Advanced Sunrise and Delayed Sunset: Understanding the Science behind it

Advanced sunrise and delayed sunset are natural phenomena that are caused by atmospheric refraction. Because of this effect, we see the Sun a little earlier before its actual time of sunrise and a little later after its actual time of sunset. You might be amazed to know that these phenomena make the duration of daylight longer than it would be without the Earth's atmosphere.

In this article, you will learn the real reason behind advanced sunrise and delayed sunset, what is atmospheric refraction and other important factors that affect sunrise and sunset timings.

Table of Contents

• What is Refraction and Atmospheric Refraction
• What is Advanced Sunrise and Delayed Sunset
• Reason Behind Advanced Sunrise and Delayed Sunset
• Other Important Factors Affecting Sunrise and Sunset
• Frequently Asked Questions on Advanced Sunrise and Delayed Sunset

What is Refraction and Atmospheric Refraction

Refraction is nothing but the bending of light when it passes from one transparent medium to another. This bending occurs because the speed of light changes when it moves between different media, such as air, water, or glass.

Atmospheric refraction is the bending of light as it passes through the different layers of the Earth's atmosphere. Since the density of air changes with altitude, light rays continuously bend while travelling through the atmosphere. 

Do you know? Because of this effect, objects such as the Sun and stars appear slightly higher than their actual positions.

Atmospheric refraction is responsible for several natural phenomena, such as advanced sunrise, delayed sunset, twinkling of stars, and the apparent flattening of the Sun near the horizon. 

What is Advanced Sunrise and Delayed Sunset

Have you ever noticed that we can see the Sun even before it actually rises above the horizon? Similarly, the Sun remains visible for a short time even after it has set. These phenomena are known as advanced sunrise and delayed sunset.

The main reason behind advanced sunrise and delayed sunset is atmospheric refraction. As sunlight travels through the Earth's atmosphere, it bends because different layers of air have their own different densities. This bending makes the Sun appear higher than its real position.

In the diagram above, we can notice that the apparent position of the Sun is higher than its actual position near the horizon. And this happens because light bends when it's travelling through layers of air with different densities. So, our eyes see the Sun in a position where it is not actually located. Interesting, isn't it? 

You might wonder, after knowing that, because of this effect, 

• The Sun appears about 2 minutes earlier than its actual time of sunrise. This is an advanced sunrise.
• The Sun remains visible about 2 minutes after its actual time of sunset. This is a delayed sunset.

Thus, atmospheric refraction increases the apparent duration of the day by about 4 minutes.

Read More: Refraction of Light 

Reason Behind Advanced Sunrise and Delayed Sunset

Have you ever noticed that we can see the Sun a little before it actually rises and a little after it has already set? This interesting phenomenon is known as advanced sunrise and delayed sunset. This happens because of atmospheric refraction, which bends the Sun’s rays as they pass through the atmosphere of our planet, Earth.

Reason Behind Advanced Sunrise 

During sunrise, the Sun is still slightly below the horizon. However, the light coming from the Sun passes through several layers of the Earth's atmosphere before reaching our eyes.

The atmosphere is denser near the Earth's surface and becomes less dense at higher altitudes. As sunlight moves from denser air to rarer air, it bends continuously due to atmospheric refraction.

Our eyes always assume that light travels in a straight line. Therefore, when the refracted rays enter our eyes, we trace them back in a straight line and perceive the Sun to be at a higher position than it actually is.

As shown in the above diagram, the observer sees the apparent position of the Sun above the horizon, even though the real Sun is still below it. Because of this effect, the Sun appears to rise about 2 minutes earlier than its actual geometrical sunrise. This phenomenon is known as advanced sunrise.

Reason Behind Delayed Sunset 

A similar effect occurs during sunset. After the Sun has moved slightly below the horizon, its rays continue to pass through the Earth's atmosphere and get refracted towards the observer.

Due to atmospheric refraction, these bent rays reach our eyes even when the Sun is no longer physically visible above the horizon. Since the human eye interprets the rays as travelling in straight lines, the Sun appears to be above the horizon.

As a result, we continue to see the Sun for about 2 minutes after its actual sunset. This phenomenon is known as delayed sunset.

But why does this happen

We know that the main reason behind both advanced sunrise and delayed sunset is atmospheric refraction. Since the Earth's atmosphere contains different types of layers of air with different densities, sunlight bends continuously while travelling through them. 

Interestingly, this bending changes the apparent position of the Sun, making it appear higher than its actual position near the horizon.

Because of advanced sunrise and delayed sunset, the length of the day increases by approximately 4 minutes (about 2 minutes earlier sunrise and 2 minutes later sunset). This is why the Sun remains visible for a little longer than its actual position would suggest.

Other Factors Affecting Sunrise and Sunset

Now we know that atmospheric refraction is the main reason for advanced sunrise and delayed sunset. But there are several other factors that also influence the timing of sunrise and sunset.

Earth's Axial Tilt

The Earth is tilted at an angle of about 23.5° on its axis. This tilt causes different seasons throughout the year. During summer, a region receives sunlight for a longer period, resulting in earlier sunrises and later sunsets. In winter, the duration of daylight decreases, leading to later sunrises and earlier sunsets.

Latitude

The location of a place on Earth also affects the length of the day. Areas closer to the poles experience greater variations in daylight hours throughout the year compared to places near the equator. As a result, sunrise and sunset timings change more significantly at higher latitudes.

Elevation

People living at higher altitudes, such as on mountains or hills, can see the Sun earlier in the morning and later in the evening. This happens because their horizon is lower than that of an observer at sea level, allowing them to view the Sun for a longer time.

Earth's Orbit Around the Sun

The Earth revolves around the Sun in a slightly elliptical orbit rather than a perfect circle. This causes small variations in the timing of sunrise and sunset during different parts of the year. Although the effect is not very large, it contributes to seasonal changes in daylight duration.

And together, these factors, along with atmospheric refraction, influence the real timing of sunrise and sunset observed from different locations on Earth.

So far, we have understood that the atmosphere of Earth bends light that is coming from the Sun, making the Sun appear earlier at sunrise and later at sunset. The interesting reason behind advanced sunrise and delayed sunset is atmospheric refraction, apparent flattening of the Sun, and the factors that affect sunrise and sunset timings. These magical phenomena show us how the Earth's atmosphere gets influences the way we observe the Sun every single day.