Advanced Sunrise and Delayed Sunset

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Sunrise and Sunset

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Have we ever wondered that we actually see the sunrise about 2 minutes and before the sun is actually at that perceived position? Did we ever know that the sunset that we actually see is of a sun that has already set? So why do these different perceptions occur? It is just because of a phenomenon which is termed as refraction of light.

What is refraction of light if we Simply put a light  ray that ‘bends’ when it travels from one medium to another. Depending on the density of the various medium the speed of the light which is travelling ray keeps varying and this causes it to speeded up or slow down therefore bending in the process takes place.

So how is this light refraction connected to our advanced sunrise and the delayed sunset? Imagine the situation or the journey of light rays from the sun. At first we shall see the journey of light is through vacuum and then through the atmosphere of the earth and then it is finally seen by us. At first the vacuum which is present will act as a rarer medium and the earth’s atmosphere with all its temperature changes or winds and different gases will be denser in that medium in comparison to the same.

During the process of sunrise the light rays bend due to our atmosphere and we see the sun early even though the sun is just below the horizon. Similarly at the time of sunset due to the same bending of light rays we can see the apparent position of the sun which is not the actual position.

By summing up all details which are due to refraction we can see or observe that the sun rises about two minutes before it’s actual time and sunset around two minutes later. even though it has already now moved from its initial position.


In physics the process of refraction is the change in direction of a wave which is passing from one medium to another medium or from a gradual change in the medium. How much a wave is refracted is determined by the changes in wave speed and the initial direction of the wave which is propagating relative to the direction of change in speed.

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Refraction of light can be seen in many places in our day to day life. It makes objects under a water surface appear closer and clearer than they really are. It is the thing on which  the optical lenses are based on, allowing for instruments such as cameras, glasses, microscope, binoculars and the human eye. Refraction is also responsible for some opticles which are natural phenomena including mirages and rainbows.

Atmospheric Reflection 

The air's refractive index depends on the air density and thus vary with temperature of air and pressure as well. Since the pressure is lower than expected at higher altitudes, the refractive index that RI is also lower causes light rays to refract towards the surface of earth when traveling long distances through the atmosphere. This whole process shifts the apparent positions of stars slightly when they are close to the horizon and makes the sun visible before it actually rises above the horizon during a sunrise.

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Temperature that varies in the air can also be the cause of refraction of light. This can be seen as a haze of heat when cool and hot air is mixed over a fire in engine exhaust, or when opening a window on a very cold day. This makes objects viewed through the air which is mixed air and which appear to shimmer or move around randomly as the cold or hot air moves. This effect is also widely visible from normal variations in air temperature during a day which is sunny day when using high magnification telephoto lenses and is often limiting the quality of the image quality in these cases.  In a similar way the turbulence of atmosphere gives rapidly varying distortions in the images of astronomical telescopes limiting the resolution of terrestrial telescopes but by not using adaptive optics or other techniques for overcoming these atmospheric distortions or the disorders.


The temperature of air variations which are close to the surface can give rise to other optical phenomena for example such as mirages and Fata Morgana. This makes the road appear reflecting in nature by giving an illusion of water covering the road.

It is a clinical test in which a phoropter may be used by the appropriate eye care professional to determine the reflective error of the eye and the best corrective lenses to be prescribed.

FAQ (Frequently Asked Questions)

Q1. What is the reason behind the Delayed sunset and Advanced sunrise?

Ans: The Advancement in sunrise and delayed sunset are the two phenomena that are caused due to the atmospheric refraction process. In this process the Sun appears to rise early by 2 minutes and set by 2 minutes late.

Q2. Why is the sun seen a few minutes before it actually rises while in the evening a few minutes longer after it sets?

Ans: From the sun When the rays of light enter the earth's atmosphere from outer space, they travel from a medium which is rarer medium to a denser medium.

Q3. What is the cause of apparent flattening of sun at Sunrise and Sunset?

Ans: The flattering of the sun at sunrise or sunset is due to refraction, because the sun becomes little visible a little before actual sunrise that  remains visible a little after actual sunset that is because of the atmosphere's refraction process of light.

Q4. Why is the sunrise visible before the sun?

Ans: The sun rising is visible to us before the actual sunrise and after actual sunset because of the process of atmospheric refraction of light. The rays From the sun which are coming through space enter from the rarer medium to the denser medium and hence bend slightly towards normal at each refraction point and become visible to us before actual sunrise time.