MEASURING THE OBLIQUITY
THE MERIDIAN INSTRUMENT AT ALEXANDRIA, 31° N
0°
10°
20°
30°
40°
50°
60°
70°
80°
Zenith
Summer
solstice
Winter solstice
Equator
7½° from zenith
54½° from zenith
54½° − 7½° = 47°
23½°
23½°
31° = local
latitude
Meridian instrument
South horizon
The instrument measures each solstice sun's distance from the zenith.
The difference between the two readings, divided by two, is the obliquity.
Their average is the local latitude.