The Earth

Our Planet is the third from the Sun and has a shape of a tilted ellipse, because when spinning in space is not completely vertical, but it has an inclination on its own axis. That axis is called rotational or polar axis and it is an imaginary line around which the Earth rotates. Even when revolving around the Sun, the Earth always keeps the same axial tilt. It is represented with a straight line crossing the planet by the poles. Then, the Earth has two fundamental movements: rotation and revolution.

In the rotational movement, the Earth moves around the polar axis following a west to east direction, and this causes the Sun to rise on the east (orient) and to set on the west (occident). The exact duration of this spin is called a sidereal day, and lasts exactly 23 hours, 56 minutes and 5 seconds; but for practical reasons, the day is adjusted to 24 hours in what we call a civil day.

The rotational movement on the Earth’s axis is very important, because it determines:

1. The day and the night.
2. The Earth’s poles flattening.
3. The apparent movement of the stars and the Sun from east to west.
4. The deviation of the bodies during free falling; if you let an object fall freely no matter its weight, it does not fall completely vertically, but suffers a slight deviation to the East that is imperceptible when the time is very short.
5. Coriolis Effect; which is the deviation happening to winds and ocean currents to the right on the North hemisphere and to the left on the South hemisphere. Consequently the water swirls in different directions in every hemisphere.
6. The time differences around the globe; the time measuring on Earth is organized in time zones, which are 24 different. They begin from the Greenwich meridian (also called “Z time”).

Then, the Earth’s revolutionary movement follows an elliptical orbit around the Sun. The whole loop is completed in 365 days, 5 hours and 48 minutes, and starts on March 21st. This is called a tropical year or Solar year. Also, for practical reasons, there is a civil year of 365 days and starts on January 1st.

The revolutionary movement determines the seasons of the year; due to the revolutionary movement, the Earth moves closer and farther from the Sun, and then light does not distribute homogeneously on the terrestrial surface. This is the reason of the season changes. According to the seasons, days and nights have different duration; the Sun has a different pathway closer to the south or north.

Nutation.  This refers to the oscillations done by the Earth’s axis while doing the precession movement. That causes that the Earth follows a sinuous path while moving its rotation axis.

Precession. This Earth’s movement is also called wobbling and describes the circular tilting of the Earth’s polar axis, which is the position change of the Earth’s North Pole in relation to the Earth’s revolution ellipse. A precession cycle lasts 25,780 years.