Why Does Earth Have Day and Night?
While you don't feel it, Earth is spinning. Once every 24 hours Earth turns — or rotates on its axis — taking all of us with it. When we are on the side of Earth that is facing the Sun, we have daylight. As Earth continues its spin, we are moved to the side facing away from our Sun, and we have nighttime. If we were looking down on Earth from above the north pole, we could see that Earth rotates counterclockwise, and we would watch daylight and darkness sweeping across our globe from east to west.
Do other planets have day and night?
Yes! All the planets in our solar system spin on their axes (so does our Sun!) and so they have day and night cycles. There are differences, however, in the length of day and night — the cycles are made even more complex by the tilt of a planet's axis and its rate of orbit. Some planets rotate faster than Earth and some rotate slower. Mars has a day and night cycle similar to Earth. Mars rotates on its axis once every 24.6 hours. Venus turns once on its axis every 243 Earth days (which is only slightly longer than it takes for Venus to go around the Sun!). Mercury's day and night cycle is more complex. Mercury rotates one-and-a-half times during each orbit around the Sun. Because of this, Mercury's day — from sunrise to sunset — is 176 Earth days long. The larger planets spin much faster. Jupiter rotates once every 10 hours, Saturn spins once every 11 hours, and Neptune completes a rotation in 16 hours. Pluto, at the farthest reaches of our solar system, spins on its axis once every 6.4 days.
Something to ponder: Does Pluto even have a “day” and “night” like we think of on Earth? Pluto is so distant from the center of our solar system that our Sun would look like a very bright star in its sky!
Why does Earth's day length change during the year?
Every location on Earth experiences an average of 12 hours of light per day but the actual number of hours of daylight on any particular day of the year varies from place to place. Locations around Earth's equator only receive about 12 hours of light each day. In contrast, the north pole receives 24 hours of daylight for a few months in the summer and total darkness for months in the winter. These two annual times of light and dark are separated by a long sunrise and a long sunset.
Earth rotates on its axis; this causes us to experience day and night. But Earth's axis is tilted 23.5 degrees (the angle is measured between Earth's equatorial plane and the plane in which it orbits our Sun). As Earth orbits our Sun, the axis points toward the same location in space — almost directly toward Polaris, the North Star. This means that during Earth's movement around our Sun each year, our polar regions spend loooooooong periods pointed toward our Sun in the summer (for example, July in the northern hemisphere, or December in the southern hemisphere) and long periods pointed away from our Sun during the winter. At latitudes greater than 66.5 degrees (90 degrees minus 23.5 degrees, the tilt of the axis), the regions above the Arctic and Antarctic circles on our globe, days of constant darkness or light occur.
Tilted Earth at summer, spring, fall, and winter positions around the Sun,
clearly showing polar regions in daylight and dark at winter/summer positions.
Because of this tilt and Earth's movement around our Sun, there is a time when Earth's north pole is tilting 23.5 degrees toward our Sun. This is the summer solstice, the first day of the northern hemisphere summer and the longest day of the year in the northern hemisphere. On December 21 or 22, Earth's north pole is tilting 23.5 degrees away from our Sun and the south pole is tilted toward our Sun. This is the winter solstice, the shortest day of the year in the northern hemisphere. Twice each year — during the equinoxes (“equal nights”) — Earth's axis is not pointed toward our Sun. The spring equinox in March marks the beginning of the transition from 24 hours of darkness to 24 hours of daylight at the north pole. The fall equinox in September marks the shift into 24 hours of darkness at the north pole. During the equinoxes every location on Earth (excluding the extreme poles) experiences a 12-hour day length.
Other planets also experience these changes in day and night length because they too are tilted on their axes. Each planet's axis is tilted at a different angle. Jupiter is tilted only 3 degrees, so its change in day and night length as it moves around the Sun is less extreme than that of Earth. Neptune's axis is tilted 30 degrees, so day and night changes would be more extreme on Neptune than on Earth. Uranus presents an interesting case because its axial tilt is even more extreme — 98 degrees! This means that the north pole of Uranus is pointed at the Sun during the north polar summer; the south pole is in total darkness. During the north polar winter, some 42 Earth years later, the south polar axis points at the Sun and the north polar region is in total darkness. During the spring and fall, when its axis is perpendicular to the incoming rays of the Sun, Uranus experiences a 17-hour day and night cycle as it spins on its axis.
Good news for overachievers: Earth's days are getting longer!
Researchers examining ancient corals noted that annual growth patterns suggested there were more days in a year in Earth's distant past. Fossil corals, 380 million years old, from the Devonian Period recorded 400 daily cycles. About 290 million years ago in the Pennsylvanian Period, there appear to have been 390 daily cycles each year. Assuming that Earth's revolution around our Sun has not changed dramatically, this means that the number of hours per day has been increasing and that Earth's rotation has been slowing. Today's day length is 24 hours. During the Pennsylvanian Period a day was ~22.4 hours long. In the Devonian Period, a day was ~21.8 hours long. Earth's rotation appears to be slowing approximately 2 seconds every 100,000 years. Why are Earth's days getting longer? Some scientists suggest that tidal cycles create a “drag” on Earth, causing it to slow down.