The Reasons For Seasons
– By Rishabh Dev for HGS
PS : The topic has been created as a webpage as well as converted to a pdf format since powerpoint was not installed on the system and the browser was outdated and did not support the online presentation software. A horizontal rule is used to mark the end of a slide.
The Reasons For Seasons
All of us have observed the weather changes and how these changes occurring in specific periods called seasons come and go every year. The objective of this text is to look into why these changes occur – or in other words, what causes the seasons.
Back To School : The Hemispheres
To understand how seasons change, we must first review a few fundamentals of geography we learnt at school. The Earth can be divided into four regions called hemispheres – namely the Northern, Southern, Eastern & Western hemispheres. These are shown in the image below:
You will soon be able to relate the hemispheres to the change in seasons, but at this point, if we say that the Northern & Southern hemispheres have opposite seasons, then we’ll just be getting ahead of ourselves.
Fun Activity : For now, let’s just try and figure out which hemisphere we live in! Try finding out which part of the hemisphere your country belongs.
Back To School : Latitudes & Longitudes
Fun Activity : Try to remember how you used to find a point on a map. Try drawing a map and finding the point (a, b) on it. See how it works. You may even get a graph paper to get a more intuitive idea of what we’re trying to say here!
Just like we find a point on a map, we need to be able to locate and mark or refer to a point on the earth. Hence, we need to first divide the earth into lines to form a grid just like we do in a map. Now, note that the earth is not flat like most maps! Hence, not all lines will be parallel to each other. The two kinds of lines to form the grid are called Latitudes (horizontal) and Longitudes (vertical).
The Latitudes – Latitudes or parallels are horizontal lines that run parallel to each other. A degree of latitude means about 111 ‘human’ kilometres.
The Longitudes – These are the vertical lines. Since the earth is not flat – but a sphere, these lines converge at the poles – and hence, are not parallel to each other.
Fun Tool : Try this out! You can find the latitude and longitude of any location using this tool – http://www.worldatlas.com/aatlas/latitude_and_longitude_finder.htm
Back To School : The Equator & The Prime Meridian
Now, We can imagine a line (latitude) that divides the earth into the Northern & Southern Hemispheres. This imaginary line is called the Equator and is the 0º latitude at the center of the earth.
Similarily, we can also imagine a line (longitude) that divides the earth into the Eastern & Western hemispheres. This imaginary line is actually a combination of two lines – called the Prime Meridian at 0º and the prime meridian at 180º.
The Earth’s Axial Tilt
There are two kinds of motions that are observed with respect to the earth – rotation about it’s axis and revolution around the sun. Both of these factors contribute to the change in seasons – the axis as well as the orbit around the sun.
Myth :
Note that it is a myth that the elliptical orbit of the earth causes seasons. If this would’ve been the reason, both the Northern and the Southern hemispheres would have the same seasons at the same time. Moreover, there is not much of the difference in the length of the two axes of the earth’s orbit abound the sun. Hence, it is not about the shape of the orbit.
Now Note that the axis of the earth is the line about which it rotates. And this axis, in case of the earth, has a 23.4º tilt. This means that the earth will always be tilted by an angle of about 23.4º when it revolves around the sun.
Creating The Four Seasons
We are now ready to understand how the seasons really change. Keep in mind the tilt of the earth as well as the hemispheres that we talked about. (Remember?) For now, consider the Northern Hemisphere region. Now, as the earth moves around the sun, there will be a period of time when the northern hemisphere will be tilted towards the sun and another time when it will be tilted away from the sun.
Important Concept : A concentrated beam of light gives more heat than a dispersed beam of light. Take your flashlight as an example.
Case 1 : Northern Hemisphere tilted towards the sun (gets concentrated sunlight) & Southern titled away from the Sun – Means Summers in the Northern Hemisphere & Winters in the Southern hemisphere.
Case 2 : Northern Hemisphere tilted away from the sun & Southern titled towards the Sun – Means Winters in the Northern Hemisphere & Summers in the Southern hemisphere.
Case 3 & 4 : When the earth is neither tilted towards or away – both hemispheres receive equal emounts of dispersed sunlight. These are called the Spring & Autumn seasons.
There you go! Here’s the answer to why the northern & southern have opposite seasons – and the answer to why we have seasons at all!