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About the Astrolabe
The Astrolabe was the mathematical jewel of the Middle Ages and was to the end
of the 17th century the most used instrument in the history of Astronomy.
It allows us to understand, in a clear and simply way, the celestial mechanism
and the apparent movement of the stars.
Its origin could be far back as Alexandria, (c9th century). It was brought to
perfection by the Arabs and Persians, who brought it into Spain at the start of
the 10th century.
It comprises a hollowed disc called the mater, in which sits one or more
(this astrolabe has a permanent, fixed tympan).
A tympan is made for a specific latitude and is engraved with a stereographic
projection of circular lines of equal azimuth and altitude representing the
portion of the celestial sphere above the local horizon.
The rim of the mater is graduated into hours of time. Above the mater and tympan
is the rete, a framework bearing a projection of the ecliptic plane (surrounded
by the signs of the Zodiac) and several pointers indicating the positions of the
brightest stars (the curved protrusions), which is free to rotate.
The rete represents the sky and has the function of a star chart. When it is
rotated, the stars and the ecliptic move over the projection of the coordinates
on the tympan. A complete rotation represents the passage of one day.
This astrolabe also has a narrow ruler that rotates over the rete, marked with a
scale of declinations.
On the back of the mater there are a number of scales used to assist in the
astrolabe's various applications, which vary from design to design. This one has
a calendar for converting the day of the months of the year to the sun's
position on the ecliptic, trigonometric scale.
The alidade is attached to the back face. When the astrolabe is held vertically,
the alidade can be rotated and a star sighted along its length, so that the
star's altitude in degrees can be read ("taken") from the graduated edge of the
astrolabe – hence "astro" = star + "labe" = to take.
Using the Astrolabe
- Line up the alidade with the current month and day of month.
- Note the respective Zodiac sign (and position within Zodiac segment).
- With midnight shown at top, align the ruler with the current time (e.g. half
way between one and two for 1:30).
- Move the rete so that the Zodiac sign/position noted from (2) aligns with the
The positions of the stars in the sky results (since this astrolabe has a fixed
tympan, universal, accurate usage is not possible).
Travel Bug Challenge
This particular astrolabe can be used to solve 43 astronomic/mathematic
problems. Your challenge is to find a (new) one of these and log it!
Travel Bugs need maintenance from time to time (as do we all?!). If this one looks in need (or if you would like to know how to make your own
capsule), please take a look at the Faber Optimé Travel Bug Maintenance Manual.
A PDF file of the custom stash note for this travel bug can be obtained here.
About Faber Optimé
We aim to deliver excellence, in all of our
activities - such as bringing to you curious, interesting and
innovative geocaches and geocache items, like this one.
Keep your eyes peeled for more of the Faber Optimé
We're always happy to help fellow geocachers! Please do feel free to get in touch with us on
the following contact details.