Lengths of Day and Night
The longest day is longer than the longest night, and the shortest day is longer than the shortest night. This is because sunrise occurs when the upper edge of the disk of the Sun appears on the horizon, and sunset is the moment when the upper edge disappears below the horizon. Consequently, the length of every day exceeds the time that the centre of the Sun is geometrically above the horizon by the interval of time required for the Sun to move the distance from its centre to its upper edge at both rising and setting.
For the same reason, day and night are not of equal length at the time of the March and September equinoxes.
Furthermore, atmospheric refraction causes the sun’s disk to appear higher in the sky than it would if the Earth had no atmosphere. Thus, in the morning, the upper edge of the disk is visible for several minutes before the geometric edge of the disk reaches the horizon. Similarly, in the evening, the upper edge of the disk disappears several minutes after the geometric edge of the disk has passed below the horizon.
For locations north of 66°34'N or south of 66°34'S latitude, the Sun will be above the horizon all day in the summer and below the horizon all day in the winter.