What Would Happen If The Earth Was Displaced Slightly?


The Sunday Nation


15 May 2005


A radio advert of a certain brand of bottled water says; “If the Earth was moved slightly closer the sun, IT would all evaporate, and if the Earth was moved slightly away, IT would all freeze out. IT is water…” Millicent Onyiego would like to know if this is true? The straight answer is no!

First of all, the distance between the Earth and the sun varies throughout the year. In December, it is 147 million kilometres and in June it is 152 million kilometres. This is a variation of five million kilometres, yet there is no discernible change in the state of water on the planet.

It may be argued that these variations are too small – about 1.5 percent of the average distance – but then again, the advert in question says “move slightly”. Nevertheless, the intensity of solar energy falling on the Earth is 7 percent greater in December than in June.

Interestingly, on a global average, June is a hotter month than December even though the Earth is closer to the sun in December. The reasons for this strange effect were discussed in a previous article; for now we may want to know how close the Earth would have to be for all the water to evaporate.

The current average global surface temperature is about 15.5 degrees celcius. For all the water to evaporate, it would need to rise to 100 degrees celcius. The question then is: what amount of solar radiation would maintain the Earth at 100 degrees?

The present intensity of solar radiation on Earth is about 1,000 watts per square metre. This maintains 15 degrees on the surface. To raise the temperature to 100 degrees, the intensity would have to increase to 2,810 watts per square metre.

Next we find out at what distance from the sun would the radiation intensity be 2,810 watts per square metre. The answer is about 90 million kilometres. That’s closer to the sun than planet Venus – not a “slight” displacement!

However, the surface temperature on Venus is 460 degrees celcius (much higher than 100) because its atmosphere is predominantly comprised of carbon dioxide. Incidentally, Venus is even hotter than Mercury.

On the other hand at what distance would the earth have to be for all the water to freeze? For that to happen, the surface temperature would have to drop to zero degrees celcius. That temperature would be maintained if the intensity of solar radiation reduced to 800 watts per square metre.

The distance at which the solar intensity is 800 watts per square metre is 230 million kilometres from the sun. This is more than 50 percent of the present position – again, not a “slight displacement”.

The moral of the story is: Don’t believe everything you get from an advert.

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