The Astronomical Observatories of Jai Singh c6.s1.t1.85

c6.s1.t1.85

15. The Alhidade (*‘iḍāḍah*) or Sighter.—In all old oriental astrolabes the alhidade is of the type attached to Jaipur A and shown in figure 7. It is fixed on the centre pin so that its graduated edge lies on a diameter of the circle. Half of the bevelled edge (the right upper edge in figure 7) is divided into 60 equal divisions, every third division being numbered. The left upper edge is divided into six equal divisions, corresponding to the divisions for the signs of the zodiac in the south-west quadrant, and each division is marked with its two proper signs and divided into 15 parts. The right lower edge is divided into six divisions numbered 1 and 12, 2 and 11, etc. Near to each end is fixed a sighting tablet, each having two holes, the upper pair generally being the larger. On modern Hindu instruments is another type of revolving index. It has no sighters and is the length of the radius only. In mediaeval European instruments we often find such a marker or ‘label,’ as it was called,² used on the front of the astrolabe. (Figure 24.)