PERIHELION AND APHELION

The orbit of the earth is an ellipse. Its length is 93,80,51,827 kilometres. The sun is located in the centre or in the focus of the ellipse.

  • Perihelion: In the process of revolution around the sun, the earth on 1st to 3rd January reaches in such a position that the distance between the sun and the earth becomes the lowest. It is about 14 crore 65 lakh 83 thousand 5 hundred kilometres. This position of the earth on that date is known as Perihelion. In Perihelion position the axis of the earth lie aslant outside the orbit.
  • Aphelion: As the orbit of the earth is ellipse, the distance between the sun and the earth does not remain fixed for all the time of the year. Sometimes the distance increases and other times it decreases. In the first half of July, particularly on 4th July, the distance between the sun and the earth becomes the highest being 15 crore 14 lakh 20 thousand and 500 kilometres. This position of the earth is known as Aphelion.
perihelion and aphelion

 

The process of changing of season

Normally, we know that there are four seasons in the earth viz., the summer, the autumn, the winter and the spring. Now we will try to understand how the changes of season do occur. The explanations for the change of seasons may be obtained from four situations of the earth during revolution around the sun.

  1. Summer in the Northern Hemisphere and winter in the Southern Hemisphere: We know that the North Pole comes nearer to the sun once in a year. During revolution on her orbit, the north pole of the earth after 21st March, starts showing inclination towards the sun. Consequently, a large portion of the northern hemisphere is being illuminated by the rays of the sun. The area of illumination gradually increases. As a result, the days become longer and the nights shorter in the northern hemisphere. As the days become longer the sun gives its rays for a longer time in the northern hemisphere. The earth gets more time to receive heat and the heated earth in turn makes the surrounding air to be hot. Nights are short and the amount of radiation from the soil is less in comparison to the amount of heat that is stored during day time. Thus hot weather prevails in the northern hemisphere and so it is summer there. At this time opposite condition prevails in the southern hemisphere. Since the southern hemisphere remains far from the sun and it gets sunshine for a shorter time. So the nights are long and the days are short. The earth radiates the heat which she has stored at day time. As a result, the earth becomes cold. Cold weather prevails in the southern hemisphere and it is winter there.
  1. Autumn in the Northern Hemisphere and spring in the Southern Hemisphere: After 21st June, the length of the day starts decreasing and the length of night get increasing in the northern hemisphere. But until 23rd September in comparison to night, the length of the day still remains shorter. The North Pole which is inclined towards the sun starts moving away from the sun and from a distant position the southern hemisphere gradually inclined towards the sun i.e. the South Polar Region comes nearer to the sun. Consequently temperature gradually decreases in the northern hemisphere and it increases in the southern hemisphere. This condition is known as autumn in the northern Hemisphere and spring in the southern hemisphere. On 23rd September the day and night are equal throughout the world.
  1. Winter in the Northern Hemisphere and summer in the Southern Hemisphere: We know that after 23rd September, the southern hemisphere starts showing inclination towards the sun. One can perceive that every day the sun moves southward. This is known as southwardly movement of the sun. In comparison to night, the days become longer in the southern hemisphere as the South Pole shows inclination towards the sun. As the sun’s ray fall directly on the Tropic of Capricorn, the southern hemisphere gets sun rays for a longer period of time. Consequently, the temperature increases in the southern hemisphere. On 22nd December, the South Pole approaches to the nearest position towards the sun and it is the longest day in the southern hemisphere. So, one and half months preceding 22nd December and the same period following that date the temperature remains high in the southern hemisphere. Naturally, during this period, the temperature remains very low in the northern hemisphere. So, one and half months preceding 22nd December, it is summer in the southern hemisphere and it is winter in the northern hemisphere.
  2. Spring in the Northern Hemisphere and autumn in the Southern Hemisphere: The sun starts moving towards the north after 22nd December. The length of the day starts decreasing in the southern hemisphere and increasing in the northern hemisphere. Consequently, the temperature increases gradually in the northern hemisphere and decreases in the southern hemisphere. Again day and night become equal in duration on 21st March. On this date the sun’s position is vertical on the equator. This is known as autumnal equinox. This is spring in the northern hemisphere and autumn in the southern hemisphere.