4. Days, seasons and years
Contents
4.1 Earth rotates around its axis
Planet Earth rotates around its axis. It takes 24 hours for the Earth to complete one of these rotations. This amount of time is called a day. As the Earth rotates, different sides of the planet receive sunlight. The side of the planet that faces the Sun experiences daytime, whereas the side of the planet that faces away from the Sun experiences nighttime.
As our planet rotates from the west to the east, the Sun is said to rise from the east and set in the west.
4.2 Earth orbits around the Sun
Galileo Galilei was right to think that the Earth moves and rotates in more than one way. Even as you are reading this, the planet is actually orbiting around the Sun at great speed. Planet Earth orbits the Sun on an almost circular track. A complete cycle around this track takes Earth approximately 365 days. This is called a year.
An imaginary axis can be drawn from between the Earth's North and South Poles. This axis is set at a 23,5 degree angle. This means that our planet is slanted relative to its orbit around the Sun.
The slanted axis of the planet combined with Earth's orbit around the Sun creates the seasons.
4.3 From one season to the next
Because Earth has a globular shape, the Sun's radiation does not divide equally to the planet's surface. The closer an area is to Earth's equator, the more solar radiation the area receives. This concentrated solar radiation results in a high amount of light and heat. In contrast, near the polar regions of the planet, the solar radiation has to cover a larger surface area, and is therefore less concentrated.
You can investigate this phenomenon by shining a flashlight on a soccer ball: when pointing the flashlight at the center of the ball (near its "equator"), the result is a small, concentrated spot of light. When you move the light towards the top or bottom of the ball (closer to its "poles"), the resulting spot of light becomes larger and less bright. A similar phenomenon takes place between the Sun and the Earth.
The imaginary axis between the Earth's North and South Poles is slanted at a 23,5 degree angle. When our planet orbits around the Earth, this angle remains the same. This, in combination with the way in which solar radiation is concentrated differently between the planet's equator and its polar regions, creates the planet's seasons. The equator receives high amounts of solar radiation throughout the year, resulting in an endless summer. In contrast, the North and South Poles receive only little sunlight, and as a result experience no summer.
The amount of daylight varies depending on whether an area is slanted towards or away from the Sun. When a polar region is slanted towards the Sun, it experiences more daylight hours than during other times of the year. The result is the season we call summer. However, as the solar radiation still has to cover a larger surface area than near the equator, the summers of polar regions remain relatively cold compared to areas closer to the equator.
Europe is located closer to the North Pole than the equator. This results in clearly defined seasons.
Earth's slanted axis also results in differences between the times of seasons on different hemispheres. When the Northern Hemisphere faces the Sun and experiences its summer, the Southern Hemisphere faces away from the Sun and experiences its winter. Likewise, when the Southern Hemisphere receives more sunlight and experiences its summer, the Northern Hemisphere faces less sunlight and experiences its winter.
You can investigate this phenomenon by shining a flashlight on a soccer ball: when pointing the flashlight at the center of the ball (near its "equator"), the result is a small, concentrated spot of light. When you move the light towards the top or bottom of the ball (closer to its "poles"), the resulting spot of light becomes larger and less bright. A similar phenomenon takes place between the Sun and the Earth.
The imaginary axis between the Earth's North and South Poles is slanted at a 23,5 degree angle. When our planet orbits around the Earth, this angle remains the same. This, in combination with the way in which solar radiation is concentrated differently between the planet's equator and its polar regions, creates the planet's seasons. The equator receives high amounts of solar radiation throughout the year, resulting in an endless summer. In contrast, the North and South Poles receive only little sunlight, and as a result experience no summer.
The amount of daylight varies depending on whether an area is slanted towards or away from the Sun. When a polar region is slanted towards the Sun, it experiences more daylight hours than during other times of the year. The result is the season we call summer. However, as the solar radiation still has to cover a larger surface area than near the equator, the summers of polar regions remain relatively cold compared to areas closer to the equator.
Europe is located closer to the North Pole than the equator. This results in clearly defined seasons.
