Photo-op on the Tropic of Cancer

On our way to the Rann of Kutch, I spotted a blue signboard which said:

It caused me to yell at the driver to stop for a photograph. I was even more excited as it was Makar Sankranti day. Makar Sankranti marks the end of the month with the winter solstice and signals the beginning of longer days. It is considered an auspicious day when the sun first transits into Capricorn. And it seemed symbolic to me that I was standing at the Tropic of Cancer when the Sun was passing through Capricorn.

I remember most of my geography lessons from school. I had a fabulous teacher: Ms Manju Bhandari (now Manju Singhwi) who brought these classes to life. And I recollect being taught about how the Tropic of Cancer and the Tropic of Capricorn are two of the major circles of latitude which are marked on the map of the earth. The positions of these two Tropics is determined by the tilt of the earth’s axis relative to the plane of its orbit. The word “tropic” itself comes from the Greek “trope”, meaning turn or change of direction (or circumstances), referring to the fact that the sun appears to “turn back” at the solstices. During the summer solstice in June, the Tropic of Cancer is the north-most circle of latitude where the sun can be directly overhead.

What is interesting is that the positions of the Tropics can shift. I remember learning in school that the Tropic of Cancer is located 23.5° north of the equator. However the Tropic of Cancer is presently located at 23°26’12.5′ (or 23.43681°) north of the equator. Why does this happen?

The earth’s tilt around its own axis varies over a 41000 year period from 22.1° to 24.5°. This means that the Tropic of Cancer is currently drifting southward at a rate of almost half an arc second latitude or around 15 metres a year. So in the year 2045, the Tropic of Cancer will be located at 23°26’N. On Federal Highway 83 in Mexico, the change in the positions of the Tropic of Cancer is marked yearwise, and here one can appreciate the shift in the position clearly. In Gujarat, the Tropic of Cancer is marked at the coordinates 23°26’N68°23’E. The other states in India where the Tropic of Cancer passes are Rajasthan, Madhya Pradesh, Chattisgarh, Jharkhand and West Bengal.

This also explains the nomenclature. When the name ‘Tropic of Cancer’ was given centuries ago, the sun was in the constellation Cancer during the June solstice. However now the Sun is in Taurus. So should this line correctly be called the Tropic of Taurus?

While we are discussing this, let’s also discover another phenomenon related to the sun that happened in 2019. Unlike most Indian festivals which are celebrated according to the lunar calendar, Makar Sankranti is observed according to solar cycles on a fixed date on 14th January. This year however Sankranti was celebrated on 15th January. What caused the shift in date? This is because the date needed to be adjusted for the precession of the equinoxes.

Let me try and explain that in simpler terms. The earth gradually shifts the orientation of its axis of rotation and moves westwards. This causes some observable changes. Such as: the apparent position of the Sun relative to the backdrop of the stars seems to change every year. Therefore each year the sun takes 20 minutes longer to come to the same location than the previous year. This means after every 72 years, the sun needs an extra day to arrive to the same spot. That explains why Sankranti was celebrated a day late this year.

As we left after the photo opportunity, I could still sense our driver wondering what all the fuss was about. I heard him grumble: “Itne saal se dekh rahe hain. Kabhi humein to yahaan koi rekha nikalte huye dikhi nahi!” (We have been seeing this for so many years. I haven’t seen any Rekha pass from here in all these years!)


Leave a Reply

%d bloggers like this: