How were the values in the Equation of Time table first determined?

Question

What is a good source on the subject?

Another way of asking the question is "How did they arrive at civil time in the first place"? Doesn't the calculation of civil time depend on knowing the Equation of Time which was determined by some other means?

Answer 1

During the course of the year, the time as read from a sundial can run ahead of clock time by as much as 16 min 33 s (around October 31–November 1) or fall behind by as much as 14 min 6 s (around February 11–12). This difference, known as the equation of time, results from an apparent irregular movement of the Sun caused by a combination of the obliquity of the Earth's rotation axis and the eccentricity of its orbit. The equation of time is visually illustrated by an analemma.

Naturally, other planets will have an equation of time too. On Mars the difference between sundial time and clock time can be as much as 50 minutes, due to its orbit's considerably greater eccentricity.

See: http://en.wikipedia.org/wiki/Equation_of_time

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980116c.html

http://www.minasi.com/doeot.htm

Answer 2

The short answer is: by comparing apparent solar time (as shown by a sundial) with time as shown on a good clock. So you do need a good clock to do it.