Read this Post in Kannada – ಈ ಲೇಖನವನ್ನು ಕನ್ನಡದಲ್ಲಿ ಓದಿ
What comes to our mind when we think about the first rays of sunlight at dawn?
If you thought of the word “Sunrise”, then yes! Sunrise, it is.
And when the last directly hitting rays of sunlight disappear, it is “Sunset”.
At some point in our lives, all of us have observed the Sun’s movement across the sky. We know that it’s actually the Earth that’s moving around the Sun while it also rotates on its own axis. It’s daytime on the part of the Earth that faces Sun and it’s nighttime on that part of the Earth that faces away from the Sun. We receive heat and light from the Sun which are very essential for life on Earth. This light from the Sun travels a distance of about 1AU (Astronomical Unit) which is equal to 1.495×1012 meters, before reaching our retina.
The sunlight travels through the empty space and through the Earth’s atmosphere. So it takes (1.495×1012 m)/(2.997×108 m/s), which is approximately 8 minutes and 20 seconds, for sunlight to reach us. The speed of light (c) in space (vacuum) is 299,792,458 m/s, but when the light enters the Earth’s atmosphere, it no longer travels at the speed ‘c’. It is less than c as it slows down once it reaches the Earth’s atmosphere.
The Illusion
We know that refraction is the bending of light when the light travels from one medium to another. Where in, the optical density of the media is not same, and which in turn makes the light slow down. The same happens when the light enters the Earth’s atmosphere from the empty space. We have to note that the optical density of the outer space is less than that of the earth’s atmosphere.
So now comes the illusion of the Sun you see at sunrise which is not the actual Sun. The Sun is still below the horizon and in reality, the Sun takes 5 minutes more to rise above the horizon. This means that the earth’s atmosphere bends sunlight and we see this apparent sunrise in the morning a little earlier than the actual one.
This is true with sunset too. Even at the time of sunset the sun has already set 5 minutes ago. So the Sun we see at sunset is an apparent one. The Sun is already below the horizon 5 minutes before we actually see it. Because of this beautiful phenomenon, we get few extra minutes of light at the time of sunrise and a few more extra minutes of light at the time of sunset. So if we wait for equinox, with equal day and equal night, i.e., 12 hours of day and 12 hours of night, we get 10 minutes extra light because of this phenomenon. So it’s actually equal daytime and nighttime a few days before and after Equinox. In fact the stars also undergo the same process.
In the figure below, red lines indicate the normal and tangent, while the green light indicates the light path in the absence of earth’s atmosphere.
