International Year of Astronomy

What is an Analemma?

An analemma is the beautiful figure-of-eight on the front cover of this issue of JM. It is actually the path that the Sun makes in the sky over a year. The photo on the cover was got by taking several (in fact 47) multiple images of the Sun at the same time each day over the year. This image was taken multiple times on the same film, with the Temple of Apollo (550-540 BC) in the background.  This temple is one of the famous structures built in ancient times by the Greek.
While the temple is a marvel of its times, the analemma that is overlaid on it is an equally marvellous thing. How does it happen?
You might have noticed that the Sun's position varies in the north-south direction over the year. We discussed this when we read about Solstices in the last issue of JM. For all of us in India the Sun moves from the South in winter, passing directly overhead, to the North in Summer.
This means that at noon the Sun is either to the South or North of us. Indeed, shortly after you read this, it will be overhead in South India! Also, the Sun is much lower down in the sky even at noon in more northern latitudes like in Kashmir or Delhi. The Sun is rapidly rising higher and higher over Chennai (13° 04' N, 80° 17' E) and Bangalore. A look at the figure will tell you how the Sun's movement between the Tropics of Capricorn and Cancer cause the Sun to drift south-to-north in the sky.
We also saw that this movement happens because of the tilt of the Earth on its axis.
In addition it may surprise you to learn that the Sun also drifts in an east-west direction over the year. This happens because the Earth moves around the Sun in an elliptical path and not a circular one.
Why does this make a difference? Indeed why does this happen?
The Earth is not moving at uniform speed all the year around. Depending on whether it is in a flatter portion, or a more curved portion of the orbit, it moves faster or slower. However as a matter of convenience we define a mean solar day and a mean solar year as having 365.25 mean solar days. While this is fine on the average, at different times of the year, the solar noon, for example, will be off the noon for that day.
The graph shows the time when solar noon occurs over Chennai, that is when the Sun is maximally overhead. (There are some corrections because noon is fixed with respect to Indian Standard Time which is slightly different from the longitude of Chennai, but this is a very small correction; it will be much larger for Delhi or Mumbai).
You can see that sometimes solar noon occurs earlier and sometimes later. There is a drift of about 8 minutes.
The two effects :
. north-south drift due to tilt of Earth's axis
. east-west drift due to Earth's elliptical orbit 
combine to give the peculiar observed path of the Sun in the sky.
expect with the change of the seasons, but also slightly east and west.  This figure-8 path that the sun makes in the sky is called the analemma.
Map your own Analemma
Find a place where the Sun shines on the ground at noon all year long.  Place a rod about 3 feet tall into the ground, being very careful not to bump the end of it during the year. If you were constructing a sundial, this rod would be called a gnomon.
On the first day of each month, at the same time every day, mark the shadow of the tip of the rod as cast by the Sun. Mark it with a clear marker that will not get erased by rain or people!  At the end of 12 months, you will see that the marks make a figure-of-8 pattern on the ground.
Why don't you involve your school friends in making this experiment?  Start now and send us a picture or a note of your experiences and experiments when you have finished.
Figure Captions: Solar noon at Chennai over the year. The height of the Sun in the sky (in degrees) is shown. Notice that the Sun is overhead twice in a year (and nearly at 90 degrees altitude) when the Sun crosses Chennai on its journey to the Tropic of Cancer and back.
The time of solar noon at Chennai. This is when the Sun is directly overhead (but may be at high or low altitude) in the sky, or in the same longitude; hence it will appear to be directly North or South.  
Note on the cover photograph:
The top and bottom of the figure-8 correspond to the Solstices -- the Northern and Southern limits of the Sun's sky motion. The two Equinoxes find the Sun at points along the anelemma curve exactly half way between the Solstices.
Furthermore, this loop will be inclined at different angles depending on one's geographical latitude.
As suggested by the relatively few number of successfully completed analemmas (less than ten including the pioneering photo in 1979), the imaging of the sun over local skies during the course of twelve months is considered one of the most difficult and demanding astronomical phenomenon to image.
Photograph and text extracts kind courtesy of Anthony Ayiomamitis, Greece.