Solar Constant Variation Calculator
Our planetary & earth system science calculator computes solar constant variation accurately. Enter measurements for results with formulas and error
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
S = S0 / (r/a)^2 + cycle_variation
S0 is base solar constant (1361 W/m2), r/a is normalized Earth-Sun distance accounting for eccentricity, cycle variation adds +/-0.5 W/m2 over 11 years.
Worked Examples
Example 1: Summer Solstice at 45N
Problem:June 21 (day 172) at 45N, standard eccentricity, solar minimum.
Solution:r/a = 1.0164, flux factor = 0.9679, S = 1361*0.9679+0.5 = 1318 W/m2\nDeclination = 23.44 deg, noon flux = 1318*cos(21.56) = 1226 W/m2
Result:Adjusted constant: ~1318 W/m2, noon flux at 45N: ~1226 W/m2
Example 2: Perihelion vs Aphelion
Problem:Compare solar flux at day 3 vs day 186.
Solution:Perihelion: r/a=0.9833, S=1361/0.9833^2=1408 W/m2\nAphelion: r/a=1.0167, S=1361/1.0167^2=1317 W/m2\nDifference: 91.9 W/m2 = 6.75%
Result:Perihelion: ~1408 W/m2, Aphelion: ~1317 W/m2, diff 6.75%
Frequently Asked Questions
What is the solar constant?
The solar constant is the incoming electromagnetic radiation per unit area at the top of Earth atmosphere perpendicular to the rays at one astronomical unit from the Sun. Its accepted value is approximately 1361 W/m2 as measured by satellite instruments. Despite the name it varies by about 0.1 percent over the 11-year solar cycle and by larger amounts on longer timescales. This variation, though small, has measurable effects on Earth climate system.
How does orbital eccentricity affect solar radiation?
Earth slightly elliptical orbit with eccentricity 0.0167 means it is closest to the Sun in January (perihelion) and farthest in July (aphelion). This causes total solar irradiance to vary by about 6.9 percent between these extremes, with flux around 1413 W/m2 at perihelion and 1321 W/m2 at aphelion. The inverse square law governs this variation so the Southern Hemisphere receives more intense solar radiation during its summer than the Northern Hemisphere does.
What causes the 11-year solar cycle?
The 11-year solar cycle is driven by periodic reversal and regeneration of the Sun magnetic field through the solar dynamo process. During solar maximum the Sun has more sunspots, flares, and coronal mass ejections with total luminosity increasing by about 0.1 percent. The magnetic field becomes complex before eventually reversing polarity. During solar minimum the surface is calmer with fewer sunspots and slightly lower irradiance. The full magnetic cycle is actually 22 years.
How is the solar constant measured?
The solar constant is measured by satellite instruments called Total Solar Irradiance monitors positioned above Earth atmosphere. Key missions include ACRIM, VIRGO on SOHO, TIM on SORCE, and TSIS on the International Space Station. These use active cavity radiometers that absorb all incoming radiation and measure heating with high precision. The accepted value was revised from approximately 1366 to 1361 W/m2 after the TIM instrument provided more accurate measurements starting in 2003.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy