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.

3 hours because it is a decent distance.

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