We all know that the earth revolves
around the sun in an elliptical path.
Most of the people believe that summer happens
when the earth is closer to the sun
and winter happens when the earth is farther
from the sun in its elliptical path.
If you are one among them, "you are wrong".
You might have noticed that the sun is not rising
at the same point in the east throughout the year.
It changes its position gradually in the east.
But do you know that this change in sun rise location
is the reason for the change of seasons on earth?
All of you would have studied that the earth is tilted
23.5 degrees in its axis.
We will look at the reason for this tilt
at the end of this video.
Tropic of cancer is the highest latitude
in the northern hemisphere beyond which
earth doesn't receive vertical solar radiation.
Sun rays hit the tropic of cancer 
vertically on 21st of June every year.
Corresponding latitude in the southern 
hemisphere is called the Tropic of Capricorn.
The Tropic of Capricorn receives 
vertical solar rays on 22nd of December.
If you look at the map or a globe,
you can see these latitudes 
are marked with 23.5 degrees
which is equal to the amount 
of earth's tilt to the vertical axis.
When the earth's northern 
hemisphere is tilted towards the sun,
it gets more amount of solar 
radiation than that of the south.
As vertical rays travel less 
distance in the earth's atmosphere,
it is scattered less and the hemisphere 
of the earth facing the sun receives more heat energy.
It leads to summer season in the northern hemisphere
and winter season in the southern hemisphere.
During the summer season in the northern hemisphere,
the Arctic pole continuously receives 
sunlight for a period of 6 months.
If you are staying beyond the 
latitude of 66.5 degrees North,
you can't differentiate between day and night
and you will need the help of a 24 hours watch.
As Norway is located closer to the Arctic circle,
it is called the land of midnight sun.
Sameway, summer happens in the southern hemisphere
when the sun rises vertically above 
the tropic of Capricorn on December 22.
The Antarctic circle continuously 
receives sunlight for 6 months.
When this is happening in the southern hemisphere,
it's winter in the northern hemisphere.
So what about the other two 
seasons, spring and autumn?
During September, the sun rises 
directly above the equator.
Months of September to November are 
autumn in the North and spring in the South.
During March, again the sun 
rises directly above the equator.
Months of March to May are spring season 
in the North and autumn season in the south.
Autumn is the season when plants shed their 
leaves in preparation for the upcoming winter.
Spring is the season of rejuvenation, 
renewal, rebirth and regrowth.
These two seasons are majorly felt in the 
temperate regions of the two hemispheres.
As this changing effect of sun rays 
is not felt in the equatorial region,
it doesn't witness different seasons and 
it is warm and humid throughout the year.
Duration of days and nights are 
almost the same in equatorial regions.
Now it's time to look at the reason for the earth's tilt.
Earth’s tilt angle was accurately measured 
as early as 1100 BC in India and China.
The ancient Greeks had good measurements 
of obliquity since about 350 BC.
Scientists believe that the earth was hit by a big object 
called 'Theia' during its initial phases of formation.
That impact led to the tilt of the earth.
It also created a big hole and sent a huge 
amount of dust particles and rubble into the space.
These dust particles slowly combined 
to form a satellite planet and we humans,
named it as the moon.
What would have happened to the 
earth if it had not faced that impact?
We won’t have changing seasons across the year
and there will be no season 
changes like the equatorial region.
