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.

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periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.