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The Question

(Submitted February 21, 1998)

I have heard two conflicting reasons explaining why winter is cooler.

(1) Because of the slight pivot of Earth's on its axis, the Sun is farther away during winter because part of the planet is pointing away from the Sun, hence, less energy reaches that surface.

(2) The Sun is actually CLOSER to the surface during winter but light hits the planet at an obtuse angle which "skims" the surface. Direct rays are not hitting the surface which brings about cooler temperatures.

Which explanation is correct?

The Answer

The second. Winter is colder because the earth's axis is tilted. Winter occurs for the hemisphere which is tilted away from the Sun (the northern hemisphere in January, the southern in July). This has two main effects on the winter hemisphere. First, the Sun is above the horizon for fewer hours each day, so that hemisphere receives less heat from the sun. Also, sunlight strikes the ground at a shallower angle so that less energy per unit area is intercepted by the winter hemisphere. It is true that the earth is closest to the Sun in January. However, the distance from the earth to the Sun varies by only about 2% over a year. This causes a change of only 4% in the amount of solar radiation hitting the earth so this effect is not significant compared to the other two.

Damian Audley and Kevin Boyce
for Ask an Astrophysicist.

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