21 JUNE - 'THE ZERO SHADOW DAY' ON TROPIC OF CANCER

Finding Tropic of Cancer through GPS

The Tropic of Cancer is a line of latitude circling the Earth at approximately 23.5° north of the equator. It is the northernmost point on Earth where the sun's rays can appear directly overhead at local noon. It is also one of the five major degree measures or circles of latitude dividing the Earth (the others are the Tropic of Capricorn, the equator, the Arctic Circle and the Antarctic Circle).

Some of the Earth's largest countries and/or cities are at or near the Tropic of Cancer. For example, the line passes through United States' state of Hawaii, portions of Central America, northern Africa, and the Sahara Desert and is near Kolkata, India. It should also be noted that because of the greater amount of land in the Northern Hemisphere, the Tropic of Cancer passes through more cities than the equivalent Tropic of Capricorn in the Southern Hemisphere.

Naming of the Tropic of Cancer

At the June or summer solstice (around June 21) when the Tropic of Cancer was named, the sun was pointed in the direction of the constellation Cancer, thus giving the new line of latitude the name the Tropic of Cancer. However, because this name was assigned over 2,000 years ago, the sun is no longer in the constellation Cancer. It is instead located in the constellation Taurus today. For most references though, it is easiest to understand the Tropic of Cancer with its latitudinal location of 23.5°N.

Significance of the Tropic of Cancer

The Tropic of Cancer is significant to Earth's geography because, in addition to being the northernmost point where the sun's rays are directly overhead, it also marks the northern boundary of tropics, which is the region that extends from the equator north to the Tropic of Cancer and south to the Tropic of Capricorn.

A Tropic of Cancer latitude passing through Ormanjhi, Ranchi, Jharkhand, INDIA

In addition to being used to divide the Earth into different parts for navigation and marking the northern boundary of the tropics, the Tropic of Cancer is also significant to the Earth's amount of solar insolation and the creation of seasons.

Solar insolation is the amount of incoming solar radiation on the Earth. It varies over the Earth's surface based on the amount of direct sunlight hitting the equator and tropics and spreads north or south from there. Solar insolation is most at the subsolar point (the point on Earth that is directly beneath the Sun and where the rays hit at 900 to the surface) which migrates annually between the Tropics of Cancer and Capricorn because of the Earth's axial tilt. When the subsolar point is at the Tropic of Cancer, it is during the June solstice and this is when the northern hemisphere receives the most solar insolation.

Sun's ray situation on 21 June

We can readily see that the Sun's vertical ray is striking the Earth overhead at 23.50N (Tropic of Cancer). The tropic marks the northern-most point at which the Sun will be overhead during the year. Poleward of 23.50N there is never a day when the Sun is directly overhead. Poleward of the Tropic of Cancer we will always have to look south to see the Sun. On June 21, when the Sun is directly overhead 23.50N, note that in shining 900, the Sun will extend not only to the North Pole (66.50 away from the Tropic of Cancer), but in fact an additional 23.50 (to make a total of 90 degrees) beyond the pole to the back side of the Arctic Circle (66.50N). Light shines 900 in all directions from the point where the Sun's ray is vertically overhead.

 

Light also shines 900 to the south from the Tropic of Cancer. This means that light will extend to the Equator (23.50) plus an additional 66.50 (to total 900) to the front side of the Antarctic Circle (66.5 0S).

 

If we could spin the Earth one rotation/one day, we would find that if we are anywhere between 66.5 degrees N and the North Pole, we would always be in light (24 hours). No darkness would occur because at no time would you rotate into the dark area. So with 24 hours of light, why is it not warm to hot in these polar areas? Keep in mind that we are a long way from the vertical ray of the Sun (it is at 23.50N). We have long days, but the low Sun angle means little heat energy is actually being received at the Earth's surface. Now, if we are located between 66.50S and the South Pole, we will on this date experience 24 hours of darkness because such locations are more than 900 away from the Sun's vertical ray.

An Example from INDIA: 

On 21 June, 2021 we decided to go to find 'Tropic of Cancer' which was nearby us. Our living address's latitude is 23045.417'  that means our location was just few minutes northward from Tropic of Cancer latitude, therefore we decided to find the exact place from which this important line is passing through. 

