Oblique dif­fer­ences in sun’s rays

The Star Early Edition - - OPINION & ANALYSIS -

IN THE north­ern hemi­sphere’s win­ter the Earth is closer to the sun, so why is it colder than sum­mer?

Imag­ine a beam of sun­shine as wide as the planet. When it reaches the planet some rays will strike the Earth full on, but as you go fur­ther north or south, the an­gle be­comes more oblique. The more oblique that an­gle, the larger the area that that unit of the sun’s en­ergy is spread around, which means those places will be less warm.

The an­gle is most oblique at the poles, mak­ing them the cold­est ar­eas on Earth.

There are two com­pli­cat­ing fac­tors: the tilt of the Earth, and its slightly el­lip­ti­cal or­bit around the sun – the Earth is about 3% closer to the sun in Jan­uary than in July.

The planet is also tilted roughly 23.5ºC to the imag­i­nary ver­ti­cal line run­ning from the North to the South Pole.

As we or­bit, the di­rec­tion in which this imag­i­nary line is point­ing changes grad­u­ally, like a spin­ning top whose spoke is slightly off-ver­ti­cal.

As the Earth or­bits the sun, grad­u­ally the area which re­ceives the most heat changes. Near the equa­tor, there are no great sea­sonal dif­fer­ences in tem­per­a­ture. As the sun’s rays strike al­most full-on all year round, it con­stantly ex­pe­ri­ences high tem­per­a­ture.

The tem­per­a­ture is thus an in­ter­play of two fac­tors: our dis­tance from the sun, and which hemi­sphere is tilted to­wards it.

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