The number of square degrees in the whole sky is 129,600/π (that is, approximately 41,253 sq.deg.) which can be derived from that the fact that the whole sky covers 4π steradians, and one degree is 180/π radians. Thus, one square degree is approximately 1/3283 steradian or 305 microsteradians. The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ... The radian (symbol: rad) is the SI unit of plane angle. ...
Multiply the number of stars per squaredegree by the estimated angular area (in squaredegrees) per star to find the fraction of a typical squaredegree that is covered with star disks.
The 1/2 centimeter squares marked in red on the centimeter side of your transparent rulers correspond to 1/120 of a squaredegree on the scale of the Palomar prints, so you need to multiply your counts by 120 to get the estimated number of stars per squaredegree.
The smaller squares 1/8 inch on a side (found on the inch side of the ruler) correspond to 1/300 squaredegree, so multiply counts made in them by 300.
A degree (in full, a degree of arc, arc degree, or arcdegree), usually symbolized °, is a measurement of plane angle, representing 1／360 of a full rotation.
For the number of degrees in a circle to be divisible by every number from 1 to 10, there would need to be 2520 degrees in a circle, which is a much less convenient number.
When this is not the case, as in astronomy or for latitudes and longitudes on the Earth, degree measurements may be written with decimal places, but the traditional sexagesimal unit subdivision is commonly seen.
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