Zero Shadow Days in Tanzania

Zero Shadow Days in Tanzania

If someone was to ask you, “Where does the Sun set?” you would immediately say “west” without even waiting to think because that is what we have been taught in our elementary school.  However when we are a little older, in our secondary schools, we are taught that the Earth rotates on its own axis which is tilted at 23.5 degrees angle to the flat plane of our solar system.  

 

The tilt of the axis was given as the reason for the Sun shifting north to south and back during the year causing four seasons – summer, winter, autumn and spring.  These seasons did not make sense to most of us in Tanzania because our experience was of only three main seasons – hot, rainy and cool. Though summer and winter had some connection with hot and cool seasons that we experienced, we just memorized autumn and spring as words that we needed to pass tests.  

 

It is not clear to everyone that when the Sun shifts its position in the skies over the year from north to south and back it shines directly overhead or at slanted angles at different latitudes and thus gives rise to seasons.  We can appreciate the difference in heat between the hot Sun that shines concentrated rays above us at midday, and the cool evening Sun whose rays are spread over the ground.

 

Seasons are even more pronounced in places outside the tropics far north or south of the Equator.  They have very cold winters because firstly, the Sun is never overhead there so sunrays always fall at slanted angles throughout the year, and even more so during winter when the rays are extremely slanted.  Near the North or South poles, sunrays are spread almost parallel to the ground hence very little energy is absorbed by the ground and it becomes extremely cold.  Also, very close to the poles, the Sun goes below the horizon for half the year, which means darkness and hence there is permanent ice there.

 

The orbits of all planets revolving around the Sun in our Solar system are restricted to a flat plane hence our solar system is flat.  The tilted axis of the Earth makes the Equator circle to be at an angle of 23.5 degrees below the Sun, on June 21 (see diagram), while six months later on 22 December, after it has moved half way around in its orbit around the Sun, it is tilted above the Sun.  In between these two dates, that is, on 21 March and 23 September, the tilt of the circle of the Equator is neither towards nor away from Sun.  These changes in the tilt of the Equator circle cause the Sun to be overhead at the different latitudes on Earth at different times of the year.

 

On March 21 and September 23 the Sun is overhead at the Equator, zero degrees latitude.  After September 23, the Sun’s position moves south, until it is overhead the Tropic of Capricorn on December 22.  After this it shifts northwards to be overhead at the Equator on March 21, then moves north to become overhead at the Tropic of Cancer on June 21, and on September 23 it is again overhead at the Equator, repeating the cycle every year.

December 22 is called the Southern Solstice because on that date the Sun is at its southernmost position, while June 21 is called Northern Solstice when the Sun is in its northernmost position.  March 21 and September 23 are called Equinoxes since day and night are exactly equal on those dates throughout the world.  During Northern Solstice days are longest and nights shortest in the northern hemisphere while during Southern Solstice days are longest and nights shortest in the southern hemisphere.  Note that this means that at the Equator, day and night are equal throughout the year.


Notice that at the June Solstice the north end of the tilted axis of the Earth is pointing towards the Sun while at the December Solstice the north end of the axis is pointing away from the Sun.  At the March and the September Equinoxes, the north end is tilting to the side, that is, neither towards nor away from the Sun.

 

The movement of the Sun north to south is never appreciated by us in the tropics since we think that the Sun always rises in the east and sets in the west and passes directly above us at midday.  However if you were to follow this closely when the Sun shines through your window,  you will remember that it shines from different angles at different times of the year, making shadows shift from one part of the room to another.  This movement means that the Sun’s position in the skies changes regularly throughout the year hence the Sun DOES NOT set in the west and rise in the east at all times throughout the year – it changes!!

 

When viewed from Earth looking towards the horizon at sunset, you will see the Sun  south of west  on December 22, after which the Sun’s sunset position will shift northward until it reaches exact west on March 21; then it will shift northward and by June 21 its horizon position at sunset will be furthest north of west.  Its position will then shift southwards, and on September 23 it will again be exactly west at sunset.  By December 22 it will once again be south of west.  If you watch sunrises during the year, the position of the Sun will also show a similar shift, but they will be exactly east only on March 21 and September 23.

