### Official degrees of earth’s rotation per day

• What is the official degree to one decimal point please, of the earth’s rotation in one single day. Can it be confirmed that it is exactly 360.0 degrees using official data? Thank you in advance.

Official according to what or whom?

A quick search shows no one has used the phrase "leap second" yet, so I will: https://en.wikipedia.org/wiki/Leap_second Also, for officialness: http://hpiers.obspm.fr/eop-pc/models/constants.html

• FSimardGIS Correct answer

3 years ago

First, we need to decide which definition of "day" to employ. There are several types of days:

Apparent solar day: the time between two successive culminations of the Sun (apparent Noon) from an fixed Earth-based observer;

Mean solar day: a more uniform, averaged solar day without seasonal variations;

Stellar/Sidereal day: the time needed for the Earth to rotate once relative to the stars;

SI day: a unit of time containing exactly 86,400 SI seconds defined by caesium atoms.

Since we generally refer to the traditional day/night cycle when we say "day", this means a form of solar time. To get a more averaged value, let's use the mean solar day.

The current formula linking the Earth Rotation Angle (ERA) to the modern approximation of mean solar time, UT1 (basically the Earth's clock following the mean day/night cycle), is by definition :

$$ERA = 2π(0.7790572732640 + 1.00273781191135448 T_u) \text{ radians}$$

Where $$Tu$$ is the Julian UT1 Date - 2451545.0

So according to this formula, a (UT1) day is 1.00273781191135448 Earth rotations, which multiplied by 360° is about 360.98561°. However, the Earth's rotation and revolution are not constant, and are always changing at somewhat unpredictable rates, so the angle is not perfect, but the changes are very slow. So this is more a modern approximation rather than an exact value. Rounded to one decimal place, this gives you 361.0°, a figure that will likely remain true for at least several millenia.

If you want to know the amount of Earth rotation for every SI day, you're in luck: it is possible to consult reports of the Earth's orientation (rotation and polar motion) thanks to the IERS. Values are tabulated for each 0h UTC every day in the IERS publications, allowing to derive the angle that Earth has rotated every 86,400 SI seconds, allowing to scientifically monitor variations in Earth's rotation compared to a very constant unit of time realized by atomic clocks. Nowadays, the Earth's rotation is measured and reported thanks to radio telescopes and VLBI observing distant objects in the universe. Official reports of its orientation, the Earth Orientation Parameters are published on the IERS Website.

As an example, here is a graph showing the amount of Earth rotation every 86,400 seconds constructed with IERS data of the last year:

As we can see, there are several seasonal, periodic and unpredictable variations in Earth's rotation.