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Beach science;Let there be light

John Stringer and Dinah Starkey explore religious festivals and the science of illumination

If you've stood on a sunny beach, you have felt the full heat of the summer Sun. If you are fair-skinned, it may have burnt you. Which places did you soothe with aftersun that night? They will have been your shoulders and the tops of your ears; your nose if you weren't wearing a hat, and the instep of your feet if you had no shoes.

You invited this direct heat to these parts of your body by putting them at right angles to the Sun's rays. This is exactly what your part of the tilted Earth experiences every summer - the direct heat of the Sun. In the winter, when your part of the Earth is tilted away from the Sun, the light strikes obliquely, and the heat and light are less fierce.

The Sun is highest in the summer, and the days are longer. The path of the Sun across the sky - its apparent movement - is lower in the winter, and the light and heat are less intense. The days are shorter, too. On two days in the year - the equinoxes - the day and night are of equal length.

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