A day is the time period of a full rotation of the Earth with respect to the Sun, averaging 24 hours or 86,400 seconds. This cycle has governed life since the planet formed and serves as the most fundamental unit of existence before humans built clocks or calendars.
The average length of a solar day is about 86,400.002 seconds, which varies by up to 7.9 seconds more or less than 24 hours throughout the year. This fluctuation occurs because the Earth travels along an eccentric orbit while spinning on an inclined axis.
The Hebrew Bible defines a day in terms of evening and morning, establishing a tradition where the Jewish day begins at sunset or nightfall when three second-magnitude stars appear. Medieval Europe followed this pattern known as Florentine reckoning where a reference like two hours into the day meant two hours after sunset.
There were 27 leap seconds inserted between 1972 and 2022 to keep atomic time in sync with Earth's rotation. The International Earth Rotation and Reference Systems Service measures the Earth's rotation and determines whether a leap second is necessary.
Paleontologist John W. Wells discovered that the day at the Earth's formation was estimated at 6 hours. The geological record shows that 100 million years ago during the Cretaceous period the day was 23 hours and 20 minutes long while 500 million years ago during the Cambrian period the day was 20 hours and 40 minutes long.
Daytime averages slightly more than half of the 24-hour day due to the Sun's apparent size and atmospheric refraction. The first light reaches the ground when the center of the Sun is still below the horizon by about 50 minutes of arc, making daytime on average around 7 minutes longer than 12 hours.