The shortest day in the Northern Hemisphere usually is December 21 and that in the Southern Hemisphere is June 21. The length of daylight changes very little during the year at the equator. When the tilt of the earth's axis causes the North Pole to face the sun, the South Pole is continuously dark and the North Pole is always in daylight. As the North Pole is tilted away from the sun, it becomes dark there while the South Pole has constant daylight. These periods of darkness and daylight last about six months.
Astronomers use a day called a sidereal day.
It is based on the period of the earth's rotation as measured by fixed stars. This day equals 23 hours 56 minutes 4.091 seconds of mean solar time. How to cite this article.