Sunset In Mumbai — Why Is It So Late?
Last month, I moved to Mumbai for my job. Here, I have observed that even around 6:30 or 7pm the sun still shines bright in the sky. This reminded me of something my father told me during his stay in Mumbai during my school days. I was able to relate it to the concept of latitudes and longitudes I had studied in Class 6 geography.
There is actually a mathematical reason to why the sun in Mumbai sets later than most other Indian cities. This involves longitudes, time zones and the calculations involved with them
Latitudes & Longitudes
Latitudes and Longitudes are a set of imaginary horizontal & vertical lines which were visualised to make life easier. They consist of a grid of horizontal vertical lines criss-crossing each other. Of course, they are imaginary — you don’t see a set of gridlines when you look up at the sky
As everybody knows, when you move east, you move forward in time and when you move west, you move backwards in time i.e if the time in India is 6pm, the time in Japan will be around 11pm whereas the time in London will be around 12 noon. How much is this difference we will see in some time
The longitudes range from 0 degrees to +180 degrees in East and 180 degrees in west. The latitudes similarly range from 180 degrees south to 180 degrees north. This post will focus only on longitudes since that is more relevant for us
This system of latitudes & longitudes makes it easier to tell the location of a place. For example, the location of Taj Mahal is approx 27° 10′ 30″ N, 78° 2′ 31″ E meaning it is approx 27 degrees to north and 78 degrees east. This makes it easier to locate it on a map — I need to travel 27 degrees to the north and 78 degrees east. But from where? Which is my reference point (or the 0 degree point) from where I need to move in the required direction?
The 0 latitude & 0 longitude point on earth is known as the Null Island and is located on the Atlantic Ocean (as you can see in the above map). Since the point is located in the ocean, it is not of much significance. We will focus more on the 0 degree longitude line, which is known as the Greenwich Meridian or the Prime Meridian
The Greenwich Meridian Time (GMT)
The Prime Meridian passes through the Royal Observatory in Greenwich in London
The time at this place is called the Greenwich Meridian Time or GMT in short
This GMT serves as the base point for all time-zones across the world. All time-zones across the world are referenced to this. All time-zones to the east of this GMT are referred to as GMT+(time) & all time-zones to the west are referred to as GMT-(time)
For example, the time zone of India is represented as GMT+5:30 meaning the time in India is 5hrs 30 mins ahead of in Greenwich i.e if the time is 12 noon in Greenwich, it is 5:30 evening in India. (How this 5hrs 30 mins is arrived at we will look at the next section)
Similarly, the time in Washington (America) is represented as GMT-4 implying it is 4 hours behind the GMT. If the time is 12 noon in Greenwich, it will be 8 in the morning in Washington. Since America is a large country (in terms of breadth), it has multiple time zones. Hence, different cities in America (like Washington, California) will have different time zones.
The Calculations
All of us know that the earth completes one full rotation in 24 hours. In this time period, it covers a total of 360 degrees (from 180 degrees west to 180 degrees east)
This entire rotation of 360 degrees is covered in 24*60 mins = 1440 mins
=> 1 degree (or longitude) = 4 minutes
The above equation is the most important one you need to remember if you have to understand the concept of time zones. It implies that for each degree you move forward (east), you gain 4 minutes and for each degree you move west, you lose 4 minutes
Coming to India, the 82°30 E (or 82.5 degree) east longitude line is selected as the official longitude for calculating the time zone. The 82.5 degree line passes through the city of Mirzapur in Uttar Pradesh
Coming to our calculation:
1 degree -> 4 minutes
82.5 degrees -> 82.5 degrees*4 min = 330 minutes or 5hrs 30 mins
This is the reason why the official time zone of India is GMT+5:30 or 5hrs 30 mins ahead of GMT
Now, let us consider a city in the West like Brazilia (capital of Brazil). Its co-ordinates are 15° 47′ 38″ S, 47° 52′ 58″ W or approx 15.8° south & 47.8° west. We only need the longitudinal co-ordinate (47.8 degrees) for our calculation
Repeating the above calculations, we get 47.8*4 = 191.2 minutes or approx 3hrs 11 mins. Since it is west, it implies 3hrs 11 mins behind the GMT. A simple google search would confirm that the time in Brazilia is GMT-3 i.e 3 hrs behind GMT. We see that our calculation matches the result
In this way, we can calculate the time zone of any city provided you know the latitudinal co-ordinate of the place.
Let us take a city on the opposite side of the spectrum — Tokyo. Tokyo’s co-ordinates are 35° 41′ 23″ N, 139° 41′ 32″ E (or approx 35.7 N & 139.7 E)
Taking the longitude and applying our calculation, we get 139.7*4 = 558.8 mins or 9 hrs 18 mins ahead. This ties in with the fact that the official time zone of Tokyo is GMT+9
Coming Back To Mumbai
Let us come back to India. As we saw, the 82.5 E line is the official time zone of India. If you see Mumbai’s co-ordinates, its longitude is 72° 52′ 32″ E or approx 72.9 degrees east. We see that there is a nearly 10 degree difference (82.5–72.9) between Mumbai & the official time zone of India. This corresponds to a nearly 10*4 = 40 mins difference in time. What does this difference mean?
This means that though Mumbai follows the GMT+5:30 time zone, in actuality, its official time should have been 40 mins behind (maybe GMT+4:50)
For example, if the time is 7pm in Mumbai (according to IST), the actual time in Mumbai should be 40 mins behind i.e 6:20 pm. But since it is following India’s standard time zone, it has been brought forward to 7pm. That is why, even at 7pm, we see the sun shining brightly in Mumbai. The actual time that Mumbai should be following is nearly 40–45 mins behind.
The opposite happens in Eastern states of India. There is a village called Dong in Arunachal Pradesh which is called “Land of the Rising Sun” since it is one of the first cities to see the sun rise in India. It is located around 97 degrees east. Compare this with the official 82.5 E line & we see a nearly 1 hour difference ((97–82.5)*4 = 58 mins). Dong is actually 1 hour ahead than India’s official time i.e if the time in India is 5am, the actual time in Dong village should be 6am but it has been brought behind by 1 hour. That is why “it seems” that the sun rises earlier in Dong village
Why All This?
The reason why Mumbai, Dong etc have been brought forward/behind in time is to make sure that India has only one official time zone. This is to ensure there is no confusion while referring to railway timetables or flight timings.
Imagine a train/flight from Chennai to Mumbai. The timetable says that the train/flight reaches Mumbai at 8pm. Now, if Mumbai had a different time zone than Chennai, this would lead to a confusion — is this 8pm according to Chennai or Mumbai’s time zone
Another confusion would be is that people would find it a bit difficult to add or subtract the 40–45 mins to factor in the change in time zone between the cities thus adding to more confusion. It would lead to people miscalculating the time and missing their train/flight.
This is the reason why though India has a difference of nearly 2 hours in time zone from east to west, it follows only one standard time-zone — the Indian Standard Time (IST) which is GMT +5:30
That is all for this post. In my next post, I will write some more interesting facts about time zones like:
Did you know there are 2 islands separated by only 3km in distance, but are apart by nearly 21 hours in time?
Do you know which island is the first to welcome the new year and which is the last?
Do you know about the country that lost a day because they shifted time zones?
Read about them in my next post
Thanks for reading!!
Originally published at http://infinitesimallysmall.com on June 19, 2022.
