Why does the Sun seem to travel across the sky? It looks like the Sun is moving, but it is really the Earth that turns, spinning around once in about a day. Earth turns eastward, and as a town turns toward the Sun and enters its light, the Sun seems to rise in the east. In the evening the Sun seems to set in the west, after a day of crossing the sky.

Now picture the whole planet from far away. Daytime is when sunlight reaches the ground, and the side of the Earth facing away from the Sun is dark. The line between daylight and darkness is called the terminator, but seen from space that line 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.

The Sun is a star, the only star in our solar system. It is about 93 million miles (150 million kilometers) from Earth. One day and one night together take about 24 hours. Scientists call that a solar day, one turn of Earth relative to the Sun. Measured against distant stars instead, one turn takes 23 hours, 56 minutes and 4 seconds. That is a sidereal day, and it is about 3 minutes 56 seconds shorter than the mean solar day.

Nicolaus Copernicus said the Sun is at the center and the Earth rotates daily, and he published his system in De Revolutionibus in 1543. That opposed the older view of Ptolemy, who held that the Earth sits motionless at the center. The idea that Earth turns was not always the accepted one, and the Sun still looks like it moves across the sky.

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. Earth's axis is tilted about 23.4 degrees compared with its path around the Sun, and the tilt points the same way throughout the year. In March and September the Sun is directly over the equator at the equinox, and day and night are about equal. The Sun rises due east and sets due west at the equinoxes.

Before standard time, towns set their clocks by the Sun's path across their own meridian. Railway companies in Great Britain adopted Greenwich Mean Time in 1847. In the United States, standard time began on Sunday, November 18, 1883, when railroads started using time zones. Railroad travel made standard time necessary, and railroad leaders were asked to design a uniform time system.

In 1884 delegates at the International Meridian Conference voted to make the Greenwich Meridian the world's Prime Meridian. The conference chose a line through the telescope at the Royal Observatory in Greenwich. The prime meridian runs through Greenwich, England, and marks 0 degrees longitude. Time zones are areas that share one standard time, and their borders follow local laws rather than straight lines.

Other worlds turn at their own speeds. Mars has days too: one turn of Mars takes about 24.6 hours, close to a day on Earth. Jupiter spins fastest of all the planets, so a day on Jupiter is under 10 hours. One day-night cycle on Mercury, from sunrise to sunrise, lasts about 176 Earth days. On Venus one spin takes 243 Earth days, and a solar day on Venus, from one noon to the next, lasts about 117 Earth days, because Venus spins backward and the Sun's motion adds to the turn. Uranus is tipped on its side, so its poles get decades of sunlight followed by decades of darkness.

Most living things, including animals, plants and microorganisms, have body clocks that follow a roughly 24-hour cycle. In humans the master clock is in the brain, in a spot called the SCN. In the evening it tells the body to make more melatonin, a hormone that makes people sleepy. Traveling across time zones causes jet lag, because the body clock no longer matches the local day and night.

Animals that are active in the daytime are called diurnal. Humans, squirrels and songbirds are diurnal. Many animals, like owls, are awake at night and rest in the day. Some animals are active at night to escape the heat of the day, and some hunt, find mates or hide from predators at night. Bright lights at night can confuse wildlife, such as migrating birds, sea turtle hatchlings and amphibians. Humans need natural darkness for good sleep and health.

Earth spins around once in about a day, and that single turn gives every town a day and then a night. Daytime is when sunlight reaches the ground, and the side of the Earth facing away from the Sun is dark. The Sun looks like it moves across the sky, but it is really the Earth that turns. One day and one night together take about 24 hours.

Words to know

• terminator: day-night line
• twilight: glow after sunset
• sidereal: measured by stars
• equinox: equal day, night
• diurnal: active by day
• melatonin: sleepy hormone