Caption: Just considering the lower panel of image is a plot of the solar constant 1978--1999.

Features:

1. The solar constant is the power per unit area at 1 astronomical unit (AU) from the Sun: i.e., at the mean Earth-Sun distance.

2. Note the circa-2023 reference mean solar constant = 1360.8(5) W/m**2 (see Wikipedia: Solar constant: Calculation; NASA: Earth fact sheet: solar irradiance, 2023). This value is NOT what the plot suggests which is ∼ 1366.5 W/m**2. The data in the plot probably is based on an out-of-date measurement of the mean solar constant.

3. The plot shows 2 full solar cycles.

   A solar cycle is an approximately sinusoid in the value of the solar constant (forgiving the contradictio in terminis = contradiction in terms). The period of the solar cycle is on average 11 years.

4. Smoothing over short-time variations (which are seen in the plot), the solar constant varies between a solar maximum (solar luminosity ∼ 0.05 W ≅ 0.05 % above average) and a solar minimum (solar luminosity ∼ 0.05 W ≅ 0.05 % below average). At solar maximum sunspots are most numerous and at solar minimum, they are least numerous.

5. The plot actually confirms during a full solar cycle that the solar constant varies by only by ∼ 0.1 % between maximum and minimum when the short term variations have been smoothed away---which is what the black curve shows.

   The rapid time variations in the plot are mostly due to sunspots. Each one causes a slight dimming of the Sun.

   The plot clearly shows that sunspot number variation between solar maximum and solar minimum by showing more rapid variations at the former than at the latter.

6. The near constancy of the solar constant is good for life on Earth as we know it. If the solar constant had strong variations, biosphere may NOT have evolved or would be very different somehow.

7. Actually, the solar constant does NOT include the major periodic variation in the solar power per unit area received at the top of the Earth's atmosphere.

   This variation is caused by the varying distance of the Earth from the Sun.

   The solar power per unit area received at the top of the Earth's atmosphere varies over about 6.9% during a year from a low of 1321 W/m**2 in circa Jul05 when Earth is farthest from the Sun to 1412 W/m**2 in circa Jan03 when Earth is closest to the Sun (Wikipedia Solar Constant; Wikipedia: Earth's orbit: Events in the orbit).

   If the distance variation were included, then the oscillations in the plot would go off the plot.

8. Without the distance variation, only the intrinsic variations in solar luminosity are manifested in the plot.