Universe

Level 1

Day and night: the Earth rotates on its axis

The Earth rotates around an imaginary axis: this is rotation. It completes one full turn in 24 hours, which is one day.

At any given moment, one half of the Earth is illuminated by the Sun (it is day) while the other half is in shadow (it is night). It is the Earth's rotation, and not the movement of the Sun, that explains the alternation of day and night.

 

The year and the seasons: the Earth revolves around the Sun

In addition to spinning on its axis, the Earth revolves around the Sun: this is revolution. It completes one full orbit in 1 year (about 365 days).

Because the Earth's axis is slightly tilted, the Sun's rays do not strike the surface in the same way throughout the year. This explains the seasons: spring, summer, autumn, winter.

The apparent motion of the Sun and stars

From Earth, it seems that the Sun moves across the sky: it rises in the east, moves up in the sky, then sets in the west. In reality, it is the Earth that rotates: this movement is therefore apparent.

  • During the day, the shadow of an object (like a stick stuck in the ground, called a gnomon) changes in length and direction.
  • At night, the stars also seem to move slowly across the sky, for the same reason.

The phases of the Moon

The Moon orbits the Earth. It does not produce light: it is illuminated by the Sun. Depending on the position of the Moon relative to the Earth and the Sun, we see a larger or smaller part of its illuminated face: these are the phases of the Moon (new moon, first quarter, full moon, last quarter).

Earth in the solar system

Earth is part of the solar system: a star, the Sun, around which several planets orbit.

The Sun is a star: it produces its own light (primary source, see the lesson Light). The planets do not produce light: we see them because they are lit up by the Sun.

  The solar system has 8 planets. Starting from the Sun:

  • 4 terrestrial planets (small and rocky): Mercury, Venus, Earth, Mars;
  • 4 gas planets (giants): Jupiter, Saturn, Uranus, Neptune.

Earth is the 3rd planet from the Sun. A planet can have one or more satellites: the Moon is Earth's satellite.

Why is life possible on Earth?

Among all these planets, Earth combines conditions suitable for life, which are rare in the solar system:

  • a temperature neither too hot nor too cold, which allows for the presence of liquid water;
  • an atmosphere containing oxygen, the gas needed for breathing (see the lesson Atmosphere).

Even further: galaxies and the Universe

Our solar system is only a tiny part of a much larger whole. It is located in a galaxy — a huge grouping of stars — called the Milky Way. And the Universe contains billions of galaxies.

Two models in history: geocentrism and heliocentrism

For a long time, scholars thought that the Earth was stationary at the centre of the Universe and that the Sun revolved around it: this is the geocentric model (“Earth at the centre”).

Later, thanks to observations, scientists understood that it is the Earth that revolves around the Sun: this is the heliocentric model (“Sun at the centre”), the model accepted today.

  This example shows how science advances: we observe, we propose a model, then we correct it when new observations contradict it.