Astronomy

Why does the Earth have Seasons



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Earth has four seasons and each season has its own beauty and wonder. What is the reason for these changes in seasons? The main reason is the tilting of the Earth’s axis. The true axis of Earth is tilted by 23.5 degrees to the axis it goes around the sun. This tilting is always there and the North and South Poles remains fixed to the same direction, despite Earth’s movement around the sun. So that whenever a hemisphere is facing towards sun, the climate gets warmer in that hemisphere and whenever it faces away, the climate gets colder.

# Tilting of Earth

This tilting of Earth causes one hemisphere of Earth to stay closer to the sun (or in other words face the sun so that part gets more sunshine) during a certain part of the year. Then the opposite hemisphere gets its chance to face the sun. The period which each hemisphere faces the sun is the summer time to that hemisphere; and the opposite hemisphere experiences winter. This happens in a cycle and each hemisphere experiences the cycle of spring, summer, autumn, winter at different parts of the year.

Earth moves around the sun in an ellipsoidal path and in this path, Earth is closest to the sun in early January and furthest from sun in early July. These have no significant effect on seasons since; it is not the distance, but the angle of the rays of sun that determines the energy reaching the surface. Less angle means that rays from the sun is concentrated to a small surface area and they also penetrate through less atmosphere. Less atmosphere means less energy loss, so that more energy is reached to the surface.

However, since in January it is closer to the sun, winter becomes less cold in the northern hemisphere.  Vice versa, winter is colder in the southern hemisphere in July as it is away from sun. However, this is compensated by the other factors like land and sea area and ocean currents.

# Importance of Longitudinal Lines

So there are two imaginary lines on Earth’s surface. One is the true midline that is set at the middle of the Earth, equal distance from each pole. This line is called the equator. Then there is the line which is drawn according to the movement of the sun. If there was no tilting on Earth’s part, then these two lines would have been the same and there would have been no seasons. But due to the 23.5 degrees tilt, these two lines are set at 23.5 degrees to one another. This causes the existence of another two lines.

There are two parallel lines 23.5 degrees north and south of the equator. These two lines are called tropic lines. The one in the north is called the tropic line of Cancer and the southern line is called the tropic line of Capricorn. The area between these two lines is called tropics and in this region there are no marked seasonal changes, since sunshine is mainly constant to these countries throughout the year. The two tropic lines represent the maximum distance the sun moves away from the line of the equator.

# Important Days Associated with Seasons

There are four names which need to be mentioned in connection with seasons. These are the two equinoxes and the two solstices. These are named mainly for their relation to the northern hemisphere. The two equinoxes are the days when the sun line crosses the equator. The days are March 20th or the spring equinox and September 23rd or the autumn equinox. In these two days, both hemispheres get the same amount of sunshine and all countries experience a 12 hour day and a 12 hour night.

The two solstices are the 21st of June and 21st of December. 21st of June is known as the summer solstice, when the North Pole has a 24 hour long day or in other words, sun doesn’t set above the Arctic Circle on the 21st of June. Same way, on the 21st of December the sun doesn’t rise at all for the North Pole and this day is known as the winter solstice, the day with a 24 hours night. It is in these two days that the sun line reaches the Cancer and Capricorn tropic lines respectively, the furthest from the equator.

# Cause for the Tilting of Earth

So what exactly caused the Earth to be tilted? There are many theories to the cause of the tilt. In fact, all the planets in the solar systems are tilted one way or the other, Uranus being tilted almost 97 degrees. One of the most popular theories is that it was the cause of some giant asteroid or minor planet colliding with Earth and the impact even caused the formation of the moon. There are some others believing that the tilting is a result of various gravitational pulls and in fact, after a long period of time (billions of years), the geographical poles changes causing a change in the seasonal patterns.

In summary, the main reason for having seasons on Earth is its tilt. It allows one hemisphere to have more sunshine during one part of the year and the other half to have more sunshine in the other part of the year. It has added to the beauty of the human society that people change with the pass of seasons and revive with each spring, rising up for a whole new row of seasonal changes.

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