The Solar Constant (SL IB Physics)

• Since life on Earth is entirely dependent on the Sun’s energy, it is useful to quantify how much of its energy reaches the top of the atmosphere
  • This is known as the solar constant, S

• The solar constant is defined as:

The intensity of the Sun's radiation arriving perpendicularly to the Earth's atmosphere when the Earth is at its mean distance from the Sun

• The average value of the solar constant is 1.36 × 10³ W m⁻²
• The value of solar constant varies year-round because:
  • The Earth is in an elliptical orbit around the Sun, meaning at certain times of year the Earth is closer to the Sun, and at other times of year it is further away
  • The Sun’s output varies by about 0.1% during its 11-year sunspot cycle

• Calculations of the solar constant assume that:
  • This radiation is incident on a plane perpendicular to the Earth's surface
  • The Earth is at its mean distance from the Sun

• The intensity of solar radiation received by different planets in the Solar System varies depending on distance from the Sun
  • For example, the intensity of solar radiation incident on Venus' atmosphere is higher than Earth's because it is closer to the Sun

Incoming Radiative Power

• The surface area of a planet, with radius r, equals the surface area of a sphere, 4πr²
• A planet's radiative intensity covers a cross-sectional area of πr² 
• So the mean value of the radiative power or intensity is: