How Eratosthenes measured the circumference of the Earth

Eratosthenes was an interesting character

He was a mathematician who lived in the 3^rd century BC who did some very important work.

He gets a mention on this course for his brilliant measurement of the Earth’s circumference.

Despite all his great work in many fields his nickname was ‘Beta’ as he tended to be second best in a number of disciplines.

Eratosthenes lived in Alexandria in Egypt but was born in Syene (now Aswan) and it was these two places that he used in his calculation.

 

Syene lies on the Tropic of Cancer.

On June 21^st (midsummer’s day) on the Tropic of Cancer, the sun is directly overhead (at our Zenith) at noon.

Any vertical object would have no shadow.

He was in Alexandria though, which is about 800 km North.

He measured the angle of the Sun here by looking at the shadow of a vertical object and calculated that this angle must be the same angle between Syene and Alexandria from the centre of the Earth.
 

See diagram below.

 

This showed that there is 7 degrees between Syene and Alexandria.

You need about 50 lots of 7 degrees to make a full circle so you must need about 50 lots of the distance from Syene to Alexandria to get around the world.

He calculated the Earth’s circumference to be about 40,000 km, which was pretty much spot on!

If you are able to understand this, you should do very well on the course.

A* and A grade students will be able to explain how Eratosthenes did his calculation with the use of a diagram.

Using the National School's Observatory (NSO)

It is now easy to get your own images of the Universe using a professional telescope
You won't need to buy any equipment and you can do it from the comfort of your computer.

You just need to get a log in for the National School's Observatory (NSO) off Mr Williams.

It's free and it allows you to request the Liverpool John Moores University telescope, located in sunny Tenerife high up on a mountain, to take an image of a point of interest of your choosing in the night sky.

The special thing about this is that they are YOUR images.
The University got funding for the project by giving up time on their telescope to schools like ours so your image will be unique.

You can use it to get images of the Planets, Craters on the Moon, Nebulae, Galaxies or even search for asteroids.
Below are some examples of 3 colour images produced by our Astronomy class of 2012 as part of their coursework.

                

M57 (The Ring Nebula) by Lottie Aitken                                                                 NGC 6946 by Emma Irving

                     

M27 (Dumbell Nebula) by Mark Hogan                                                 M76 (Little dumbell nebula) by Oriana Gibson

Just get logged in, click on 'Go Observing' and follow the instructions.
You may have to wait a few days for the telescope to get round to taking your image.
Once you have your obesrvations you will need to download and use LT image to view them.
This page explains it all.

Enjoy!