Atmospheric Refraction and Twinkling of Stars – Explained with Examples

Atmospheric Refraction and Twinkling of Stars – When you look at the night sky, stars appear to twinkle, and the Sun seems visible even before it rises or after it sets. These beautiful phenomena are all results of a single optical effect — atmospheric refraction.

This article explains what atmospheric refraction is, why stars twinkle, why planets don’t, and how this effect causes the advance sunrise and delayed sunset, in simple and student-friendly language.

What Is Atmospheric Refraction?

Atmospheric refraction is the bending of light as it passes through the Earth’s atmosphere, which consists of layers of air with varying densities and refractive indices.

When light from a distant object (like a star or the Sun) enters the Earth’s atmosphere, it moves through these layers, each having a slightly different optical density. As light travels from one layer to another, it changes direction (refracts), causing the apparent position of the object to shift slightly.

Everyday Example

You might have noticed the flickering of objects above a hot surface, like a road on a sunny day or above a flame.
This occurs because hot air near the surface is less dense than the cooler air above it. The refractive index of hot air is slightly less, causing light rays to bend irregularly and make objects appear to waver or shimmer.

This small-scale wavering effect is the same principle as atmospheric refraction but on a local level.

Causes of Atmospheric Refraction

The Earth’s atmosphere is not uniform — its temperature, pressure, and density vary with height. These variations cause the refractive index of air to change continuously.

Key causes:

1. Change in air density: Light bends when it passes through layers of air with different densities.

2. Variation in temperature and pressure: Hotter air is less dense and has a lower refractive index. Cooler air is denser and has a higher refractive index.

3. Curvature of the Earth’s atmosphere: Light rays from celestial bodies bend gradually as they travel toward Earth.

Apparent Position Shift Due to Refraction

Because of atmospheric refraction, the apparent position of a celestial object is different from its actual position.

• The light from a star bends gradually toward the normal as it enters denser air layers near the Earth’s surface.
• As a result, the star appears to be slightly higher in the sky than its true position.
• This apparent displacement becomes more noticeable when the star is near the horizon.

Twinkling of Stars

Definition:

The twinkling of stars is due to the atmospheric refraction of starlight. As starlight passes through the Earth’s atmosphere, it undergoes continuous refraction because of changing air densities and temperatures.

Explanation:

• The atmosphere acts like a series of refracting layers, each bending starlight slightly.
• As the physical conditions (temperature, pressure, wind) change constantly, the density of these layers also changes.
• This causes the apparent position of the star to shift rapidly.
• The amount of light entering the observer’s eye varies — sometimes more (appearing brighter), sometimes less (appearing dimmer).
• This fluctuation in brightness gives the illusion that the star is twinkling.

Scientific Summary:

• The real position of the star remains fixed.
• The apparent position changes constantly due to variations in the refractive index of air.
• The amount of starlight entering the eye fluctuates, causing the twinkling effect (also called stellar scintillation).

Mathematical Idea (Qualitative)

If light rays from a star enter at slightly different paths due to changing air density, the direction of rays reaching your eyes keeps shifting. The angular shift (apparent motion) is small but noticeable because stars are point sources of light.

Why Do Planets Not Twinkle?

Planets are much closer to the Earth compared to stars and appear as extended sources of light rather than single point sources.

Each part of a planet emits light that passes through different atmospheric paths. The tiny variations in brightness from individual points average out, resulting in a steady image.

Hence, planets do not twinkle.

In Summary:

Reason	Stars	Planets
Distance from Earth	Very far (point sources)	Closer (extended sources)
Effect of Refraction	Continuous and variable	Averaged out
Twinkling Effect	Yes	No

Advance Sunrise and Delayed Sunset

Another fascinating effect of atmospheric refraction is that the Sun becomes visible to us before it actually rises and remains visible even after it sets.

Explanation:

• When the Sun is just below the horizon, its light rays entering Earth’s atmosphere bend toward the normal due to refraction.
• This bending makes the Sun appear slightly above the horizon, even though it is geometrically below it.

Thus:

• We see the Sun about 2 minutes before the actual sunrise.
• The Sun remains visible for about 2 minutes after the actual sunset.

Time difference: Approximately 4 minutes total (2 minutes early sunrise + 2 minutes delayed sunset).

Diagram Description (Figure 10.10)

When the Sun is just below the horizon:

• Its light refracts through the atmosphere.
• The observer sees the Sun at a slightly higher apparent position.
• This creates the illusion that the Sun has already risen (or has not yet set).

Additional Effect – Flattening of the Sun’s Disc

At sunrise and sunset, light from the lower edge of the Sun passes through a thicker layer of atmosphere than light from the upper edge.
This unequal refraction flattens the Sun’s disc, making it appear slightly oval.

Real-Life Examples of Atmospheric Refraction

1. Mirage: An optical illusion caused by refraction of light through hot air layers near the ground in deserts or on roads.

2. Looming: Distant ships or objects appear raised above their actual position on the sea due to refraction in cool air over warm water.

3. Starlight bending: Observing stars near the horizon shows them slightly displaced due to bending of their light path.

4. Early sunrise and late sunset: As explained above, caused by refraction of sunlight.

Important Facts to Remember

Concept	Explanation
Refraction	Bending of light when it passes through media of varying densities
Atmospheric Refraction	Continuous refraction of light through Earth's layered atmosphere
Twinkling of Stars	Caused by fluctuating refraction of starlight through turbulent air
Planets Don’t Twinkle	They appear as extended sources; light fluctuations average out
Advance Sunrise/Delayed Sunset	Sun appears earlier/later due to bending of its light rays
Flattened Sun	Unequal refraction at sunrise and sunset distorts the Sun’s shape

Summary of Key Points

• Atmospheric refraction occurs due to variation in air density and refractive index.
• It causes apparent displacement of celestial objects.
• Stars twinkle because of continuous refraction and change in apparent brightness.
• Planets don’t twinkle because they are closer and act as extended light sources.
• Atmospheric refraction also leads to advance sunrise, delayed sunset, and flattened Sun at the horizon.

FAQ

Q1. What is atmospheric refraction?

Atmospheric refraction is the bending of light as it passes through layers of the Earth’s atmosphere with varying densities.

Q2. Why do stars twinkle?

Because the atmosphere refracts starlight continuously, making its apparent brightness and position fluctuate.

Q3. Why don’t planets twinkle?

Planets are closer and act as extended sources of light, so fluctuations in brightness cancel out.

Q4. What is the cause of advance sunrise and delayed sunset?

Refraction of sunlight by the Earth’s atmosphere bends light rays, making the Sun visible even when it is below the horizon.

Q5. How much time difference does atmospheric refraction cause?

Approximately 2 minutes before sunrise and 2 minutes after sunset — a total of around 4 minutes.