English: This is a diagram illustrating the Roman system of time keeping - division of the day into 12 segments (hours) called hora and the night into 4 segments (watches) called vigilia (vigilia prima, secunda, tertia and quarta).

The surise and sunset times for this diagram are calculated for Forum Romanum 41°53′33″N 12°29′07″E / 41.892426°N 12.485167°E / 41.892426; 12.485167 AD 8 (the Julian calendar). Many sources however are ambiguous because of confusing terms sunrise with dusk and sunset with dawn. This is important, because times of dusk and dawn differ significantly from times of sunrise and sunset. The semidiameter of the sun and atmospheric refraction are taken into account. Equinoxes and solstices are computed too, not set by Romans.

Sunrise and sunset times, dates and times of equinoxes and solstices were calculated using algorithms from:
Jean Meeus, Astronomical algorithms, Second English Edition, Willmann-Bell, Inc., Richmond, Virginia, 1998, With corrections as of August 10, 2009, ISBN 0-943396-61-1.
There is an online calculator also based on Astronomical Algorithms, by Jean Meeus, computing sunrise and sunset times, sun azimuths and elevations:
http://www.esrl.noaa.gov/gmd/grad/solcalc/ NOAA Solar Calculator
(Gregorian calendar)

however calculations for this image are based directly on this Meeus's book.