Day and night. While Earth travels through space around the sun, it also spins on its own axis. A solar day is the length of time that it takes Earth to turn around once with respect to the sun. We usually say day for the time when the sun is shining on our part of Earth, and night for the time when our part of Earth is dark, or turned away from the sun. But the night is really a part of the whole day. We also say business day sometimes to mean the usual hours of business in any one day.
Each day begins at midnight. In most countries, the day is divided into two parts of 12 hours each. The hours from midnight to noon are the a.m. (before noon) hours. The hours from noon to midnight are the p.m. (afternoon) hours. The military services often designate the time of day on a 24-hour basis, such as 0000 or 2400 for midnight, 0100 for one o’clock in the morning, 1200 for noon, and 1800 for six o’clock in the evening.
The Babylonians marked sunrise as the beginning of the day. The ancient Jews used sunset. The Egyptians and the Romans marked the day’s start at midnight.
The length of daylight changes during the year in all parts of the world. It does so because the tilt of Earth’s axis causes first one pole to slant toward the sun and then the other as the planet orbits the sun. The longest day in the Northern Hemisphere usually is June 21 and that in the Southern Hemisphere is December 21. Each of these days has 13 hours and 11 minutes of daylight at 20° latitude. The same days have 15 hours and 2 minutes of daylight at 40° latitude, and 18 hours 53 minutes at 60°. The shortest day in the Northern Hemisphere usually is December 21 and that in the Southern Hemisphere is June 21. Each has only 10 hours and 56 minutes of daylight at 20° latitude, 9 hours 20 minutes at 40°, and 5 hours and 52 minutes at 60°. 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 about 23 hours 56 minutes 4 seconds of mean solar time.
See also articles on the days of the week; Daylight saving time; Sidereal time; Time; Twilight.
