Solar Intensity Worksheet printable pdf download

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Because their distances from the sun vary, the planets do not

all receive the same intensity levels of solar energy. If you

could stand on Mercury and peer at the sun (wow, that’s

blinding!), it would look almost three times as big as it does

from Earth; and the intensity of solar radiation would be almost

Intensity

seven times that on Earth. At the farther reaches of the solar

system, say on Pluto, the effect would be quite different. There

the sun would appear as a very bright star, and the light level

would resemble being in a dimly lit room.

In terms of the physics involved, we know precisely how much sunlight falls on a planetary body

at any distance from the sun. There is an unchanging mathematical relationship: The intensity of

solar energy at any point in the solar system is inversely proportional to the square of its distance

from the sun, or, expressed in symbols,

∝

where is solar intensity and d is distance from the sun.

This principle is known as the inverse square law for light intensity. To see how this principle

works, imagine a rectangular card placed one meter from a point source of light –

a high-intensity bulb for instance. Now imagine a moveable wall

located exactly two meters from the light source on

which is projected the shadow of the card.

The shadow’s dimensions will be twice the

dimensions of the card, and the area of the

shadow will be four times the area of the card.

Move the wall back an additional meter, and you will find that the shadow has grown to three

times the dimensions and nine times the area of the card.

If you take away the card, the light that illuminates the area that used to be in shadow is the same

light that fell on the card before it was removed. What’s different is not the amount of light

reaching the card or the wall, but its intensity (brightness). Double the distance and the light

intensity is reduced to one-fourth the amount. Triple the distance and it is reduced to one-ninth,

and so on. In general terms, the amount of light stays the same, but it is spread over an

increasingly larger area as you move away from the light source, so the intensity is less.

When considering the intensity of solar radiation on the nine planets of the solar system, it is

sometimes useful to think of relative intensity, a ratio comparing the solar intensity to that on

Earth. For the reasons explained above, if a hypothetical planet were twice the distance from the

sun as Earth, the solar intensity would be one-fourth the solar intensity on Earth. For any planet,

this ratio, derived from the inverse square law, may be written as follows:

Relative solar intensity

At this point it is helpful to introduce the concept of astronomical unit (abbreviated AU), as its use

in the above equation simplifies the math. One astronomical unit is defined as the mean Earth-

sun distance, which is 149,597,870.66 kilometers (rounded to 150 million km), or 92,955,807.25

miles (rounded to 93 million milies). For reasons that are probably obvious, interplanetary

distances are often written in AUs.

Solar Intensity Worksheet

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