December 21 – Still Standing Still

Today’s factismal: Today marks “Midwinter Day” in many countries.

Today is one of the more interesting days in the year. It is the day in which the Sun stops its apparent southward movement through the sky and starts to move northward once more. On this day, the Sun appears to stand still (at least as far as the North/South question is concerned), hence the name “solstice” (Latin for “Sun stand”). Today, astronomers use the change in the Sun’s apparent movement to declare the start of “astronomical winter”. But 10,000 years ago, people used it to mark the middle of winter. So today is both the start and the middle of winter!

The analemma tracked out by the Sun over the year (Image courtesy the Analemma)

The analemma tracked out by the Sun over the year
(Image courtesy the Analemma)

So how did people track the Sun 10,000 years ago? The same way that we do today; they plotted the position of the Sun at noon using a stick and pebbles (teachers: this makes a great class science project!). The pebbles created a figure-eight pattern that the Greeks named after the pedestal of a sundial; they called it an “analemma”. Of course, 10,000 years ago, they sometimes used some pretty big pebbles – Stonehenge is one example of a solstice calculator.

Stonehenge was used (in part) to calculate the winter solstice (Image courtesy English Heritage)

Stonehenge was used (in part) to calculate the winter solstice
(Image courtesy English Heritage)

Today we’ve managed to shrink the size of the stones that we use to do calculations, and we’ve found ways to get more calculations from them. And one of the best way of doing this is by linking your silicon-based calculator (that is, your computer) to others so that researchers can perform calculations that are too big or too complicated for any single computer. (Match that, Stonehenge!) One project that is using this sort of distributed computing is the SkyNet. They want to process radio astronomy data using your spare CPUs. To take part, set your browser to:
http://www.theskynet.org/

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