Earth's slanted axis also results in differences between the times of seasons on different hemispheres. When the Northern Hemisphere faces the Sun and experiences its summer, the Southern Hemisphere faces away from the Sun and experiences its winter. Likewise, when the Southern Hemisphere receives more sunlight and experiences its summer, the Northern Hemisphere faces less sunlight and experiences its winter.
4.4 Important dates in the yearly cycle
During the summer solstice (June 21st or 22nd), the Sun shines directly at the Tropic of Cancer. Because of this, the Northern Hemisphere experiences its longest daylight hours during this time. In the regions close to the North Pole, the summer solstice results in a phenomenon called the "midnight sun", where the Sun stays above the horizon even at night.
During the spring (March 20th or 21st) and autumn equinoxes (September 22nd or 23rd), the Sun shines directly at the planet's equator. During the equinoxes, the night and day are the same length everywhere on planet Earth. The equator receives its name from this phenomenon.
During the winter solstice (December 21st or 22nd), the Sun shines directly at the Tropic of Capricorn. The Southern Hemisphere experiences its longest daylight hours, whereas the Northern Hemisphere experiences its shortest daylight hours. Near the Arctic Circle, the Sun does not rise above the horizon, and the regions close to the North Pole experience a sunless period known as the polar night.
The seasons are the result of two factors: 1) The fact that planet Earth rotates around a slanted axis, and 2) the fact that this axis remains the same throughout the year. Because of these two factors, in July the amount of sunlight is higher on the Northern Hemisphere, whereas in December the amount of sunlight is higher on the Southern Hemisphere.
During the spring (March 20th or 21st) and autumn equinoxes (September 22nd or 23rd), the Sun shines directly at the planet's equator. During the equinoxes, the night and day are the same length everywhere on planet Earth. The equator receives its name from this phenomenon.
During the winter solstice (December 21st or 22nd), the Sun shines directly at the Tropic of Capricorn. The Southern Hemisphere experiences its longest daylight hours, whereas the Northern Hemisphere experiences its shortest daylight hours. Near the Arctic Circle, the Sun does not rise above the horizon, and the regions close to the North Pole experience a sunless period known as the polar night.
The seasons are the result of two factors: 1) The fact that planet Earth rotates around a slanted axis, and 2) the fact that this axis remains the same throughout the year. Because of these two factors, in July the amount of sunlight is higher on the Northern Hemisphere, whereas in December the amount of sunlight is higher on the Southern Hemisphere.
4.5 The Moon and months
The Moon, our only natural satellite, travels on an orbit approximately 350 000 kilometers from planet Earth. The Moon orbits Earth in a counterclockwise direction, and it completes one cycle around the planet in 27 days, 7 hours, and 43 minutes. This is called a month. Only one side of the Moon faces towards our planet at all times. The "dark side of the Moon" has only been investigated with space probes.
During a new moon (phase 1 in the picture), the Sun and the Moon are aligned, and the face of the Moon is dark. As the Moon increases in phase (waxes), the right side of the Moon is bright. When the Moon decreases in phase (wanes), the left side of the Moon is bright.
During a full moon (phase 5 in the picture) the whole face of the Moon is bright. When the Earth, Moon, and Sun are all aligned, a solar eclipse can be experienced on Earth. Similarly, a lunar eclipse can be experienced during a full moon.
Summary
A day is the time it takes for planet Earth to complete one rotation around its axis. This takes approximately 24 hours.- Our planet orbits the Sun on an almost circular orbit. A complete cycle around this orbit takes our planet approximately 365 days. This is called a year.
- The Earth rotates around an imaginary axis that can be drawn between the North and South Poles. This axis is slanted at a constant 23,5 degree angle. This slanted axis, combined with Earth's yearly orbit around the Sun, creates the seasons.
- During the summer and winter solstices, the light part of the day is either at its longest or its shortest on one hemispehre. During the winter solstice, the Northern Hemisphere receives the least sunlight. In contrast, during the summer solstice, it receives the most sunlight.
- During the spring and autumn equinoxes the day and night are of an equal length everywhere on Earth.
- Key words: day, month, year, seasons, solstice, equinox, eclipse.