The Sun looks like it moves across the sky, but it is really the Earth that turns, and Earth spins around once in about a day. Daytime is when sunlight reaches the ground, and the side of the Earth facing away from the Sun is dark. Earth turns eastward, and as a town turns toward the Sun and enters its light, the Sun seems to rise in the east. A spot on the equator moves at about 1,675 kilometers per hour (1,040 miles per hour) as Earth turns, a figure found by dividing the equator's length, about 40,075 kilometers, by one sidereal day, about 23.93 hours. The speed falls with latitude and is zero at the poles.
Which turn counts as one day depends on what you measure against. Earth turns once relative to the Sun in about 24 hours, and one day and one night together take about that long. Earth turns about 366 times a year relative to the stars but only 365 times relative to the Sun, because its trip around the Sun adds one turn to the count, and so a sidereal day is about four minutes shorter than a solar day. A solar day on Earth is around 24 hours, while the sidereal day, one spin measured against the stars, is almost exactly 23 hours 56 minutes.
Earth spins around once in about a day, and because Earth rotates, moving air is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Deflection of moving air caused by Earth's rotation is called the Coriolis effect. You may have heard that sinks and toilets drain in opposite directions on the two sides of the equator. In fact identical toilets drain the same way in both hemispheres, and the shape of the bowl matters most.
Earth is tilted, and the tilt gives us the seasons. Earth's axis is tilted about 23.4 degrees compared with its path around the Sun. Day and night come from Earth's spin, while seasons come from its tilt as it goes around the Sun, which takes 365.25 days. In March and September the Sun is directly over the equator at the equinox, and day and night are about equal. The solstices happen in June and December.
Near the poles, some days do not end. A hemisphere's summer solstice is its longest day, and when it is summer in one hemisphere it is winter in the other. In Svalbard, Norway, the northernmost inhabited region of Europe, the Sun does not set from about 19 April to 23 August. In mainland Norway, Tromsø, above the Arctic Circle, has midnight sun from late May to mid-July. The midnight sun reaches a little farther south than the Arctic Circle itself, because bending of sunlight by the air, called refraction, lifts the Sun into view, so it occurs from about 65 degrees 44 minutes latitude to the pole.
The line between daylight and darkness on Earth is called the terminator. Seen from space, the line between day and night is blurry because the atmosphere scatters light. Twilight is the glow in the sky after sunset and before sunrise, when the Sun is out of sight but the air still scatters its light. At sunset, the sky and clouds can glow orange and red. Daytime is when sunlight reaches the ground, and the side of the Earth facing away from the Sun is dark.
Before standard time, towns set their clocks by the Sun's path across their own meridian. Railroad travel made standard time necessary, and standard time zones began in the United States on Sunday, November 18, 1883, when railroads started using them. Today's world time standard, UTC, is based on atomic clocks that are more stable than the Earth's rotation, and it replaced GMT as the international civil time standard. UT1, the modern form of mean solar time, is based on the rotation of Earth, which is irregular. The speed of Earth's rotation is not constant, so the day does not stay exactly the same length.
Living things carry a clock for the day inside them, and in 1729 Jean-Jacques d'Ortous de Mairan kept a heliotrope plant in constant darkness and saw its leaves still open and close each day. It was the first experiment to tell an inner clock from a response to sunlight. Most living things, including animals, plants and microorganisms, have body clocks called circadian rhythms that follow a roughly 24-hour cycle. In humans the master clock is in the brain, in a spot called the SCN. Light, seen by the eyes, resets the body clock and drives how much melatonin the SCN makes, and melatonin is a hormone that makes people sleepy.
Blue LED lighting suppresses melatonin production about five times more than orange-yellow sodium lights, and light from electronic devices at night can confuse body clocks. Animals that are active in the daytime are called diurnal, and humans, squirrels and songbirds are diurnal. Bright lights at night can confuse wildlife, such as migrating birds, sea turtle hatchlings and amphibians, and even dim light at night can disturb sleep and melatonin. Humans need natural darkness for good sleep and health. Nearly half of Earth's species are nocturnal, and all species have evolved with natural cycles of dark and light.
Words to know
- sidereal: measured by stars
- deflected: turned aside
- Coriolis: spin deflection
- refraction: light bending
- heliotrope: a flowering plant
- nocturnal: active at night
