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Kate Humble and the team call out to the public to help them build a global picture. On the day of Equinox the hours of light and darkness are of near equal length, so wherever you are on the planet you will get near to 12 hours of day and 12 hours of night. But it's also a moment of change. On the day of the equinox the planet is in "neutral", each hemisphere poised at the mid point of its journey between summer and winter - at least in terms of solar radiation.