How the ancients calculated Earth's size correctly

Earth's circumference was estimated over 2,000 years ago using geometry and shadows

By measuring the angle of the shadow cast in Syene when no shadow was cast in Alexandria, Eratosthenes used the distance between the two cities and trigonometry to estimate the Earth's circumference to within a few percentage points of the actual value.

Four astronomical discoveries from ancient Greece

Modern astronomy relies on making discoveries without being present on the planets, stars, galaxies, and other objects studied. Imagine the difficulty the ancient Greeks had studying them without computers and telescopes. Yet somehow they calculated precise measurements of planetary distances, sizes, and more. Read this page to learn more about early triumphs in astronomical science.

The hour originated from an ancient Egyptian astronomical system

The movements of astronomical objects, measured in a day and a year, form the foundation of our sense of time. The Egyptians divided each day and night into 12 time eras, using arbitrarily selected stars as references rather than movements of specific celestial objects.

Explaining why the Earth wobbles

Astronomers going back to Hipparchus in the 2nd century BCE have noticed our planet has a distinct wobble. More recently, scientists have observed that the Earth is 26 miles wider at the equator than from pole to pole. The wobble results from our...

How a solar eclipse proved Einstein's theory of gravity

For hundreds of years, Isaac Newton's laws of gravity and motion accurately predicted every observation and measurement involving orbits. With the advance of technology, more precise measurements revealed anomalies, including the orbit of Mercury, which Albert Einstein's general theory of relativity could explain. This article details the two frameworks and how they went head-to-head during an eclipse for gravitational supremacy.

Ancient civilizations predicted eclipses by tracking different cycles of their recurrence

The periodic nature of the orbits of the sun, moon, and Earth results in several series called saros cycles, in which the geometric configuration of these celestial bodies that produces an eclipse recurs after about 6585 days. Because eclipses are viewable from different locations on Earth, there exist many saros cycles, one for each ecliptic configuration.

After NASA's Voyager 1 captured Earth as a single pixel as it left the solar system in 1990, astronomer Carl Sagan used the image to reflect on humanity's fragility and the responsibility to protect 'the only world we've ever known.'

Ancient Greek told to kick rocks after claiming moon wasn't a deity

About 2,500 years ago, ancient Greek philosopher Anaxagoras theorized that the moon probably wasn't a goddess. This article explains how this belief led to his exile. Anaxagoras also argued the sun was just a burning rock, writing that it was the sun that illuminated the moon. According to this resource, this theory enabled Anaxagoras to accurately explain eclipses and lunar cycles.

Earth's core is about 2.5 years younger than its surface according to relativity

Due to differences in where objects are located within gravitational fields, time will run at different rates, which has major impacts on technology that relies on synchronization. For example, GPS satellites run about 38 microseconds faster every day than clocks on Earth's surface, resulting in GPS discrepancies of about six miles, were it not for software corrections.

Mapping the shifts in ancient Greek territories

The jagged coastlines and many islands of the eastern Mediterranean peninsulas cultivated a seafaring civilization, which helped the Greeks colonize and trade in the Middle East, North Africa, and the rest of Europe. See the evolution of their territory with this quick animated map video.

What was the longest year in history?

In 46 BCE, Julius Caesar declared the year would last 445 days. This four-minute podcast and accompanying article explain why. Caesar wanted to make the following year start after the winter solstice, an achievement attainable due to his tinkering with the Roman calendar. But to start the year right, Caesar had to stretch the transitional year, leading to the creation of Caesar's Year of Confusion, the longest year in history.

Stanford built a Google Maps for the Roman Empire, revealing how much it cost to travel in the ancient world

Hear Carl Sagan reflect on the smallness of humanity

Imagining a fourth dimension

Famous astronomer Carl Sagan attempts to describe what it's like to think about the fourth dimension using the famous Flatland example from author Edwin Abbott in this classic video.

Reconstructing an average day in ancient Egypt

Egyptologists can reconstruct the intimate details of everyday life in the ancient civilization by studying the archaeological material preserved in tombs, monuments, and partially intact neighborhood layouts. Watch a comprehensive tour of the daily habits and lifestyles of ancient Egyptians with expert Dr. Joann Fletcher.

The greenhouse effect was discovered by analyzing Earth's temperature

Using mathematical techniques developed to analyze heat transfer, Jean-Baptiste Joseph Fourier showed that Earth should be significantly cooler if solar radiation were the only contributor to its surface temperature. By considering that the atmosphere could amplify the sun's warming effect, he formulated what became known as the greenhouse effect.

An illustrated guide to the ancient Greek version of the Olympics

Go back to the original time and place of the athletic contest with this illustrated guide to the Games. For 1,200 years, the ancient Greeks would gather every four years for the Games in Elis, months after heralds had spread the word of the event to the region's many peninsulas, with its greatest athletes competing for honor.

Fascinating 3D animation shows how Romans built roads

Highly efficient engineering and materials production allowed Roman administrators to lay down nearly 50,000 miles of roads across the ancient empire. These roads were built so effectively that many are still around today. Watch this quick 3-minute video to see how they did it.

The world's different calendars, explained

The calendar we use to mark the days, months, and year can feel set in stone, but the history of calendars shows these organization systems can and do change. Parse the fascinating, complex details of the Gregorian, Julian, Chinese, Hebrew, and many more calendars in this comprehensive, well-executed explainer video.

