March 16, 2009
If you stretch out your arm, your fingers are useful instruments to measure angles. An adult arm length is ~60 cm (about 24 in). Held at this distance, every small object of linear size “D” in centimeters will subtend an angle “A” in radians -- A=D/60. Coincidentally, converting this angle from radians to degrees requires its multiplication by 180/π, which results ~60. So, you just have to measure an object in centimeters in order to know the approximate width in degrees of the object in question. In the above image, my forefinger (a little less than 2 cm) was used to determine the width of a primary rainbow (2°). For comparison, both the lunar disk and solar disk measures 0.5 degrees. Photo taken on January 6, 2008, in Curitiba, Brazil.