Solar Flux at Distance Calculator
Calculate the solar irradiance at any distance from the Sun.
Compare to Earth's solar constant of 1,361 W/m².
Includes planet presets.
Solar flux (irradiance) is the power per unit area received from the Sun at a given distance.
Formula:
F = L☉ / (4πd²)
Where:
- L☉ = solar luminosity = 3.828 × 10²⁶ W
- d = distance from the Sun (meters)
- F = irradiance (W/m²)
The inverse-square law: Because light spreads out spherically, doubling the distance reduces flux by a factor of 4. This is why Mercury is much hotter than Mars — not just because it’s closer, but because it’s inside closer to the Sun.
Simplified formula:
F = 1361 W/m² × (1 AU / d)²
where the solar constant (1,361 W/m² at 1 AU) is measured above Earth’s atmosphere.
Solar flux at each planet:
| Planet | Distance (AU) | Flux (W/m²) |
|---|---|---|
| Mercury | 0.387 | 9,082 |
| Venus | 0.723 | 2,601 |
| Earth | 1.000 | 1,361 |
| Mars | 1.524 | 586 |
| Jupiter | 5.203 | 50.3 |
| Saturn | 9.537 | 14.9 |
| Uranus | 19.19 | 3.69 |
| Neptune | 30.07 | 1.51 |
Equilibrium temperature: A body at distance d with albedo A reaches an equilibrium temperature:
T_eq = (F(1-A)/(4σ))^(1/4)
where σ = 5.67 × 10⁻⁸ W/m²/K⁴ (Stefan-Boltzmann constant).
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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