Astrolabe - Blog Posts

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Ep. 7 Measuring Mechanisms - HD and the Void

Learn about four ancient (and contemporary, if you work on a ship) devices that utilize very specific areas of astronomy to help you figure out where you are and what time it is, and also predict where celestial bodies will be on other dates and a...

Observing stars is all well and good, but how can I use stars to make my life easier? With a few handy tools and a lot of complicated math and careful table scouting, of course! Okay, it’s not actually any easier to tell where you are, predict when the Sun will rise or where the star Rigel will be at 11:36pm EST, or guess when the next eclipse will be using these tools, but if you don’t have a computer handy maybe it will help.

I did my best to describe all these odd devices in the clearest terms I could but you can hit me up with questions if you have them! Definitely check out some of the video links if you can’t quite picture what I said. I’m also on Twitter at @HDandtheVoid if you’d rather ask me there. And please check out the podcast on iTunes, rate it or review it if you’d like, and subscribe! I’ll always post all the extras here on Tumblr but iTunes is probably more convenient for downloading.

Below the cut are my sources, music credits, vocab list, and the transcript. I mention a play and a story/book and quote an astronomy book in this episode so if you want to see that written down, those sources are there as well. Let me know what you think of this episode, let me know what you think I should research next*, tell me a fun space fact… anything’s helpful!

*(My thoughts were planets, spectroscopy, or Edmond Halley. Let me know by the 6th and I’ll have the next podcast up by July 17th!)

Glossary:

armillary sphere - a device showing the apparent daily motion of the Sun depending on the season, the date, and the latitude of observation. See example video in the link.

Antikythera Mechanism - a device used to establish a calendar based on the Metonic Cycle; eclipse prediction; the location of planets, the Sun, and the Moon on a particular day; and determine the phase of the Moon on a particular day. See example video in the link.

astrolabe - a device for measuring the altitudes of certain celestial objects and for calculating latitude before the development of the sextant. One side is indented, the space called the mater, and can hold a plate depicting the local latitude. Over this plate is a rete, which points out different fixed stars as well as the Sun’s ecliptic, divided into 30 degree sections representing the zodiac signs. On top of the rete was a clock-like hand that stretched the diameter of the astrolabe, called the rule. The rule and rete could be rotated over the face of the plate. See example in the link.

azimuth -  a section of the horizon measured between a fixed point and the vertical circle passing through the center of an object. See example in the link.

declination - the angle of the Sun relative to the equator. The Sun’s angle changes with the seasons.

ecliptic - the path of the Sun over the course of a year.

exeligmos cycle - a cycle that is 3 times the saros cycle, or 669 months. It is more accurate means of predicting eclipses and additionally predicts eclipses that will be visible from a location close to the initial eclipse.

kamal - an Arabic navigation tool consisting of a knotted string and a piece of wood. A navigator would tie a knot in the string and, by holding it in their teeth, sight the North Star along the top of the wooden piece and the horizon along the bottom. To return home, the navigator would sail north or south to bring Polaris to the altitude they had observed in their home port, then turn left or right and sail down the latitude, keeping Polaris at a constant angle. Over time, Arab navigators started tying knots at regular intervals of a fingerwidth, called an issbah, that’s about 1 degree and 36 minutes.

metonic cycle - a 19-year cycle developed by the Babylonians to sync their lunar months with the solar year. In the Metonic cycle, there would be 12 years that lasted 12 lunar months and 7 years that lasted 13 months.

saros cycle - a cycle of 223 months that is used to predict eclipses.

sextant - a device used to determine an observer’s location based on the observation of a known celestial object and a lot of calculation. It is still in use by sailors.

stereographic projection - a process for depicting a spherical, 3-dimensional object on a flat surface. An imaginary line is drawn from one point on the object to a point on the flat surface, following an angle to achieve the same relationship between each point on the object. See example in the link

Script/Transcript

Sources:

Video of how to use an armillary sphere

History of the armillary sphere via University of Cambridge

Video lecture on using an armillary sphere. It sounds like he’s trying to sell it.

Video of how to use an astrolabe

Make your own astrolabe suggestions via In the Sky.org

An old guy kept up a website on astrolabes but he died in April 2016, it’s very sad. Excellent info though.

Explanation of unequal hours

Pullman Car Hiawatha summary, just to prove it’s a real play

Chaucer’s Canterbury Tales with its brief astrolabe mention

Video on how to use a sextant

The many uses of a sextant via Classic Sailing

Why a sextant works via Trailnotes

The history of the sextant

The definition of azimuth

The definition of declination

Video of Antikythera Mechanism’s virtual model based on a theoretical and mechanical model. Just a theoretical model!

Antikythera Mechanism via Smithsonian Magazine

The Antikythera Mechanism Research Project website

Antikythera Mechanism via The New Yorker

Saros cycle via NASA

Saros and Exeligmos cycles

Crouper, Heather and Nigel Henbest. The History of Astronomy. Firefly Books: Buffalo, NY, 2007.

“The circular gear wheels of the Antikythera Mechanism reflect the ancient Greeks’ preoccupation with circles—and with the idea that everything in the sky moves around in circular paths, because the heavens are the home of perfection, and a circle is the ideal shape.” (59)

Intro Music: ‘Better Times Will Come’ by No Luck Club off their album Prosperity

Filler Music: ‘Brooklyn Nights Guitar’ loop from Garageband

Outro Music: ‘Fields of Russia’ by Mutefish off their album On Draught