Astrolabe – Amalie Lambert

How to tell the time during the day:

Hold the astrolabe at eye level.
Using the sights on the alidade, align them so the sun passes through both of them.
Note the angle of the alidade at this point (let’s say it’s at 30 degrees, in the morning).
Align the alidade with the calendar date. Note the corresponding (aligned) zodiac date. This
indicates the Sun’s position on the ecliptic (let’s say it’s Pisces 25).
5. Turn the astrolabe over. Align the zodiac date on the ecliptic with the correct angle of the sun.
(Pisces 25 on 30 degrees). We will use the East/left side of the plate because it is the morning
(we would use the West side in the afternoon).
6. Rotate the rule so it aligns with Pisces 25. This gives us the current solar time on the outer scale.
As my astrolabe does not have time markings, the user would find the time by assigning the 12
am value to the 180 degree marking, and counting each subsequent hour at 15 degree intervals,
going clockwise.

How to tell the time at night:

1. Hold the astrolabe at eye level.
2. Using the sights on the alidade, align them so that a chosen star (let’s say Cor Leonis)is seen
through both.
3. Note the angle of the alidade (let’s say 40 degrees).
4. Align the alidade with the calendar date. Note the corresponding zodiac date (once again, let’s
say Pisces 25).
5. Turn the astrolabe over. Align Cor Leonis’ star pointer with the 40 degree almucantar (or
altitude circle). There are once again two options: one can align on the East or the West side
of the plate. The University of Hawaii website recommends doing both the readings (East
and West) with two different stars. From the four results (completing all 5 steps), two will
correspond, thus giving the correct time.
6. Finally, rotate the rule so it aligns with Pisces 25 on the ecliptic. By following the above
recommendations, we shall once again find the solar time.

To find “legal time” (the one on your watch), the modern user would take into account (a) latitude, (b)
the difference between Real (or Apparent) Solar Time (as calculated on the astrolabe) and Mean Solar
Time (the time on the clock), and finally, (c) daylight savings.

Phillipe Dutarte’s website explains this well for modern astrolabes: http://dutarte.perso.neuf.fr/
instruments/how%20to%20use.htm

One thought on “Astrolabe – Amalie Lambert

  1. Aur

    This is a great project and your astrolabe looks fantastic. As for the instructions though – I would never encourage anyone to look through t the sights of the alidade to read sun’s altitude! This could result in damaged or lost eyesight! Also, the way you have the sights positioned on the alidade – you’ll never get an accurate reading. The line between the holes should pass between the tips of the ruler where you take your reading. Yours doesn’t. See, on your picture the ruler reads 16 degrees but if you draw a line between the holes it actually points to 10 degrees. Other than that – good job!

    Reply

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