5. THE EARTH'S MOVEMENTS

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The Earth, like the rest of the planets in the Solar System, moves around the Sun. It also moves on its own axis. 

5.1. The Earth's revolution

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The Sun is not located in the middle of ellipse because the distance between the Earth and the Sun varies as it moves along its orbit.

• The maximum distance of the Earth from the Sun is 152 million km, reached in the summer in the Northern Hemisphere.

• The minimum distance of the Earth from the Sun is 147 million km, reached in the winter.

Because takes a little more than 365 days to complete its revolution (almost a quarter of a day more), we have to add a day to the month of February every 4 years. This is called a leap year.

• The Earth's revolution has some characteristics that permit light and temperature conditions that are ideal for life to exist.
• The Earth is tilted 23.5º perpendicular to the ecliptic plane. As it moves, the Earth keeps its tilt.
• The Earth moves around the Sun on a plane called the ecliptic plane.
• Because the Earth is almost spherical, the Sun's rays don't hit all parts of the planet at the same angle.

5.1.1. The Earth's seasons

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Characteristics of the Earth's revolution cause the amount and intensity of sunlight that hits a part of the Earth to vary during the year.
These variations cause the seasons.
During the summer, the sun's rays hit the Tropic of Cancer in the Northern Hemisphere and the Tropic of Capricorn in the Southern Hemisphere perpendicularly.
Due to the tilt of the Earth's axis with respect to the ecliptic plane, the sun's rays hit the Northern Hemisphere directly and the Southern Hemisphere at an oblique angle. This is why when it is summer in one hemisphere, it is winter in the other.
In spring and summer, the sun's rays hit the Equator perpendicularly.
The change from one season to the next is marked by the length of day and night. The first day of summer is the longest day of the year and the first night is the shortest.
Winter begins with the longest night. These days when the difference between the length of night and the length of day is the biggest, are called solstices.
The day and night that mark the beginning of spring and autumn are the same length. They are called equinoxes.

5.2. The Earth's rotation

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5.2.1. Time zones

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Because the Earth is a sphere, not all the areas of the planet receive sunlight at the same time. As a result, the time of the day varies between some continents and even between countries of the same continent or between cities in the same country.
To unify time, in the 19^th century, the Earth was divided into 24 meridians. These are imaginary lines that go from pole to pole, in semicircles. Each area between two meridians was called a time zone.
All the countries and cities that belonged to the same time zone had the same time. Later, the time zones were changed so they were adjusted to borders of countries or regions. This is why the borders of the time zones are so irregular.
Starting from the Greenwich meridian (0º), an hour is added for each time zone going east and an hour is subtracted going west. The calendar day starts at 180º east meridian and ends at 180º west meridian.

5.3. The importance of the Earth's movements for living things

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The movements of the Earth cause days and nights and also the seasons. We have seen that the number of hours of daylight (day) and darkness (night) vary depending on the season and the hemisphere we are in. In addition to these variations, there are also changes in the average temperature in an area during the year.
Living things can detect these variations. They influence their behaviour and cause them to adapt to their environment.
These are some examples of adaptation:

 

 

 

 

 

⁹sprout: start to grow

Many trees lose their leaves in autumn, when they detect a decrease in the hours of daylight. In the same way they sprout⁹ new leaves in spring, with the increase in the hours of daylight.

Flowering and forming of fruits also depend on temperatures and hours of daylight and darkness.
Not all plants flower and form fruit in the same season.

Animals are also sensitive to the consequences of the Earth's movements. For example, the mating and breeding seasons of many animals start when the days get shorter and temperatures lower. In this way, the babies will be born during the ideal season for feeding them.

Another example of adaptation is migration. Some birds migrate at certain times of the year.
For example, the majority of the white storks, populations spend the winter in sub-Saharan Africa, and migrate to Europe for the summer.