky. These main parts are what make an astrolabe. There are very plain ones, while others can be quite ornate and have levers and chains attached to them, as well as decorative carvings and metalwork. Using an Astrolabe Astrolabes are somewhat esoteric in that they give you information that you then use to calculate other information. For example, you could use it to figure out the rising and setting times for the Moon, or a given planet. If you were a sailor back in the day you would use a mariners astrolabe to determine the latitude of your ship while at sea. What you would do is measure the altitude of the Sun at noon, or of a given star at night. The degrees the Sun or star lay above the horizon would give you an idea of how far north or south you were as you sailed around the world. Who Created the Astrolabe? The earliest astrolabe is thought to have been created by Apollonius of Perga. He was a geometer and astronomer and his work influenced later astronomers and mathematicians. He used principles of geometry to measure and try to explain the apparent motions of objects in the sky. The astrolabe was one of several inventions he made to aid in his work. The Greek astronomer Hipparchus is often credited with inventing the astrolabe, as is the Egyptian astronomer Hypatia of Alexandria. Islamic astronomers, as well as those in India and Asia also worked on perfecting the mechanisms of the astrolabe, and it remained in use for both scientific and religious reasons for many centuries. There are collections of astrolabes in various museums around the world, including the Adler Planetarium in Chicago, the Deutsches Museum in Munich, The Museum of the History of Science at Oxford in England, Yale University, the Louvre in Paris, and others.