d ~ 61'747'200 km: day 24 of Earth's orbit

"Drivetime" seemed a misnomer for Simon Mayo's early evening show on Radio 2 as my car inched out of BBC TV Centre and into the homeward crawl.

Twenty minutes and 200 yards later, Simon read out a question from a listener whose five year-old daughter had asked "What would be different if the Earth spun the other way?" Easy, I thought - everything would travel the other way across the sky and we would greet the sun from the west every morning.

My smugness lasted until Chiswick, when I realised I'd forgotten the little matter of Coriolis.

The Coriolis effect transfers the spin of the earth into the circular motion of winds around a weather system. Storms spin anti-clockwise in the northern hemisphere and clockwise in the south. Reverse the rotation of the Earth and you put the storms into reverse too. Interesting, but apart from confused weather forecasters I couldn't imagine a huge impact.

Reaching the M4, the pace of traffic is picking up to a gentle jog and this seemingly simple question is quickening my thought processes too. What about the jet stream?

This river of high altitude, fast-moving air steers the mid-latitude depressions across the planet from west to east. Swirling masses of cloud and rain are pushed from Japan to the Pacific coast of America, and from Newfoundland to Cornwall. Reverse the flow and climate changes dramatically. The British Isles loses the moderating effect of weather from the Atlantic. A harsher continental climate becomes more likely, with a predominantly easterly flow bringing bitter Siberian winds in winter and hot, dry weather in summer. Goodbye green and pleasant land.

Finally on to the M4. Now that we're really moving, the constant stream of traffic reminds me of the trade winds, another crucial part of our planet's circulation system.

The sun heats the atmosphere more at the equator than it does at the poles. On a stationary Earth the warm air would rise at the equator, moving to the poles where it would sink and flow back to the equator along the surface. Nice and simple.

Rotation complicates things. The flow breaks up into three separate cells known as the Hadley cell, the Ferrell cell and the Polar cell. Northward and southward-moving surface winds generated by the cells are then deflected to right or left by our old friend the Coriolis effect and we end up with the trade winds.

These constant easterly winds in the tropical regions were the motorways of the seas for sailing ships. A captain heading out of southern Spain could depend on picking up the northeast trades for a free ride to the Caribbean. Again, reverse the Earth's spin and the whole thing switches. Patterns of human discovery, subsequent empire-building and the resulting political geography would all be different.

The trade winds also affect the distribution of rainfall across large parts of the planet, influencing the position of some deserts and rainforests and interacting with periodic events like El Nino. It's reasonable to assume that a reversal would alter the pattern of habitable land.

Conclusion - change something as simple as direction of rotation and you change the planet we know.

And beware the innocent questions of five year-olds!

Peter Gibbs is a BBC weather forecaster and Met Office meteorologist

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