PRODUCTIONS of the activity:
Eratosthenes in 360 BEST.pdf
And a "sample" in flipsnack for an embed version(free version of flipsnack is up to 30 pages)
WORKING PROCESS
A summary and easy-accessed shared working space is this slide. Please use the links in the first slide for guidelines and share this link to your students https://bit.ly/30DJw7K :
Eratosthenes historic experiment explained on this video:
More details generally for the experiment that can be conducted ant day
(you could join Ertaosthenes eTwinning project as well)
Christiane's experience:
"How to procede to take a measurement for the Eratosthenes activity"
- choose an object that will produce a shadow (wooden stick, book, box, etc)
example: 
- make sure to have a horizontal soil
- make sure to have a vertical line from the top of the object to the ground (it is easier if you choose a gnomon like the stick as the shadow that you will measure must be of an object vertical to the ground)
- take the measure from the vertical line to the end of the shadow at solar noon
(solar noon is when the shadow of the object is the shortest but you can check the exact time of your solar noon in this link https://www.esrl.noaa.gov/gmd/grad/solcalc/ if you enter your latitude and longitude)
-check on google maps for the coordinates (Latitude and Longitude) of your place (...° N , ...° E): https://www.google.com/maps
a) right click on your place
b)click on "what's here?"
c) on the popup window, under the name of your place you can see the coordinates.
- post the measurement:
- date, time your measurement (remember that you must make the measurement +/- 10mins around solar noon),
- town, country, school, longitude and latitude,
- height of your gnonom (object) in centimeters,
- length of the shadow of the object in centimeters
When do we have to measure?
Especially for the solstice, we can make a measurement any day (or days) from 15 June till 23 June. The partner for each one of us will be the cancer tropic(Latitude 23.437° any point Νorth of the Equator).
RESULTS???
The results will be announced in this page every evening by the admins so nothing more for you to do
And...what comes next?
You could try the measurement ANY DAY of the year, too! And in that case you need Collaboration among the partners
IF other schools will do the measurement on the SAME DAY at THEIR time of solar noon, then admins can find for your data the best partner to calculate the circumference of the earth. How? Nothing you need to do about that. But you can check the following video in a language that you understand better, if you want to see how this collaboration works:
An example of collaborative measurement (Lafrancaise and Petroupoli) of Earth's circumference in this video in English
and in this one in French (Lafrancaise and Ioannina)
So, when we start our measurements?
ANY DAY starting from now.