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— Dorsum = Astrolabe: Usage —

Determination of the Declination of the Sun

Hold dorsum at the armilla and adjust alhidate at the backside until a ray of sunlight shines through both diopters. To see the tiny sun disk, hold a hand behind the nearer hole of the diopter. The ruler at the far end of the alhidate points to the declination angle.

Measurement of the Declination of a Star

Hold dorsum at armilla and find one of the 14 stars through the diopter. Read angle as for the suns declination.

Approximation of the height of a building with the help of the Shadow Square

Hold dorsum at armilla and look at the highest point of the building by peeking through the diopter. On the shadow square at the nearer side of the alhidate, read the value on the scale. Each division is a twenty-fourth. Count the steps when walking to the building. Divide the number of steps through 24 and multiply with the value read off the scale. This will give the height of the building in steps.

The shadow square corresponds to the cotangent of the angle in 24ths. 30° correspond to cotan 14/24 = 0.583, arc cotan 0.583 = 30.25°; cotan 30° = 0.577, 0.577 x 24 = 13.85/24 = 14/24 (7/12).

Determination of the Astrological Date

Up to 24 October 1582, the Julian Calendar was in use and it was replaced by the Gregorian Calendar at this date, continuing directly with 15 October. This astrolab was constructed around the year 1500 and thus based on the Julian Calendar.

Point with the alhidate on the innermost ring scale at the backside to the desired date, then turn back 11 days. On the middle scale, read the corresponding astrological date.

5 October - 11 days = 24 September = Libra (balance) 11°. Please note: constellations start at 0° while months start with the 1st day.

Determination of the Time of the True Sun

Find astrological date and move ostensor to the corresponding date on rete. Determine declination of sun. Move ostensor and rete together until rete and ostensor meet the almucantar of the suns declination. At left in the morning, at right in the afternoon. The end of the ostensor points to the limbus (degrees circle at the edge of the mater).

270° corresponds to 6 a.m., 0° to noon (12 o'clock), 90° evening (6 p.m. or 18 o'clock) and 180° to midnight at 0 o'clock. One degree equals 4 time minutes. Value read x 0.0667. Between midnight up to near noon: result - 12 h = time. Between noon and up to midnight, result + 12 h = time.

The following «errors» must be accounted for: time equation for the date, longitude (degrees / 15: + for east of Greenwich, - for west of Greenwich). The result corresponds to the mean time for the location. Consider daylight saving time if appropriate.

Measurement of the Time at Night

Proceed as if it were day. Instead of the sun, use one of the 14 stars to measure the declination. Rotate rete until star cuts the measured almucantar (left after midnight, right before midnight). Move ostensor over the date on the rete (do not move rete). On the limbus, read the degrees and calculate time.

Time Equation

The Time Equation shows the difference between the «True Sun» and the «Medium Sun». Minutes (Min) rounded and shown for each 15 days.

   Date Min    Date Min    Date Min    Date Min   
   01. Jan - 03    01. Apr - 04    01. Jul - 03    01. Oct + 10   
   15. Jan - 09    15. Apr +- 0    15. Jul - 06    15. Oct + 14   
   01. Feb - 14    01. May + 02    01. Aug - 06    01. Nov + 16   
   15. Feb - 15    15. May + 03    15. Aug - 05    15. Nov + 15   
   01. Mar - 13    01. Jun + 03    01. Sep +- 0    01. Dec + 11   
   15. Mar - 09    15. Jun +- 0    15. Sep + 04    15. Dec + 05   


The star names on the astrolabe correspond to those in use in the 16th century. Rectascension (RA) and declination (δ) of the stars were approximately as indicated in the blue columns.

The star names in use today and their positions (aequinoctium 2000) are shown in the red columns. For the apparent brightness of the variables (α Her, α Lyr, α Aql und β Peg), the averages are noted.

1500 A.D m Today
Stella RA δ Constel. Name alt. Name RA δ
Mirac 00h44 +35° +2m0 α And Alpheraz Sirrah 00h08 +29°
Aldebaran 04h08 +15° +0m9 α Tau Aldebaran   04h36 +17°
Alhaiot 04h40 +45° +0m1 α Aur Capella   05h17 +46°
Rigil 04h52 -09° +0m1 β Ori Rigel   05h15 -08°
Alhabor 06h24 -16° -1m5 α CMa Sirius   06h45 -17°
Algomeisa 07h12 +06° +0m4 α CMi Prokyon   07h39 +05°
Cor Leonis 09h40 +14° +1m4 α Leo Regulus   10h08 +12°
Azimech 13h04 -09° +1m0 α Vir Spica   13h25 -11°
Alramech 13h56 +22° +0m0 α Boo Arcturus   11h16 +19°
Alfeta 15h16 +29° +2m3 α CrB Alphekka Gemma 15h35 +27°
Razdalgeti 16h52 +17° +3m3 α Her Rasalgethi   17h15 +14°
Vega 18h20 +38° +0m0 α Lyr Vega Wega 18h37 +39°
Alkair 19h20 +08° +0m8 α Aql Ataïr Altair 19h51 +09°
Sceat 22h28 +24° +2m5 β Peg Scheat   23h04 +28°
  © 2004 - 2017 by Horo Wernli.