When we started saying that we wanted to measure the size of the earth, our friends wondered whether we were insane.
After all, if you really want to know it, you can simply check in any basic atlas of astronomy.

But that was too obvious. Instead we wanted to actually measure it ourselves, because we were fascinated by the stories of those men who did the same thing for the first time thousands of years ago.
We wanted to experience their same emotions as they were discovering more and more about the world around them.
For instance that the earth is not flat but is rather a sphere and that it can be measured using simple instruments available to everyone.

We decided to use the following method:

- chose two locations, possibly very close to the equator and aligned more or less along the same parallel (= along a direction east-west)

- measure the time when each of the two locations passes in front of the sun (= local midday)

- calculate the time interval between these two events

- calculate the ratio between the time that the earth takes to do a complete rotation (= 24 hours) and the above time interval

- measure the distance between the two locations

- calculate the earth circumference by multiplying this distance times the above ratio

earth_circumference = D x 24_hours / (T2-T1)

The two locations that we chose were:
- the Boat Yard "Knorr Marine", in Victoria Island
- the Beach Hut of our friend Vivian, at Eleko

We measured the distance between them using the odometer of our car:
- 0 km at the Beach Hut
- 2.9 km at P2 (entrance gate to Eleko Beach)
- 6.9 km at P1 (junction along the road to Epe)
- 55.8 km at the Boat Yard

The road is not very straight, but we considered this inaccuracy acceptable for our method.
We did not count the piece of road between P1 and P2, because it runs North-South.
We also removed the piece between the Beach Hut and P2 from the total count.

The distance that we calculated was therefore:
  (Boat_Yard - P1) - (P2 - Beach_Hut)

  D = (55.8 - 6.9) - (2.9 - 0) = 46 km

We later checked on a map and we measured the actual distance of 43 km between the two points.
Thus we were confident that our rudimental method was accurate enough.

For this measurement we split in two groups. One stayed at the Boat Yard and the other one went to the Beach Hut.

Each group set up a device that allowed to project a clear shadow of a ping-pong ball onto the ground.

With the help of a plumb line we marked the position of the vertical projection of the ping-pong ball on the ground.

Then we started to observe the shadow moving slowly and we marked its position on the ground at regular intervals of five minutes.

We marked the movement of the shadow for an hour and a half and then we measured all the distances between each mark and the vertical projection of the ping-pong ball.

We prepared a table where, for each position of the shadow, we wrote down the corresponding time and the distance.

Where the curve is lowest, there is the local midday.

For these data that were collected at the Beach Hut, one group calculated the local midday at 12:41

Unfortunately no measurements could be taken at the Boat yard, because it kept on raining all the time.
We decided therefore to get this second value using a software that calculates the local midday of any place in the world, given the coordinates of the place and the day of the observation.

With the help of a GPS we obtained the input data:
- Longitude: 3° 26.198'
- Latitude: 6° 26.403'
- Date: 11th May 2003

And from the software we obtained the local midday at the Boat Yard: 12h:42min:41sec or 12h:42.68min

In conclusion, the time interval between the local midday at the two location is:
12:42.68 - 12:41.00 = 1.68 minutes

  X axis: Time of measurement
  Y axis: distance between Vertical Projection and Sun Shadow

The more you are away from the equator, the shortest is the radius that you will measure with our method.

In this case you might need to correct your measurement (r) using the latitude of your location (λ):

R = r / cos(λ)

We also applied this correction, but it turned out to be not very significant, since we are so close to the equator.

The average latitude of the Boat Yard and the Beach Hut is: 6° 26' = 6.43°

R = 6263 / cos(6.43) = 6303 km