Earth's solar constant a. is actually variable. b. refers to the intensity of solar radiation that reaches Earth's surface. c. is about 0.5 calories per square centimeter per minute. d. must be measured at perihelion. e. must be measured at aphelion.

a. is actually variable.

Explanation:

Earth's solar constant  -

It refers to the total amount of radiations coming from the sun per unit area and per unit time on the surface of the earth , is referred to as Earth's solar constant .

There is an increase of about 0.2 percent during the peak of every 11 year solar cycle  and hence ,

The amount is variable in nature .

Since , the sun is composed of hydrogen gas , which increases to around 10 percent on every billion year .

Hence , from the given question ,

The correct option is a. is actually variable.

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.