The Ancient Chinese harnessed solar power for heating and cooling 6,000 years ago

Neolithic Chinese homes faced south for winter sun warmth, while overhangs shielded them from high summer sun. These efforts also influenced urban planning, with cities aligning streets east-west for optimal solar exposure.

This flat map is still wrong, but it's the least wrong of all flat maps

While flat maps can never be as perfectly scaled as a globe, they're far more practical than carrying around a sphere. Researchers set out to minimize six major errors of flat maps, including boundary cuts and skewed proportions, by simply squashing...

What Stonehenge sounded like

Researchers built the first scale model of the world's best-known prehistoric stone circle, Stonehenge, to study its acoustic properties. Built approximately 5,000 years ago, the landmark in southern England has puzzled historians for centuries, but...

Voyager 1 captured Earth as a 'Pale Blue Dot' from 6 billion kilometers away

On Feb. 14, 1990, NASA's Voyager 1 space probe took the farthest photo of Earth ever captured. Earth appears so small in the image that it takes up less than a single pixel out of 640,000 in the photo's frame.

The team responsible for the first interplanetary sundials included Bill Nye

While reviewing designs for instruments to be carried by future Mars rovers, Nye suggested adapting the function of a calibration tool, which would be used by the Spirit and Opportunity rovers to help calibrate the brightness and color of their panoramic cameras on the Martian surface.

With the exception of Earth, the planets are named after Roman gods and goddesses

Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune are named after the messenger of the gods, the goddess of love, the god of war, the king of gods, the god of agriculture, the god of the sky, and the god of the sea, respectively. "Earth" comes from Old English and Germanic words meaning "ground."

The ancient Greek philosopher who lived in a barrel

Out with green puppets living in trash cans, and in with ancient Greek philosophers living in barrels. This article details the life and beliefs of Diogenes the Cynic, a philosopher who reportedly lived in a barrel. According to this peer-reviewed article, it's unclear if Diogenes recorded any of his beliefs in writing. If he did, no texts have been recovered.

Visualize the size of countries on the sun and other planets

Pluto is roughly the size of Australia. What about Namibia and Nepal? How small would North America appear on the sun? Compare the relative size of continents and countries with this easy-to-use interactive.

Visualize the path of every solar eclipse from 1900 to 2099 and see what each looks like when observed from different locations on Earth.

While the sun's gravity keeps the planets in orbit, their gravity also pulls on the sun, causing it and everything else in the solar system to orbit around a mutual balance point called the barycenter—the true center of the solar system.

Scientists can determine Earth's past climate conditions by analyzing air trapped in microscopic bubbles within ice cores—columns of compressed snow layers that build up over hundreds of thousands of years.

The term 'algorithm' originated from the Latinized name of al-Khwārizmī, a 9th-century mathematician whose works introduced Arabic numerals and the decimal-based numeral system to Europe and whose treatise 'Al-Jabr' gave algebra its name.

Clever Hans was a 1900s horse that was believed to be able to solve math equations, tell time, and read and comprehend German, until a psychology student discovered how he got his answers.

Learn more about the history of fireworks, which dates back to an accidental invention during an alchemist's quest for information about immortality.

Explore chaos in this simulation of two double pendulums, where small differences in the release angles produce wildly different outcomes.

For centuries, it was common to sleep in two shifts: a few hours at the beginning of the night, and a longer period beginning around midnight

Stars appear to twinkle because turbulence in the motion of molecules in Earth's atmosphere causes light to bend and wiggle rapidly on its way to the surface—a phenomenon called scintillation.

SpongeBob exemplifies Aristotle's philosophy for living well because he understands that happiness doesn't come from external factors like money, recognition, or power.

See a 3D reconstruction of Tenochtitlan, the Aztec capital that was home to an estimated population of 200,000 to 400,000.

Meet the 56 signers of the Declaration of Independence—most of whom didn't put pen to paper until August 2, 1776—and see their original signatures.

Sony set the standard for cylinder-shaped batteries when it repurposed manufacturing equipment used to make rolls of magnetic recording tape.

The Mona Lisa's celebrity status and repeated restorations have obscured what made the painting so revolutionary for its time

Waves off the coast of Nazaré, Portugal, can reach over 100 feet and are caused by the area's deep underwater canyon geography

How the most famous Ebola book, The Hot Zone, which sold 3.5 million copies and was translated into 30 languages, got aspects of the disease so wrong.

As of 2025, the most cited papers in science include a review of graphene, methods to improve image recognition, streamlining electron density calculations, and optimizing protein studies.

Long before Major League Baseball began allowing instant replay for balls and strikes, an umpire reluctantly used the technology to determine whether a hit was a home run, a move that inadvertently ushered in a new era for the sport.

Alzheimer's can lead to brain shrinkage, with an average of around 2% of volume reduction per year

Planarian flatworms are small, aquatic worms with exceptional regenerative capabilities. When cut, the worms can regenerate multiple bodies in a matter of weeks.

Gerrymandering works through two techniques: cracking opposing voters across districts to dilute their power, and packing them into a few to waste their votes in lopsided majorities.

Famed poet TS Eliot rejected the manuscript for George Orwell's political fable 'Animal Farm,' writing that its criticism of authoritarianism was not 'the right point of view.'

NBA superstar Victor Wembanyama spent the 2025 summer training with monks in central China to enhance his body and mind.