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# Is sunlight a constant?

## Is sunlight a constant?

These measurements show that the solar constant is not constant. It varies with the 11-year sunspot solar cycle. Over billions of years, the Sun is gradually expanding, and emitting more energy from the resultant larger surface area.

What is called solar constant?

Solar constant, the total radiation energy received from the Sun per unit of time per unit of area on a theoretical surface perpendicular to the Sun’s rays and at Earth’s mean distance from the Sun. The value of the constant is approximately 1.366 kilowatts per square metre.

Why is the solar constant not really constant?

Now this solar constant is actually not a true constant. It varies by +/- 3% because of the Earth’s slightly elliptical orbit around the Sun, being larger when the Earth is at perihelion (currently the first week in January) and smaller when the Earth is at aphelion (currently the first week in July).

### What keeps the size of the Sun constant?

Through nuclear fusion, the sun is constantly using up the hydrogen in its core:Every second, the sun fuses around 620 million metric tons of hydrogen into helium.

Which is the correct definition of the solar constant?

The solar constant (GSC) is a flux density measuring mean solar electromagnetic radiation (solar irradiance) per unit area.

How does the distance of the sun affect the solar constant?

This is because the solar constant is evaluated at a fixed distance of 1 Astronomical Unit (au) while the solar irradiance will be affected by the eccentricity of the Earth’s orbit. Its distance to the Sun varies annually between 147.1·10 6 km at perihelion and 152.1·10 6 km at aphelion.

#### How is the Earth’s distance from the Sun expressed?

So Earth is 1 AU from the Sun and receives 1 solar constant. This will help keep the math easy. The relationship can be expressed most simply as: 1/d^2 (one over the distance squared) where d = distance as compared to Earth’s distance from the Sun (for our first examples). Let’s start with sunlight as an example.

How to calculate the amount of sunlight reaching the ground?

To calculate the amount of sunlight reaching the ground, both the eccentricity of Earth’s elliptic orbit and the attenuation by Earth’s atmosphere have to be taken into account. The extraterrestrial solar illuminance ( Eext ), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation by