Why is it that the earliest sunrise and latest sunset do not coincide with the longest day of the year?







            The time of rising and setting of any celestial body is a function of its position in the sky defined by right ascension and declination in the celestial coordinate system, position of the observer on the Earth and the zenith distance of the body adopted to define the phenomena. Due to Earth’s motion in its orbit around the Sun, the geocentric right ascension of the Sun increases at the rate of 1 degree per day with slight variation during the course of the year.

            However, the declination of the Sun varies from about 23.5 degree south in winters to 23.5 degree north in summer during the course of the year. The rate at which the declination varies changes very much from about 0.4 degree on vernal equinox to 0 degree on summer and winter solstices. The longest day occurs in the Northern hemisphere when the declination of the sun is maximum north on summer solstice.

Around the time the daily rate of change in declination goes through zero value. Therefore, around this time the rising and setting times are affected mainly due to change in right ascension which can be about 1/2 degree from sunrise to sunset. It is for this reason that earliest sunrise does not coincide with latest sunset for a given place.

The difference is more pronounced for places in lower latitudes on earth over which the diurnal path of Sun is at a greater inclination to the horizon and its motion in right ascension takes it more to the East, away from the horizon.