The analemma

If you photograph the sun at the same time every day (or every few days), you will find that the sun traces out a path in the sky, called the analemma. György Soponyai, in Budapest (Hungary), did exactly that at 8 AM each morning between 29 January last year and 6 January this year, to produce the wonderful photograph below (click to zoom):

More analemma photographs (by Anthony Ayiomamitis) can be found here. The shape of the analemma results from the fact that (1) the Earth is tilted on its axis by 23.5° and (2) the Earth orbits the sun in an ellipse, rather than a circle. The diagram below shows the calculated analemma for 12 noon at the Royal Observatory, Greenwich (latitude 51.48° N):

The concept of the analemma can also be used in constructing sundials. If an appropriate analemma is placed in the centre of the sundial, a gnomon placed at the right point on the analemma will correctly tell the time with its shadow (except for daylight-saving, of course).

Such sundials are popular in parks, because the viewer can stand on the analemma at a position corresponding to the current date, and his or her shadow will tell the time, without the need for additional time-of-year correction. I photographed the sundial above and below at Mt Stromlo Observatory in June 2012. It can be seen that the time was about 2:20 PM.

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