The Astrolabe: How to Make One and Understanding Its Use
What is an Astrolabe?
The mariner's astrolabe was developed over two thousand years ago. It was an instrument of navigation used to measure celestial altitude. Celestial altitude is the relative 'height' of a star, planet or other celestial object above the horizon. Why would "celestial altitude" be important to ancient mariners? Ancient navigators could not measure longitude. However, it was quite easy to determine latitude. Geographic latitude, or distance from the equator, therefore, was very important to sailors and determining celestial altitude was the means by which geographic latitude could be estimated.
Ancient mariners used the following method to determine their latitude at sea:
- The noon altitude of the sun was measured during the day or,
- the altitude of a star of known declination was measured when it was on the meridian (due north or south) at night.
- Using an almanac, the Sun's or star's declination for the date was determined.
- The following formula was then used: Latitude = 90° - measured altitude + declination.
Φ Declination is like latitude. It tells a navigator how far a star is from the celestial equator.
History of the Mariner's Astrolabe
Classical Greece was the birthplace of the astrolabe. A number of developments in math and astronomy provided the tools necessary for its development.
- 225 B.C.: Apollonius, a mathemetician known for developments in geometry, probably studied the astrolabe projection.
- 160 B.C.: Hipparchus working in Rhodes, determined the precession of equinoxes, made developments in trigonometry and fine tuned projection as a way of solving astronomical problems.
- 88 B.C.: Marcus Vitruvius Pollio describes a device using stereographic projection in a machine, an anaphoric clock
- 150 A.D.: Claudius Ptolelmy hints in his writings of an instrument he possessed which may have operated as an astrolabe.
It is unknown when the application of stereographic projection ( a way of picturing a sphere on a plane) was modified into the astrolabe.
- 390 A.D.: It is said that Synesiius of Cyrene had an instrument constructed that was a type of astrolabe.
- 6th Century A.D.: John Philoponos of Alexandria provides the earliest descriptions of actual astrolabes.
- 7th Century A.D.: It has been definitely documented that astrolabes existed by this century.
- Mid-8th Century: The Islamic world was introduced to the astrolabe where the astrolabe was fully developed. Because the astrolabe could be used to determine astronomically determined prayer times and it could also be used to find direction to Mecca, the astrolabe was extremely valuable to Islamic society. In addition, astrology was a rich part of ancient Islamic culture and the early use of the astrolabe was principally in astrology.
- 11th Century: Astrolabe technology spread from Islam, through North Africa into Spain whereupon Europe cultures gained access to the technology through Christian monasteries in northern Spain.
- 13th and 14th Centuries: European usage of the astrolabe became widespread.
- 15th and 16th Centuries: The astrolabe became one of the basic tools of astronomical education.
- Mid-17th Century: The use of the astrolabe declined due to the development of more specialized and accurate scientific devices including the telescope.
- 19th Century: Astrolabe production, especially in the early part of this century continued mainly in the Arab world.
The astrolabe was instrumental in the development and history of astronomy.
- Astronomers used it to measure the positions of stars and planets.
- They kept track of eclipses.
- Ancient astronomers using astrolabes developed terminology, measurements and techniques which became the foundation of later astronomical knowledge.
How to Make an Astrolabe
- plastic protractor
- large plastic straw
- 12 inch piece of string
- a small bolt or washer (or other metal weight that can be tied to a string)
- clear tape
How to Make the Astrolabe:
- Tie one end of the string to the hole in the middle flat-edged side of the protractor. If there is not a hole carefully drill one.
- Attach the metal weight to the other end of the string.
- Tape the straw to the flat edge of the protractor.
- Looking North, find the constellation the Big Dipper. It looks like a large spoon or wheelbarrow and is the easiest constellation to find.
- The Big Dipper is composed of seven stars. Locate the two that form the outer edge of the 'spoon' Connect these front stars of the Big Dipper and continue this line off to the upper right. The first bright star you come to is Polaris, the North Star.
- If you are still having trouble locating Polaris see the following link for clarification: How To Find Polaris the North Star.
How to Determine Your Latitude Using the Astrolabe
- Locate the star Polaris at night.
- Sight the star through the straw.
- Note what degree the string lines up at on the protractor using the set of numbers from 0-90 degrees. This number is the zenith angle.
- To find the altitude angle: 90° - zenith angle. This number will be the equal to or very close to your sighting location.
Determining Latitude Using an AstrolabeClick thumbnail to view full-size
Diagram Illustrating Height Determination Using Trigonometry
How to Determine the Height of An Object using your Astrolabe with and without Trigonometry
- Walk away from your object to be measured until your view through the sight vane shows a 45° measurement on the astrolabe.
- Measure the height of the astrolabe above the ground.
- Measure the distance to the base of the object.
- Height of the object = height of astrolabe above the ground + distance to the base of object.
With Trigonometry: (used it you can't get far enough away from the object to line up the sight vane)
- A “right triangle” has two sides that meet at a 90° angle.
- The side of the triangle opposite the 90° angle is the hypotenuse.
- The tangent of one of the other angles is defined as the length of the side opposite the angle divided by the side closest to the angle (not the hypotenuse).
Using the diagram to the right, I will illustrate determining height of an object using your astrolabe and the principles of trigonometry:
- The height of the tree is side T plus 5 feet. The 5 feet measurement is the measurement of the height of your eyeball above the ground.
- The tangent of angle determined by using your astrolabe to sight the top of the tree, in this case 38°, is equal to side T divided by 20 feet (the adjacent angle).
- Then, Tan 38° = T/20 feet
- Using a scientific calculator, Tan 38° is found to be 0.78. So,
- 0.78 = T/20 feet; therefore,
- T = 0.78 x 20 feet; therefore, T = 15.6 feet
- The height of the tree equals T plus the height of your eyeball above the ground.
- Therefore, tree height = 15.6 feet + 5 feet. The tree is thus, 20.6 feet.