Why Do We See the Sun Before Sunrise and After Sunset?
Have we ever wondered that we actually see the sunrise about 2 minutes 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 that 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 mediums the speed of the light which is traveling 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 moved from its initial position.
Refraction
In physics, the process of refraction is the change in direction of a wave that 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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The 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, allowing for instruments such as cameras, glasses, microscopes, binoculars and the human eye. Refraction is also responsible for some articles which are natural phenomena including mirages and rainbows.
Atmospheric Reflection
The air's refractive index depends on the air density and thus varies with the 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 the 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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The temperature that varies in the air can also be the cause of the 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 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 the 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.
Significance
The temperature of air variations that are close to the surface can give rise to other optical phenomena for examples 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.
FAQs on Advanced Sunrise and Delayed Sunset: Physics Made Easy
1. What is the primary reason for advanced sunrise and delayed sunset?
The phenomena of advanced sunrise and delayed sunset are caused by atmospheric refraction. As sunlight enters the Earth's atmosphere from space, it travels from a rarer to a denser medium. This causes the light to bend downwards. Due to this bending, we see the Sun's apparent position above the horizon about two minutes before it physically rises and about two minutes after it physically sets.
2. Can you explain what atmospheric refraction is and why it occurs?
Atmospheric refraction is the bending of light as it passes through the Earth's atmosphere. The atmosphere is not uniform; its density is highest near the surface and decreases with altitude. Because of these changes in density (and thus the refractive index), light rays from celestial objects like the Sun or stars bend continuously. This bending is most noticeable for objects near the horizon, as their light travels through a greater thickness of the atmosphere.
3. Why does the Sun appear oval or flattened during sunrise and sunset?
The apparent flattening of the Sun is also a result of atmospheric refraction. Light from the bottom edge of the Sun is refracted more strongly than light from its top edge because it passes through denser layers of air near the horizon. This greater bending makes the Sun's vertical diameter appear shorter than its horizontal diameter, giving it an oval shape.
4. How much extra daylight do we get because of atmospheric refraction?
Due to atmospheric refraction, the Sun is visible about 2 minutes before the actual sunrise and remains visible for about 2 minutes after the actual sunset. This results in a total of approximately four extra minutes of daylight each day. The apparent shift of the Sun's position is about half a degree, and since the Earth rotates 1 degree every 4 minutes, this 0.5° shift corresponds to a 2-minute time difference.
5. If Earth had no atmosphere, how would sunrise and sunset be different?
If Earth had no atmosphere, there would be no atmospheric refraction. Consequently:
- There would be no advanced sunrise or delayed sunset. The Sun would appear exactly when it crosses the geometric horizon and disappear the moment it sets.
- The transition from day to night would be abrupt, without any twilight.
- The sky would appear black, not blue, as there would be no air molecules to scatter sunlight.
- The Sun would appear perfectly circular at all times, without any flattening at the horizon.
6. What is the fundamental difference between the reflection and refraction of light?
The main difference lies in how light interacts with a medium:
- Reflection is the bouncing back of light from a surface into the same medium. For example, a mirror reflects light. The angle of incidence equals the angle of reflection.
- Refraction is the bending of light when it passes from one medium to another with a different density. For example, a straw in a glass of water appears bent due to refraction. This phenomenon is responsible for advanced sunrise.
7. Does atmospheric refraction affect our view of other celestial objects like stars?
Yes, atmospheric refraction affects all light from celestial objects. It causes stars to appear slightly higher in the sky than their actual position, especially near the horizon. It is also the reason why stars twinkle. The continuous changes in the density of atmospheric layers cause the light from a star to bend randomly and rapidly, making its apparent position and brightness fluctuate.
