Category Archives: Physics

How Eratosthenes calculated the Earth’s circumference

In 240 BC, the Greek astronomer, geographer, mathematician, music theorist and librarian Eratosthenes of Cyrene (c. 276 BC – c. 195/194 BC) calculated the circumference of the Earth without even leaving Egypt. Here’s how:

Eratosthenes knew that at local noon on the summer solstice (at the time of the longest day, about 21 June in the northern hemisphere) in Syene (modern Aswan, Egypt), the Sun was directly overhead – Syene was in fact slightly north of the tropic, though (1). Local noon is – technically when the sub-solar point is somewhere over your meridian, it’s noon for you. So, on that day, Syene is the sub-solar point of Earth (the sub-solar point on a planet is where its sun is perceived to be directly overhead). To learn more about the local noon and the subsolar point, see the article titled “How Earth Moves“.

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Are we the first?

In 1950, the Italian physicist Enrico Fermi was chatting with his colleagues at the Los Alamos National Laboratory(1). They talked about the recent rise in UFO reports, then the conversation shifted to other subjects. But during the lunch, Fermi suddenly asked: “Where is everybody?”. He was talking about the extraterrestrial life, especially intelligent life. He made some calculations on the probability of Earth-like planets, the beginning of life, the probability of intelligent life and high technology and concluded that we ought to have been visited long ago and many times over.

But, there is no reliable evidence aliens have visited Earth and we have observed no intelligent extraterrestrial life with current technology nor has SETI found any transmissions from other civilizations. The Universe, apart from the Earth, seems “dead”.

So, where is everybody?

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Your Life On Earth (presented by BBC)

A nice interactive webpage by the BBC – The British Broadcasting Corporation: how you and the world have changed since you were born? You’re simply entering your birth date, gender, and height; selecting units (metric or imperial/US) and then watching how our planet (and you) has changed in your lifetime.

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No! The Earth Would NOT Look Like This Without Water

No, no no, and again: no! The Earth would NOT look like this without water. Please stop sharing this nonsense.

Here some numbers: the Earth has a diameter of about 12,735 kilometers (on average). The highest point on Earth is the top of Mt. Everest, at 8.85 km. The deepest point on Earth is the Mariana Trench, at about 11 km deep. Make the calculations and you can see, the Earth definitely would NOT look like below without water:

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A Bowling Ball and Feather Falling in a Vacuum (video)

It’s one of the basic Newton laws: how fast something falls due to gravity is determined by a number known as the “acceleration of gravity”, which is 9.81 m/s2 at the surface of Earth. The acceleration of gravity, shortly “a” means that in one second, any object’s downward velocity will increase by 9.81 m/s because of gravity. The gravity accelerates everything at exactly the same rate.

This also means a heavy object like a bowling ball and a lightweight object like a feather should fall down with the same speed, regardless of their shapes. But we see this phenomenon very rarely in our daily lives. The reason is the air resistance. That’s why people (including Aristotle) thought that the heavier objects fall faster for thousands of years.

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