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 How did ancient scientists conclude that the earth is a sphere?

 

First of all, does the shape of the Earth really matter? Who cares if it's flat, round, or oval shaped? Will it interfere with our day to day life? Actually, the shape of the Earth matters a lot. The GPS system relies heavily on the Earth's roundness. Earth's shape affects tides and weather patterns, allowing us to predict weather and tides more accurately. Very large scale engineering projects, such as the towers of the Humber Bridge in the UK, are influenced by the Earth's spherical shape. Without understanding this, engineers couldn't have built such masterpieces. Additionally, telecommunications, astronomy, celestial navigation, and other fields are heavily affected by the Earth's spherical shape. This concludes that knowing the Earth's shape is important, even in our daily lives. So, let's embark on the journey of discovering the Earth's shape.

Observations Of The Moon

The first real proof of the Earth being a sphere came from the Moon. To measure an object, we need a reference object at some distance for comparison. To determine the Earth's shape and size, the Moon serves as a suitable reference. There's a theorem stating that if every projection of a 3D convex body is a flat disk, then the 3D object must be a sphere. Suppose you bring a spherical 3D object near a mirror; from every angle and position, the reflection in the mirror is always a flat disk. This proves that the 3D object you're holding is a sphere. In two dimensions, this isn't true. For example,

 

 

 The Reuleaux triangle projects the same line from every angle. You can see this in a GIF where if we put a light above the object, the object will always project a line type shadow because it's in two dimensions.

 

If you look at the Earth from any direction in outer space, it always appears as a flat projection, confirming that the Earth is a sphere. But of course ancient scientists didn't go to outer space. They used the moon. A lunar eclipse reveals the Earth's shape. A lunar eclipse occurs when the Earth comes between the Sun and the Moon, causing Earth's shadow to fall on the Moon. This alignment can only happen during a full moon when the Sun, Earth, and Moon are closely aligned in a straight line. Lunar eclipses aren't always total; sometimes, they're partial or penumbral.

 

 

 

Here, In this composite lunar eclipse pic, we can see the Earth's shadow is a perfect circle. Eclipses don't always happen at the same place or time; they occur at random locations and times. This indicates that all the Earth's shadows falling on the Moon always depict a flat disk. You will always see a round shadow from every part of the world. This indicates that the Earth's shadow is always flat on the moon regardless of where you are watching it from. Using the previous theorem we talked about we can say the earth is a 3d sphere.

 

Aristotle's Contribution

 

Now, you may wonder which great mind discovered this. The great mind, sometimes called one of the greatest scientists to ever exist, is Aristotle of Stagira, an ancient Greek philosopher and polymath. Aristotle proposed this idea around 350 BC. You might also wonder why this idea wasn't popular in the past. Contrary to popular myth, almost all educated people in the Middle Ages (5th to 15th centuries) believed the Earth was a sphere. Aristotle's proposed idea is the earliest scientific proof of the Earth being a sphere. It's true that many cultures accepted the idea of a spherical Earth very late. This was because they didn't want to deviate from their cultural beliefs, not because Aristotle's work was debunked.

Initial Skepticism and Acceptance

Aristotle's discovery wasn't fully accepted at first. Critics argued that the flat disk shadow of the Earth was due to atmospheric distortion. Some skeptics pointed to the appearance of crepuscular rays sunlight beams that seem to diverge from the Sun as evidence against a spherical Earth. In certain periods, especially during the Middle Ages, literal interpretations of religious texts led some to believe in a flat Earth. Despite this heavy criticism, Aristotle's discovery eventually prevailed because it was the ultimate truth.

Eratosthene’s Measurement

Building upon Aristotle's work, Eratosthenes, a Greek mathematician and the chief librarian at the Library of Alexandria, made a significant contribution by calculating the Earth's circumference. He observed that at noon during the summer solstice in Syene (modern-day Aswan, Egypt), the Sun was directly overhead, casting no shadow. However, at the same time in Alexandria, shadows were cast at a specific angle. By measuring this angle and knowing the distance between the two cities, Eratosthenes used geometry to estimate the Earth's circumference with remarkable accuracy. This calculation provided quantitative proof of a spherical Earth. We will talk about how Eratosthenes calculated the circumference of the earth very soon.

Modern Evidence Supporting a Spherical Earth

In the 1800s, Samuel Rowbotham reignited flat Earth theories. His book, Zetetic Astronomy, attempted to debunk the spherical Earth theory but ultimately failed. Nowadays, we can easily prove the Earth is a sphere through various experiments and technology. Photographs from space, composite lunar eclipse photos, airplane flight paths, time zones, and the Coriolis effect all demonstrate the Earth's sphericity. The most interesting phenomenon that disproves the flat Earth theory is the 24-hour sun in Antarctica. During the summer months, the Sun remains visible for 24 hours a day in regions within the Antarctic Circle. This continuous daylight occurs because the tilt of the Earth's axis allows the Sun to circle around the sky without setting. Such an occurrence is impossible on a flat Earth model, where the Sun would have to move in a completely different manner. This phenomenon serves as clear and observable proof of the Earth's sphericity.

Conclusion

Understanding the Earth's shape is not merely an academic pursuit but a practical necessity that impacts various facets of our lives. From navigation and communication to weather prediction and engineering, the spherical nature of our planet is fundamental. The journey to this understanding began with Aristotle's observations and has been continually reinforced by subsequent scientific discoveries. Today, the evidence supporting the Earth's sphericity is overwhelming and irrefutable. As we continue to explore and understand our planet, it is essential to recognize the importance of this knowledge and the role it plays in shaping our world. The discovery of the Earth's shape was also a stepping stone in the development of the cosmic distance ladder. Without understanding the Earth's shape, we wouldn't have been able to determine distances in the universe. 

 

 

 

 

 

 

 

 

Reference : 

https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question54.html 

https://www.astronomypassion.com/the-spherical-earth-revelation-aristotles-lunar-eclipse-insight/

https://en.wikipedia.org/wiki/Spherical_Earth

https://www.astronomy.ohio-state.edu/thompson.1847/1101/lecture_aristarchus.html

https://www.youtube.com/watch?v=YdOXS_9_P4U&t=733s