Day and night

Earth is always spinning.

When your home faces the Sun, it is daytime. When your home turns away from the Sun, it is nighttime.

Why does the sky go dark every evening? NASA explains it simply: day and night happen because Earth spins. When places on Earth are facing toward the Sun, it is daytime. When they are facing away from the Sun, it is nighttime.

The Sun does not switch off at night. It keeps shining all the time. It is your part of Earth that has turned away from it.

Day and night are a direct result of Earth's rotation. NASA's description for students is precise: at all times, half of Earth is lit by the Sun and half is in darkness. Areas facing the Sun experience daytime; areas facing away experience nighttime.

The Sun is effectively a constant source; what changes is the orientation of your location. As Earth spins on its axis, the imaginary line through the planet from the North Pole to the South Pole, each place is carried from the lit hemisphere into the dark one and back. The boundary between them sweeps steadily around the globe, bringing sunrise to one region as it brings sunset to another.

The Sun lights up half of the Earth.

The other half is dark. So right now, it is daytime for some children and nighttime for others.

At any moment, the Sun lights up half of Earth. On that half it is day. On the other half it is night. So while you eat breakfast, children on the opposite side of the world may be getting ready for bed.

That is also why different parts of the world use different clock times. NOAA explains that Earth turns 360 degrees in 24 hours, which is 15 degrees every hour, so noon arrives at different moments in different places.

Because the Sun always lights one hemisphere, Earth always has both day and night somewhere. NASA notes that as the planet spins, most places cycle through day and night once every 24 hours.

The rate is steady. NOAA's National Ocean Service points out that Earth rotates 360 degrees in 24 hours, or 15 degrees per hour, so local noon, when the Sun is highest, arrives an hour later for every 15 degrees you go west. That simple arithmetic is the basis of time zones, though real time-zone boundaries, NOAA adds, depend on local laws.

In the morning, the Sun comes up in the east.

In the evening, it goes down in the west. It looks like the Sun is moving. Really, the Earth is turning!

Every day the Sun seems to rise in the east, climb across the sky and set in the west. But the Sun is not traveling around us. NASA's StarChild explains that Earth spins toward the east. As it turns, it carries you around, so the Sun comes into view over the eastern horizon in the morning and disappears below the western horizon in the evening.

The Moon and stars rise in the east and set in the west for the same reason.

The Sun's daily path across the sky is an apparent motion. NASA's StarChild explains that Earth rotates toward the east; an observer facing east is carried eastward, so whatever lies beyond the eastern horizon eventually comes into view. That is why the Sun, the Moon, the planets and the stars all rise in the east and move westward across the sky.

We do not feel the motion, although it is fast. NASA notes that at the Equator, Earth spins at just over 1,000 miles per hour, and StarChild adds that the speed decreases toward the poles. A person standing exactly at a pole is simply turned in a circle once a day.

One whole spin takes about 24 hours.

That is one day and one night together. Then the spin starts again.

One day and one night together make one full day. NASA's Space Place explains that on Earth a solar day, one cycle of daytime and nighttime, is around 24 hours.

Other planets spin at different speeds. Space Place lists a day on Mars as about 25 hours and a day on Jupiter as only about 10 hours. A day on Venus lasts 5,832 hours!

There are two ways to measure a day. A solar day, one cycle of daytime and nighttime, is around 24 hours on Earth, though NASA's Space Place notes that Earth's elliptical orbit makes some solar days a few minutes longer or shorter. A sidereal day, one full rotation relative to the stars, is almost exactly 23 hours 56 minutes; NASA's Earth fact page gives the rotation as 23.9 hours.

So "one day" depends on what you measure against: the Sun or the distant stars. Other planets show how variable "a day" can be: Space Place lists Mars at 25 hours, Jupiter at 10, Saturn at 11, Uranus at 17, Neptune at 16, Mercury at 1,408 and Venus at 5,832 hours.

Near the North Pole and South Pole, days can be very long.

In summer the Sun stays up all day and all night. In winter it stays down. Near the middle of the Earth, days and nights are about the same.

Days and nights are not the same length everywhere. Earth's axis is tilted, NASA explains, by 23.4 degrees. Because of the tilt, the North and South Poles have continuous daylight or continuous darkness, depending on the time of year.

Near the Equator it is very different. The U.S. National Weather Service says that on or close to the Equator, daylight lasts about 12 hours all year round, give or take a few minutes.

Rotation sets the rhythm of day and night; the 23.4-degree tilt of Earth's axis (NASA) sets how that rhythm is divided. As each hemisphere leans toward or away from the Sun over the year, days lengthen and shorten. At the extremes, NASA notes that the poles have continuous daylight or darkness depending on the time of year; National Geographic adds that the North Pole sees only one sunrise and one sunset a year.

At the other extreme, the National Weather Service notes that on or very close to the Equator daylight stays within a few minutes of 12 hours year-round. Living with months of darkness is described in the culture topics on the South Pole and on polar night in Tromsø.

You can make day and night!

In a dark room, shine a flashlight on a ball. Put a sticker on the ball for your home. Turn the ball slowly. Day, night, day!

You can see how day and night work with a flashlight and a globe or ball. Britannica Kids describes shining a flashlight on one side of a globe in a dark room to show how sunlight reaches half of Earth and leaves the rest in shadow.

Put a small sticker on the ball to stand for your home. Hold the flashlight still: that is the Sun. Now turn the ball slowly. Watch your sticker move from the light into the dark, and back again. That is a whole day and night.

A flashlight and a globe in a dark room make a good model, as Britannica Kids describes: the light falls on half the globe and leaves the other half in shadow. Mark your location, keep the light fixed and rotate the globe toward the east, the way Earth turns, and you will see your location carried through sunrise, noon, sunset and midnight.

The model can test ideas too. Tilt the globe's axis and keep rotating: places near one pole may stay lit for a whole turn while places near the other stay dark, which is the pattern NASA describes for the poles. Places on the Equator, meanwhile, spend about half of each turn in the light.