We started our journey on almost 9:30 am on 21 June to go Adra (we calculated that Tropic of Cancer is passing through between Joychandi Pahar and Adra city). This month was very busy month for Monsoon rainfall, before today since three days we experiencing 4 days continuous rainfall. Today we seen sunny weather, so we were sure that it may possible to see the sun under clear cloud and weather that is why we also decided to go there to find the latitude. During our journey (one hour journey) we were so exited to see that the shadow of any two wheeler or four wheeler was accruing just vertical (on time 11:00 am). 

Tropic of Cancer is passing through Adra (Purulia, West Bengal, INDIA) and ZERO SHADOW can be see through our bodies shadow

We reached Adra on about 11:40 am and very quickly we found the location from where Tropic of Cancer is passing with the help of GPS (Garmin). We drawn a line (Tropic of Cancer) and put a Sun dail (made by us). On accurate 12:00 pm we seen a slightly angled shadow on our Sun dial (this is because of IST is 82030 degree and we reached here late by 16 minutes 40 second) but We seen almost vertical (Zero shadow) here.  We can understand this by following calculation:

                    Longitudinal difference Between Adra and Allahabad (IST)

                    Longitude (on GPS point) in Adra        :     86⁰40’283”

                    Longitude of IST (Naini, Allahabad)    :     82⁰30’000”

                    Hence longitudinal difference is            :     4⁰10’283 or 246 km

                    Time distance                                        :     1⁰ = 4 minute difference

                    Therefore, difference will be  4⁰10’ = 16 minute 40”

Solar day (Vertical Sun’s ray falling time on Tropic of Cancer) on Adra: 12:00 pm – 16:40 =  11 O'clock 43 minute 20 second 

In this video we can see 'Zero Shadow' phenomenon; See attention fully on vehicle's shadows 

(Because at 11:35 O'clock on 21 June, 2021 we were on the road journey; just few km before reaching Adra)

Bolero shadow falling vertical on the road

Omni shadow making zero shadow on the road

A car's shadow became zero

An Auto shadow possesses no any side angle

Measuring Tropic of Cancer by GPS and Verifying Zero Shadow by Sun Dail

Tropic of Cancer confirmation by our mobile's GPS

We were so happy to see the yearly phenomenon on Tropic of Cancer with our students. We read all this theoretical aspects but never verified this; it was first time when we implemented it in the field to check it out and we saw it really works. Therefore, we  found that on 21 June, their is occurring 'Zero Shadow Day' on Tropic of Cancer. 

How to Calculate the summer Solstice Sun Angle: During summer solstice, which occurs around June 21 every year, the Earth's axis is positioned relative to the sun such that northern hemisphere is closest to the sun and sourthern is farthest from the sun. The hemisphere closest from the sun experiences the summer solstice, with the direct rays of the sun falling 23.5⁰ (Tropic of Cancer) north of the equator. Calculate the sun angle during the summer solstice for your location by determining your latitude and doing two simple calculations.

Consult an atlas or a geographical website to find the latitude of your location on Earth. For example, we are living in Asansol, our latitude is 23° 43' 42" N, or approximately 23.43°.

Longitudinal position of Asansol and Adra

Add 23°.5’ to your latitude to compensate for the fact that the sun's direct rays fall on one of the tropic lines during the summer solstice: the Tropic of Cancer for the northern hemisphere and the Tropic of Capricorn for the southern hemisphere. For example, if we live in Asansol, add 23°.5’ to 23°.4’ to get 56°.9’.

Subtract this value from 90° to get the angle of elevation from the horizon of the sun during midday on the summer solstice. In the above example, subtract 56.9° from 90 to get 33°.1’. This is the angle of elevation of the sun in Asansol at midday.

I am thankful for assisting to our lovable students during the field study e.g., i) Raja Chaurasia, ii) Arjun Nonia and iii) Abhijeet Kumar Shaw; I hope you all enjoyed and became evident this celestial phenomena first time in your life. 

Tropic of Cancer passing through Ormanjhi, Ranchi, Jharkhand, INDIA