Its outer surface is covered in scales made of keratin, the material of hair, horns, and claws. Turtle shells are made mostly of bone the upper part is the domed carapace, while the underside is the flatter plastron or belly-plate. Like other amniotes (reptiles, birds, and mammals) they breathe air and do not lay eggs underwater, although many species live in or around water. They are found on most continents, some islands and, in the case of sea turtles, much of the ocean. There are 360 living and recently extinct species of turtles, including land-dwelling tortoises and freshwater terrapins. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked turtles), which differ in the way the head retracts. We can measure this very accurately using satellites, which is important for all sorts of applications including understanding climate, volcanoes and earthquakes.Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs.

It is also slightly lumpy, due to variations in the density of the material it is made up of.

These days, we also know that the Earth isn’t really a sphere – it’s a slightly flattened ellipsoid (egg shape), mainly due to the fact that it is spinning. We now know the Earth’s circumference is just over 40000 km, so Eratosthenes was amazingly close. To find out more about Eratosthenes Measuring the Earth, read this article. This meant the circumference of Earth was about 50 x 790 km = 39500 km. He picked Alexandria, about 790 km from Syene (measured by camel travel!), and he showed the angle at Alexandria was about 7.2 degrees, or about 1/50 of a complete circle. Eratosthenes realised that if he could measure the angle of a shadow cast by a stick at noon on the same day somewhere much further north (or south), and he knew the distance to that place, he could calculate the curvature of the Earth. He had heard about a well in Syene (now Aswan, Egypt) where at noon on the summer solstice (21st June), the sun would be directly overhead and so shine straight down the well, without casting any shadow. Then about 240 BC Eratosthenes devised an experiment to estimate the Earth’s circumference. The Ancient Greeks noticed that the shape of the Earth passing across the moon during a lunar eclipse was a crescent, which means the Earth has to be curved. Read this article from NASA which includes an answer to 'how do we know the earth is round?'.Īmazingly, we’ve known the Earth is round for over 2000 years.

Flat earth turtle