WHY IS THE EARTH TITLED ? AND HOW SEASONS OCCUR ON THE EARTH ?

Have you often wondered why the earth is tilted instead of perpendicular to the orbital plane ( Plane in which earth orbit around the sun )? Because of inclination, our earth remains worth it for living. So what was the reason behind for tilt of the earth? This we are going to discuss today and also discuss about the consequence caused by this tilt of the earth. A lot of scientific theories have been given behind the Earth's tilt. But the most widely known theory is Giant Impact Hypothesis. It is also known as Big Splash or Theia Impact. 

According to this hypothesis, earth titled from the collision between proto-Earth ( Planet earth is in the early stage ) and Mars sized planet approximately 4.5 billion years ago. The Mars sized planet or colliding body is called as Theia, from the name of the Mythical Greek Titan who was the mother of Selene ( In Greek mythology, Selene is the goddess of the Moon ). According to astronomer, this collision happened between 4.4 to 4.5 bya ( bya abbreviate form of billion years ago ) after solar system began to form.  In astronomical term, the impact between the young earth and theia would have been at moderate velocity. When the earth was fully formed, Theia have struck at oblique angle (Any angle other than right angle ; may be acute and obtuse). Computer simulation are often used in the astronomy field to study the stellar collisions and black hole interaction. Computer simulation suggest that impact angle of Theia is nearly equal to 45 degree and velocity of Theia not more than 4 km/sec.

Core of Theia generally made up of iron. During this collision, iron core of Theia would have mixed with the young Earth,s core. This is the main reason why Earth's core contain iron. Due to the collision, mantle part of Theia or debris (small pieces of rock create due to the collision) would have been ejected onto the orbit around the earth. This is possible if ejected velocity is between orbital velocity (The minimum velocity required  to place a object or satellite in the given orbit) and escape velocity (The minimum velocity which is required by the object to escape from the earth gravitational field). If ejected velocity is higher then mantle parts of Theia or debris would have been ejected onto the orbit around the sun. Within few month not more than century, debris came together to form the moon. The moon would have formed like this. Due to the collision, the earth would have gained an angular momentum [The property of any rotating object which is defined as the moment of inertia (The tendency of rotating object to remain in the state of rest or uniform motion. It is also known as angular mass) times angular velocity (The change of angular displacement with respect to time is known as angular velocity)]. From this we can conclude that the earth is tilted like this.

Now we are going to discuss how seasons occur on the earth. Before discussing it, we have to study some concepts related to th Earth.
Latitude : Latitude is an angle which range from 0 degree at equator to 90 degree at north and south pole
Equator : An imaginary line which is around the middle part of the Earth. It is the midway between north and south pole. And it divides the Earth into northern and southern hemisphere.
Northern hemisphere : The half portion of the earth that is north side of the equator.
Southern hemisphere : The half portion of the earth that is south side of the equator.
Tropics : The region of the earth which lies in the middle of globe (earth). It lies between tropic of Cancer ( It is the line of latitude circling the earth at 23.5 degree north of equator) and tropic of Capricorn ( It is the line of latitude circling the earth at 23.5 degree south of equator).
Axial tilt : It is defined as the angle between the axis of rotation (It is an imaginary line which is passing through the north and south pole and about which earth rotates) and line which is perpendicular to the ecliptic orbital plane (Ecliptic plane means an imaginary plane containing the Earth's orbit around the sun.

Now let us discuss how seasons occur on the Earth. We know that the earth rotation and revolution period is 24 hours and 1 year respectively. The earth rotates from west to east. The earth is also tilted at 23.5 degree. Due to axial tilt, the north pole of the earth always point only in one direction. The sun rays fall on the half portion of the earth at once and another half portion of the earth is in dark. During this revolution of the earth around the sun in elliptical orbit, sometimes the north pole is pointing toward th sun and away from the sun. The variation of sunlight on the earth during year cause different seasons.

WHAT HAPPEN WHEN THE NORTH POLE IS POINTING TOWARD AND AWAY FROM THE SUN :

NORTH POLE IS POINTING TOWARDS THE SUN :

During this scenario, the virtual equator is tropic of cancer. The center which is denoted by the red dot lies on the tropic of cancer. ( 0 degree equator is actual equator of the earth but due to axial tilt it does not come in direct impact of the sun rays ). Red dot which lies on tropic of cancer directly comes in the impact of the sun as shown in above figure. In this condition, north pole receives the sun rays. But at same time south pole does not receive the sun rays. From this we can say that there will be a summer in northern hemisphere and winter in southern hemisphere. This scenario occurs when the earth is at the position A as shown in below figure. This condition is also known as Summer Solstice ( During summer solstice, day is longer and night is shorter ). It mainly occurs on 21-22 June.

NORTH POLE IS POINTING AWAY FROM THE SUN :

Now let us discuss opposite scenario when the earth reach at the opposite side of sun. During this scenario, the virtual equator is tropic of Capricorn. Orange dot which lies on tropic of Capricorn directly comes in the impact of the sun as shown in above figure. In this condition, north pole does not receive the sunlight. But at the same time south pole receive the sunlight. From this we can conclude that there will be a winter in norther hemisphere and summer in southern hemisphere. This scenario occurs when the earth is at the position B as shown in below figure. This condition is also known as winter solstice ( During winter solstice, night is longer and day is shorter ). It mainly occurs on 21-22 December.

When the earth is at the position C and D, both south and north pole receive sunlight. It means both northern and southern hemisphere are in the sunlight. This condition is known as Equinox ( During equinox, day and night are same). There are two equinox. Spring equinox  occurs when the earth is at the position C on 21-22 March. Autumn equinox occurs when the earth is at the position D on 22-23 September. Thus the seasons are created on the earth.