What is an analemma? This is an intriguing shape that would be
observed, if one locates the position of the Sun in the sky, at exactly the same
time, every day, for a whole year.
One may be familiar with the changing angle of sunlight falling into one’s
living room, between summer and winter. The apparent annual movement of the Sun,
in the sky, is such that, it appears to set considerably to the North of west,
in summer and way south of west, in winter. On the days of the equinoxes –
around March 21st and September 22nd every year, the Sun
can be seen to set exactly due west.
The analemma results as a sum of the effects of the Earth’s elliptical orbit around the sun and the tilt of the Earth’s axis in relation to the plane of its orbit around the sun. As a combination of these two effects – the shape that would be seen for an analemma is in the form of a figure 8 – its extent, and tilt in the sky would depend on one’s location in the sky and the constant time in a day, that is chosen as a reference, for making the analemma.
In the absence of ellipticity of Earth’s orbit and a tilt away from the perpendicular, of its axis, with respect to its orbit – the Sun would rise and set in the same direction every day of the year and cross the meridian at the same time. However, due to these two effects there is an annual movement of the Sun in the sky and in summer, the Sun crosses the meridian, ahead of a mean schedule and behind this mean schedule, in winter. It is this difference in time of the crossing of the meridian, by the Sun, that is called the equation of Time and has to be incorporated in calibration of any sundial.
However, while making a
simple analemma, one is not bothered about calibrated time, and, if one wishes
only to have an appreciation of the annual movement of the Sun, in the sky –
any simple gnomon or a small ring suitably placed to throw the noontime solar
shadow on a vacant ground would do, to keep track of the analemma. Very
recently, a daycare center at Minnesota, USA, completed a two year long analemma
activity with the day care children and managed to obtain an analemma thrown by
the shadow of a ring mounted on a wooden fence running east-west. Michael Kauper,
the originator of this project, talks of the way the children would run out at
the appropriate time, shouting ‘Analemma! Analemma!’ and set to mark the
Sun’s shadow through the ring. A wonderful effort that could be repeated by
school students easily.
One just needs to mount a
gnomon or a ring around the school playground, keeping a sufficiently wide area
around it, undisturbed for a year. Choose a convenient time of the day when the
shadow of the ring or the tip of the gnomon may be seen on the ground easily,
throughout the year. At exactly the same time of the day, for every observation,
the position of this shadow needs to be marked, in some indelible manner.
Observations can be taken 15 days apart. However it may be worthwhile making a
draft marking a day or two before or after the predetermined day of observing,
to take care of possible cloudy days.
This is not an activity directly useful for the measurement of the Astronomical Unit, from the observations of transits. However, it is an activity that easily makes one appreciate the complicated geometrical patterns that can arise from a simple combination of an elliptical orbit and a tilt of the axis of rotation, with respect to the orbit.
A more ambitious Analemma activity would be to create an actual photographic image of the analemma, by using a fixed and undisturbed camera and giving it a one year long exposure, clicking images of the Sun at predetermined times (the same time, for every observations) spaced at 15 day intervals. May not sound a very difficult thing to do, but, it may be daunting as well as inspiring to know that only about half a dozen people in the whole world have managed to create successful photographic analemma. It would be wonderful if an Indian Amateur Astronomer would create an analemma against the background of our wealth of monumental heritage – against the backdrop of the Konark Temple, for instance!