Now this concept, albeit difficult, is very interesting to understand. To give a background of how and why I came across it, let us do a quick flashback. Once upon a time, I travelled to a place in a different latitude than mine (for our purposes we will say I moved from around 25⁰ North to 5⁰ North ) and one night when I was walking home with a friend I noticed a star similar to one I recalled seeing before. This amused me, how could I still see a portion of the same stars when I had moved to a different country down south? I mentioned it to my friend who had travelled from the same place as I, and she did not believe me. So I tried to explain her using rocks and science why this was not bizarre at all. I obviously failed to convince her and now I’m going to try making up for it by explaining to you how and why this happens.
Before we begin, I won’t say this is an intermediate level problem. Any beginner in science can comprehend it given they can visualize what I am going to try to explain. The first thing to understand would be a snapshot of the universe we see at night time. Consider the following image.
So as you can see, in the above image we observe the earth rotating on its own axis and also orbiting around the sun. The spinning takes a full day and the orbiting takes a complete year. The above image shows two possible valid positions of earth in the orbit. One instance of the earth when its on the right side and another when its on the left side (6 months later). So far so good?
First consider the earth on the right side of the sun in the above image, if we fix the earth in that spot (pause the orbital velocity) and only allow it to rotate on its own axis. Night time will be when we are on the right side and daytime will be on the left side. What this means is that all the countries in the world will see the same stars snapshot of the night sky. The countries in the north will see an upper snapshot while the countries in the south will see the lower side. But no matter what we will only be able to see a snapshot of the stars on the “dark side” of the planet, as the sun will outshine the star systems observed on the bright side.
Hope everything is making sense so far. Now consider the earth on the left side of the sun in the above image. In this scenario, the same thing will happen but this time we can see the star system that was previously difficult to observe due to the sun blocking our view. We won’t be able to observe the same star system as before. So essentially as our seasons change, we can see a 360-degree view of the universe surrounding our solar system. As our world is spherical (not actually spherical), depending on which hemisphere you live in, you are able to see a different part of the picture.
If you didn’t comprehend that, try this. Hold up a piece of paper with some writing on it and fold it partially. Now, bring your face really close to the upper part you will see that no matter how much you move your eyes down you won’t be able to read the stuff on the bottom. Thus, depending on where you live on earth you are able to observe a different portion of the sky. What is amazing about that is if you were living in say Iran and your friend was living in Malaysia and another in Venezuela, all three of you would be able to see portions of each other’s night sky! Despite you all living in different latitudes, it would still happen! (maybe even more than half depending on how far up and down you are).
Here is another picture to explain the overlap.
Equipped with this information maybe you will be able to prove your obnoxious science hating friend wrong and experience the night sky in a different light (get it?).