 

What about midday?  At the Tropic of Cancer on June 21, the Sun will be exactly overhead at midday so shadows of vertical objects will be directly below the object and we will not be able to see any shadow.  After three months, on September 23 the Sun will be exactly overhead at the Equator.  Hence in three months, from June 21 to September 23, the position on Earth for overhead midday Sun shifts day by day from latitude 23.5 degrees north to latitude zero at the Equator with a specific day of overhead midday Sun for each latitude. 

 

From September 23 to December 22, the Sun’s overhead position moves south day by day from Equator to the Tropic of Capricorn, latitude 23.5 degrees south. Hence for us in Tanzania which lies between latitudes 1 degree south to 11 degrees south, the Sun will be overhead between September 26 to October 22 from northernmost to southernmost places in Tanzania. It is again overhead at midday in Tanzania from February 20 to March 17 when the Sun’s position during the Sun’s northwards shift from the Tropic of Capricorn to the Equator.

 

So we can see that for every place that lies within the tropics, that is, between latitudes 23.5 degrees north to 23.5 degrees south, there can be only TWO DAYS IN A YEAR on which the Sun will be EXACTLY overhead.  Outside the tropics the Sun will NEVER be overhead at any place that is at high latitudes north or south of the tropics up to the poles.

 

Shadows are formed when slanted sunrays strike objects.  So when sunrays are more slanted, shadows of upright objects are longer, as we see in the evenings and mornings.  However if the midday Sun is exactly overhead then shadows of vertical objects will be directly below the object hence you cannot see the shadow.  The shadows will disappear!! Such days are called ZERO SHADOW DAYS.

 

Following Equinox on September 23 when the Sun was exactly overhead at midday at the Equator, the Sun's position in the sky is now slowly shifting south. In the coming days at midday the Sun will be overhead at progressively southern latitudes.  For Tanzania, from 26 September until October 22nd the Sun will be overhead at mid-day from Bukoba, Musoma (latitude 1 deg south) in the extreme north of Tanzania  to Newala, Tunduru (latitude 11 deg south) in the south of Tanzania. 

 

Midday times are local times that depend on the longitude of the place.  Places on the east side of the country such as Zanzibar, Dar es Salaam and Tanga will have local times of midday that are earlier than those on the west side of the country such as Bukoba, Kigoma and Sumbwanga. Since the east and west extremes of Tanzania have a difference of 10 degrees of longitude, there is a 40 minutes of difference between local times in Dar es Salaam and Kigoma.





Go out and watch all SHADOWS DISAPPEAR on the days and local times at which the Sun is exactly overhead at your place as follows.  For towns that are not listed, estimate the dates and times using data given for nearby towns.

 

Latitude

Dates (2015)

Towns(with MIDDAY TIMES in brackets)

1 deg south

26 September

Bukoba(12:44PM), Musoma(12:38PM)

2 deg south

29 September

Mwanza(12:39PM)

3 deg south

01 October

Arusha and Moshi (12:23PM)

4 deg south

 03 October

 

5 deg south

06 October

Pemba and Tanga(12:11PM), Tabora(12:37PM), Kigoma(12:48PM)

6 deg south

10 October

Zanzibar and Dar-es-salaam (12:10PM), Morogoro(12:17PM), Dodoma(12:24PM)

7 deg south

12 October

Mafia(12:10PM), Iringa (12:24PM), Sumbawanga(12:39PM)

8 deg south

14 October

 

9 deg south

16 October

Kilwa(12:09PM), Mbeya(12:32PM)

10 deg south

19 October

Lindi and Mtwara(12:08PM), Masasi(12:12PM), Songea(12:22PM),

11 deg south

22 October

Newala(12:07PM) and Tunduru(12:15PM)

 

 

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