33. The Earth and Moon system
The Earth rotates
Planet Earth rotates around its axis. The ends of this axis are known as the planet's two poles. Of course, there is no actual axis inside the planet, but the invisible axis can be imagined. The globe on the right simulates Earth's invisible axis with an axis made out of metal.A day is the time during which planet Earth rotates completely around its axis. During this time, one half of the Earth faces both toward the Sun (day) and away from it (night).
A day lasts for approximately 24 hours. The lengths of the light and dark parts of the day vary between different times of the year.
The Sun does not actually rise during the mornings. Instead, the Earth only rotates so that one part of it faces the Sun. This makes it seem like the Sun rises from the horizon.
Where is it day and night now? The year and the seasons
A year is the length of time it takes for planet Earth to complete its orbit around the Sun. This takes the Earth approximately 365 days. Our planet travels around the Sun on an elliptical orbit. The Earth is closest to the Sun during January, and furthest away from it during July.
The picture on the right shows the yearly orbit of planet Earth around the Sun. The Earth on the top shows the planet during spring. On the left, you can see Earth during the summer (when the northern hemisphere is tilted towards the Sun). The Earth on the bottom shows the planet during autumn. On the right, you can see Earth during the winter (when the northern hemisphere is tilted away from the Sun).
The yearly orbit of planet Earth results in the seasons: spring, summer, autumn and winter. The seasons are the result of two factors. First of all, the Earth orbits around the Sun. Secondly, the Earth's axis is tilted at a slight angle. This makes the planet's two hemispheres face the Sun more during different times of the year.
The movement of the solar system
The Earth rotates around its axis and orbits around the Sun. In addition to these two movements, the planet also experiences a third form of movement. Our planet moves in space at great speed as a part of a large star system or galaxy.
Galaxies are full of stars. The Sun is only one of the stars in our solar system.
During dark and bright nights, we can see our own galaxy in the form of dense star clusters. Our galaxy is known as the Milky Way. We cannot see all of it, however, because we are located inside it.
The Milky Way as seen from Earth. Movements of the Moon
The Moon orbits planet Earth. Many of the other planets in our solar system also have moons.
The Moon can be seen during the night. This is because it reflects sunlight back towards the Earth. The Moon completes its orbit around the planet Earth in a month. Only one side of the Moon is visible from Earth.
The relative positions of the Sun, the Earth and the Moon determine how much of the Moon is visible. A new moon (1) does not reflect almost any light at all.
A full moon (5) reflects light completely. At other times of the month, only a part of the Moon is visible.
Once in a while, the Moon ends up between the Sun and the Earth. When this happens, the Sun's light is blocked by the Moon. This is called a solar eclipse. Sometimes, the Sun's light can be blocked by the Earth from reaching the Moon. This is known as a lunar eclipse.
Terminology
| Term | Explanation |
|---|---|
| day | The time it takes for the Earth to complete its rotation around its axis. |
| year | The time it takes for the Earth to complete its orbit around the Sun. |
| month | The time it takes for the Moon to complete its orbit around the Earth. |
| galaxy | A star system that consists of many stars and the planets and objects that orbit them. |
| moon | A large object that orbits a planet. |