All of us know that earth rotates around its axis in 24 hours which, generally, contains 12 hours of daylight and 12 hours of night. The length of day and night varies, seasonally. The days become longer during the summer months and shorter during the winter months. However, there are two regions on earth which experience six months of day and night. These two regions are northern and southern polar region. Why? To answer that question, one must understand the process of change of seasons on the earth.
Change of Seasons on the Earth
- Earth revolves around the Sun in about 365.25 days.
- Earth is inclined on its axis at and angle of 66.5° from its orbital plain and 23.5° from the perpendicular line with respect to orbit (Fig.1).
- This inclination of earth’s axis along with revolution around the Sun leads to change of seasons.
- For instance, during summer in northern hemisphere in May-June, the earth’s northern hemispheres faces towards the Sun and the rays of Sun fall vertically over Tropic of Cancer.
- During winter in northern hemisphere in December-January, the earth’s southern hemisphere faces the Sun and rays of Sun fall vertically over Tropic of Capricorn.
- This means that when the north hemisphere experiences summer, there is winter in the southern hemisphere. Similarly, when the northern hemisphere experiences winter, there is summer in the southern hemisphere.
- When the earth revolves around the sun, the relative position of sun moves between tropic of cancer and tropic of Capricorn.
Why Six Months of Day and Night?
After studying the facts in previous section, we can now answer the question, “why there is six months of day and night in the polar region?”
- When the Sun’s position is in north of equator (21st March to 23rd September), the Sun rays reach the northern pole for 6 months. There is no night during this period because the north pole is facing the Sun as shown in fig.2. Similarly, the south polar region stays away from the sun, so, there is no day.
- A total opposite of the above situation happens when the Sun’s position is south of equator from 23rd September to 21st March. During this period, the south pole faces Sun and there is no night in the south pole even when the earth is rotating (see fig. 2)
- It must be noted that the difference between day and night is negligible near the equator during different seasons of the year. However, this difference increases as we move from equator towards the poles.
- This means that the during the summer in northern hemisphere, the days become larger and nights shorter as we move towards the pole. The hours of the daylight which are generally 12 hours at the equator becomes 24 hours at the poles during the summer. Similarly, during the winter season, the night time grows longer as we move from equator towards the pole. The night grows from 12 hours at the equator to about 24 hours at the north pole in winter season.
- Please note that position of the sun moves from tropic of cancer to Capricorn because of revolution of the earth around the Sun. Sun does not move itself.
To watch the animation of day and night at the pole during different seasons, Watch the following Video.
To conclude, we can say that 6 months of day and night is possible because the earth’s axis is inclined. In reality the 6 months of day is not really 6 months of day. It is so because there are few minutes or hours of night time in each day. The true 24 hours day or 24 hours night is possible on summer and winter solstices i.e. 21st June and 22nd December. The days and nights are not as noticeable as they are noticeable in tropical or temperate regions. So, we declare it as 6 months of day or night.
Kulwinder Singh is an alumni of Jawaharlal Nehru University, New Delhi and working as Assistant Professor of Geography at Pt. C.L.S. Government College, Kurukshetra University. He is a passionate teacher and avid learner.