You will...
- Identify the Earth's movement of rotation and how it causes day and night.
- Identify the Earth's axial tilt and the Earth's movement of revolution, and how they cause the seasons.
- Identify the Moon's movement around the Earth and its phases.
- Distinguish the types of eclipse.
- Distinguish the layers of the Earth and their characteristics.
1. The shape of the Earth
For a long time, mankind believed that the Earth was flat. However, images taken from spacecrafts show that the Earth is spherical. As early as the 4th century, the Greek philosopher Aristotle was convinced that the Earth was a sphere. He contributed three observations to prove it.
For a long time, mankind believed that the Earth was flat. However, images taken from spacecrafts show that the Earth is spherical. As early as the 4th century, the Greek philosopher Aristotle was convinced that the Earth was a sphere. He contributed three observations to prove it.
- On clear days, he could observe how boats disappeared over the horizon; first the hull and then the sails. If the Earth were flat, he entire boat would be observed, getting smaller and smaller, until it disappeared.
- Sometimes, he could observe the shadow cast by the Earth on the Moon. It was always circular shaped, the type of shadow cast by a sphere.
- When moving North or South, he could see new stars and stop seeing others.
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| Comparison showing changes in shape of Earth. |
In the direction of its rotation axis, i.e. from the North Pole to the South Pole, the Earth's diameter is 12,714 km. The diameter is 12,756 km at the Equator.
The Equator divides the Earth in two halves, called hemispheres (from hemi, half). The upper one is the Northern Hemisphere and the lower one is the Southern Hemisphere. The Earth has a spherical shape and is divided at the Equator into the Northern and Southern Hemispheres.
2. The movements of the Earth
Galileo started in the 17th century that the Earth is a planet in constant motion and performs two types of movement.
Rotation
Galileo started in the 17th century that the Earth is a planet in constant motion and performs two types of movement.
Rotation
- It rotates on its imaginary axis.
- It completes a rotation in 24 hours.
- It causes the succession of day and night.
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| Planet Earth ROTATES in a counter-clockwise direction. Earth ROTATES around its axis. |
Revolution
- It orbits the Sun, following the imaginary ecliptic plane.
- It completes its orbit in a year.
- Changes in the length of day and night.
- The seasons: spring, summer, autumn and winter. These seasons are divided according tot he length of the day, and are marked by particular days: equinoxes and solstices.
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| Differences between Earth's rotation and revolution. |
Remember! Kepler's first law of planetary motion: planets orbit the Sun in ellipses with the Sun at one focus.
Also the continuous movement can cause eclipses. They occur when the Sun, the Earth and the Moon are aligned like:
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| Moon, Planets and Starts rotate around their axis. |
- Solar eclipse: happens when the Moon interposes between the Sun and the Earth.
- Lunar eclipse: happens when the Earth interposes between the Sun and the Moon.
The Moon is the Earth's satellite. As it orbits the Earth, the part of the moon facing us changes according to how it is illuminated, causing the lunar phases:
- Full Moon.
- First quarter.
- New Moon.
- Last quarter.
3. The Earth's layers
Four layers can be distinguished on Earth:
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| The Earth consists of four layers: the atmosphere, the hydrosphere, the geosphere and the biosphere. |
THE BIOSPHERE
This includes all the living things on the planet. Life flourishes in a thin layer about 20 km thick and extends from the highest mountains to the deepest ocean floors. So it can also be termed as the zone of life on Earth.
THE ATMOSPHERE
This is the gaseous layer surrounding the Earth. It is a very thin layer, but plays an essential role for life.
- It provides us with the air we breathe.
- It filters ultraviolet (UV) rays.
- It protects the Earth from extreme temperatures.
THE HYDROSPHERE
This includes all the water of our planet. The oceans contain 97% of the water on Earth, but there is also water in rivers, lakes and polar ice caps. Water moderates temperature fluctuations on the planet, and it is an essential component for life.
THE GEOSPHERE
This is the rocky sphere located under the atmosphere and the hydrosphere. It includes all the terrestrial materials.
