Day

A day is the time required for the Earth to complete one full rotation with respect to the Sun. This rotation is a fundamental cycle that creates the transition between morning, afternoon, evening, and night. On average, a standard day lasts 24 hours, which is equivalent to 86,400 seconds. This cycle is essential for life on Earth because it drives circadian rhythms. These are internal biological processes in many organisms that rely on the daily light and dark cycle to function correctly.

There are several ways to define the length of a day depending on the scientific context. A solar day is measured by the time it takes for the Sun to return to its highest point in the sky, known as its culmination point. Because the Earth moves along an eccentric orbit, it travels at different speeds at various positions. This means a solar day is not always exactly 24 hours. Due to the Earth's tilted axis and its orbital path, a solar day can vary by up to 7.9 seconds. In recent decades, the average solar day has been approximately 86,400.002 seconds.

Astronomers often use a different measurement called a sidereal day, or a stellar day. This is the time it takes for the Earth to make one full rotation relative to the distant, fixed stars. A sidereal day is about four minutes shorter than a solar day. It lasts approximately 23 hours, 56 minutes, and 4.09 seconds. Because of this difference, there are about 366.2422 sidereal days in one mean tropical year. This results in one more stellar day than there are solar days in a single year.

Humanity has developed various systems to organize these days into calendars. A solar calendar organizes dates based on the Sun's annual cycle, which provides consistent start dates for the four seasons. In contrast, a lunar calendar organizes dates based on the phases of the Moon. Most modern civil days begin at midnight, recorded as 00:00 or 12:00am. However, different cultures and fields use different starting points. For example, the Jewish religious calendar counts days starting at sunset. In astronomy, a day may begin at noon to keep a single night of observations within one calendar day.

To manage time across the globe, the world uses time zones. These systems ensure a uniform standard time for different geographical locations. Time zones became widely adopted in the mid-19th century alongside the rise of railroads. By 1929, most major countries had implemented them. Currently, there are 40 time zones in use worldwide. The central zone is known as UTC+00, which uses Coordinated Universal Time. In some regions outside the tropics, daylight saving time is practiced. This can result in one civil day lasting 23 hours and another lasting 25 hours.

Earth's rotation is not perfectly constant over long periods. The Moon's gravitational pull causes tidal deceleration, which slowly slows the Earth's rotation. Because of this, the average length of a solar day is increasing by about 2 milliseconds per century. To keep our clocks in sync with this slowing rotation, scientists use leap seconds. A leap second adds one extra second to a UTC day, making it 86,401 seconds long. Between 1972 and 2022, a total of 27 leap seconds were inserted into the calendar.

Geological evidence shows that day lengths have changed drastically throughout Earth's history. By studying sedimentation rings in coral fossils, paleontologists have estimated past day lengths. When the Earth was first formed, a day is estimated to have lasted only about 6 hours. As time passed, the days grew longer. For example, during the Cambrian period about 500 million years ago, a day lasted roughly 20 hours and 40 minutes. During the Cretaceous period, 200 million years ago, a day lasted about 23 hours and 20 minutes.

Every large astronomical object in our solar system has its own rotation period. These "days" vary greatly depending on the planet or dwarf planet. On Mercury, a day lasts 708.7 hours, while on Mars, it is about 24.7 hours. Some worlds spin much faster than Earth. Jupiter has a day of only 9.9 hours, and Saturn's is 10.7 hours. Even the dwarf planet Ceres has a rotation period of approximately 9 to 9.1 hours.