Definitions of various terms related to data from the U.S. Naval Observatory Astronomical Applications Department.
civil-twilight-begin)Before sunrise, when the center of the Sun is geometrically 6 degrees below the horizon.
civil-twilight-end)After sunset, when the center of the Sun is geometrically 6 degrees below the horizon.
sun-transit)When the center of the Sun crosses the observer's meridian running from north to south.