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Current time:0:00Total duration:9:04

Video transcript

did you know we get to see the Sun about two minutes before the actual sunrise and it's still visible about two minutes after the actual sunset and also the Sun looks pretty squashed during the sunrise or sunset so why are these things happening well the short answer is because our atmosphere refracts light so in this video we'll explore this in detail so let's start with something that we already know we might already know that the earth keeps spinning around its own axis and that's what causes the day and the night but since we are also spinning along with the earth we don't see that instead from our perspective it feels as if it's the Sun this is the Sun and the Sun is very far away that's why it's looking smaller to us but from our perspective it looks as if it's the Sun that's going around us and that's why we use the word sunrise and sunset but at what position does the Sun appear to be rising for us that's what we're interested in right we want to know what happens at the sunrise or the sunset and you may also also know the answer to this well you may already know that when things are below the horizon we don't see them and then things are above the horizon we do see them which means when the Sun just hits the horizon somewhere over here that's when we see it rising and horizon is just a tangent line that we draw to our Earth's surface notice a tangent line that we draw to our Earth's surface at the place that where we are standing okay and so when things are below it like right now the Sun is below that we can't see it because if we try to draw any rays of light from the Sun let's say we try to draw rays of light from the top of the Sun to the point where we are standing then these rays of light will not reach us and the reason is because they're gonna hit somewhere over here so when any object let's let's try let's try to hit draw this it's not very accurate but you can sort of see it right when many object is below the horizon anyway of light from there it cannot reach our eyes because it's going to hit somewhere over here and that's why right now the Sun is not visible to us and what would we be seeing right now well we would be seeing something like this we don't see the Sun because the Sun is not visible and for us the horizon is the part where the sky and the land or the ocean meets so this represents the horizon for us so right now the Sun is below the horizon and when say the Sun comes somewhere over here now the sunrise has happened so what we would see is the Sun has just started rising like this now so far we didn't talk about the atmosphere of the earth so let's bring the Sun back down the horizon and let's say here is our atmosphere and let's again draw a couple of rays of light from the top of the Sun let's zoom in and look at this so these rays of light would have if they're if them it wasn't for the atmosphere these rays of light up just gone straight and they would have missed our eyes and you wouldn't be able to see the Sun like we discussed before but now these rays are entering from vacuum into our atmosphere and as a result the rays of light the light is going to slow down because light has the fastest speed in vacuum when it enters into any other medium from vacuum it's going to slow down and because it's gonna slow down if we draw a couple of normals over here this light will bend towards the normal so if we see the refracted light these are a couple of normals notice the light has bend towards the normal because it has slowed down and if you're wondering why why are the light bendings towards normal if this slowed down well then we've talked a lot about this in in previous videos on refraction so if you are not clear about this it'll be a great idea to go back and watch that video and then come back over here but now what's important to see is that these rays of light have bend down and so they will reach our eyes and so to understand what we see now we have to just backtrack these rays so it appears to us as if these rays of light are coming from let's zoom out now are coming from somewhere somewhere over here somewhere over here and as a result we will feel as if the top of the Sun is somewhere over here and as a result we will see that Sun somewhere over there so that Sun will look somewhere over here and so notice even when the Sun is below the horizon because the rails of light are bending towards us we see the Sun above the horizon in other words in other words even before the actual Sun Rise we see the Sun has risen so because of the atmosphere the Sun is actually below down over here but because the rays of light are bending towards us right now we are seeing the Sun somewhere over here and that's why we say that the Sun rise we get to see the Sun Rise even before the actual Sun Rise and similarly this also explains why we get to see the Sun after the sunset once the Sun has set which means it has gone below the horizon because the light bends and appears to other reaches our eyes because of that we still get to see the Sun and that's why the Sun Rise happens earlier and the sunsets are always delayed and if you're wondering exactly how earlier how delayed we see the sunrise or the sunset then if we do the calculation it turns out it's about a couple of minutes so the sunrise we get to see the sunrise a couple of minutes earlier and the sunset will be delayed by a couple of minutes so long story short due to our atmosphere bending the rays of light towards us downwards towards us the Sun appears higher in the sky and this is not just for the Sun it will happen for any celestial body so for the moon or the planets or even the stars and by the way this effect is the strongest when the objects are near the horizon again if we zoom in we can see that we only when the objects are near the horizon that's when the incident light makes a very strong angle with a normal and if the objects come overhead us so let's say during the noon time when the Sun is right above us now notice the incident light is almost perpendicular surface and as a result they would hardly Bend and so they'll be hardly any shift in the in the objects position so usually okay let's go back our atmosphere makes the objects appear higher in the sky and the effect is the strongest near the horizon are to be more specific the lower the object in the sky stronger is the shift in the object of course if the object gets too low then eventually the light will not Bend enough to reach our eyes but in general the lower the object more is this shifting effect and as the objects rise higher and higher in the sky the shifting effect becomes smaller and eventually when the object is at the highest point in the sky right above us the shifting effect is almost zero and this also explains why sailors are in the earlier times who used to use the location of the stars to navigate around would not look at the stars at the horizon for their navigation because of the same reason because the Stars which are very close to the horizon have a huge shift in their position and so that would cause a huge error in their navigation all right so the last thing is why does the Sun appear squashed during the sunrise or the sunset when it's close to the horizon the concept is exactly the same so can you pause and try and think about this for a while all right let's see now we know that the Sun is actually below the horizon and due to the atmosphere it appears higher in the sky but remember the effect is stronger as the objects are lower and lower in the sky did we just discuss that lower the object stronger the effect now the bottom part of the Sun is Lord in the sky compared to the top part isn't it because the bottom is lower and as a result the bottom part of the Sun will shift a little bit more or I will appear a little bit higher in the sky than the top part so the shift is not the same so the top got shifted this much distance the bottom will not get shifted the same distance the bottom might get shifted a little bit more and as a result that Sun will appear squash to us this and this is the reason why astronomers hate to take photos of celestial bodies when they are close to the horizon