The time it takes for the Earth to rotate completely around on its axis is what we call a day. It’s Earth’s rotation that gives us night and day. One complete rotation actually takes 23 hours, 56 minutes and 4.1 seconds.
There are other ways to measure time, as well. There is apparent solar time, mean solar time, and the equation of time.
A solar day is the length of time between the Sun reaching its highest point in the sky two consecutive times. You can learn more about a solar day here.
Apparent solar time, sometimes called true solar time, is determined by the daily apparent motion of the observed Sun. It is based on the interval between two successive returns of the Sun to the local meridian. The length of a solar day varies throughout the year, and the accumulated effect of these variations (equation of time) produces seasonal deviations of up to 16 minutes. Why, you may ask: Earth’s orbit is elliptical and the Earth’s axial tilt. Consequently, apparent solar days are shorter in March (26–27) and September (12–13) than they are in June (18–19) or December (20–21). In 2010, the greatest Universal Time(UT1) interval between apparent midnights (at Greenwich) is 86,471 seconds and the shortest interval is 86,325 seconds.
Mean solar time is the conceptual hour angle of the fictitious mean Sun. At this time, mean solar time is realized with the Universal Time(UT1) scale, which is constructed mathematically from baseline interferometric observations of the diurnal motions of radio sources located in other galaxies, and other observable information. The length of a mean solar day is constant, unlike that of an apparent solar day, which can be up to 20 seconds shorter or 30 seconds longer than a mean solar day. The difference between apparent solar time and mean solar time is called the equation of time. The length of the mean solar day is increasing due to the tidal acceleration of the Moon by the Earth, and the corresponding deceleration of the Earth by the Moon.
The equation of time is the difference between apparent solar time and mean solar time at the same location on the Earth. The need for the equation of time results mainly from the two astronomical causes mentioned above: the Earth’s axial tilt and the elliptic orbit of the Earth. Its significance is explained in detail by NASA at this link.
It can be difficult to explain “how long is a day on Earth” because there are many variables. The most convenient convention is to say that it is 24 hours for normal conversational situations.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.