By identifying patterns in the periodic movements of the sun, moon, and Earth, ancient civilizations realized that identical eclipses repeat about once every 6,585 days, known as the saros cycle.

Every eclipse is preceded or followed by another eclipse, separated by about two weeks and alternating between solar and lunar types.

Although the moon passes between the sun and Earth each month, its orbit is tilted by five degrees, preventing Earth from experiencing a solar eclipse each month.

Over thousands of years, the frequency of solar eclipses is greatest around the Arctic and Antarctic Circles and lowest near the equator.

Solar eclipses are a cosmic coincidence because the moon is 400 times smaller than the sun but 400 times closer to Earth, allowing it to completely block sunlight.

According to the Greek historian Herodotus, the first person to successfully predict a solar eclipse was the Greek philosopher Thales of Miletus in 585 BCE, with the resulting celestial event ending a half-decade-long war.

While solar eclipses were considered ominous throughout history, multiple cultures explained the celestial event as the result of mythical creatures nearly consuming the sun.

In the third century BCE, the Greek astronomer Aristarchus of Samos estimated the distance to the moon using geometry after measuring the length of Earth's shadow during a lunar eclipse.

While the sun, moon, and stars rise and set from east to west due to Earth's rotation, the moon's eastward revolution around the Earth causes its shadow to move from west to east during a solar eclipse.

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

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.

In 1676, Danish astronomer Ole Rømer determined that light did not travel instantaneously by observing a delay in the appearance of eclipses on Jupiter when it was farther from Earth.

The Aztecs synced their solar calendar with a complex 260-day ritual cycle

Researchers have uncovered how the Aztecs aligned their agricultural solar calendar with a symbolic 260-day ritual calendar, utilizing celestial tracking and monuments. Evidence from ruins and writings explains why their new year corresponded to February 23 on the Gregorian calendar.

Astronomers can identify systems with pairs of orbiting stars by observing recurring dips in the system's brightness as the stars eclipse one another.

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.

How the ancients calculated Earth's size correctly

Watch this seven-minute video to hear Carl Sagan explain how ancient Greek polymath Eratosthenes correctly calculated the size of the Earth in the third century BCE. At some point, Eratosthenes noticed a discrepancy in the length of shadows cast in different cities during the summer solstice. This observation grew into a simple experiment that enabled Eratosthenes, the head librarian at the Library of Alexandria, to calculate the Earth's size precisely.

Just before and just after a total solar eclipse, sunlight can pass through uneven terrain on the lunar surface, producing disconnected bits of light around the moon called Baily's beads.

Earth’s tilt and orbit cause the sun to create a figure-eight pattern

The shape—called an analemma—can be obtained by photographing the sun from the same location and time daily for a year. The Earth's eccentricity created the analemma's width, and its tilt shifts the sun's height in the sky over the seasons.

The ancient Roman origins of the leap year

The earth's orbit around the sun takes about 365.2422 days, leaving farmer almanacs the awful job of pretending the calendar year fits into a whole number. This article explores the modern origins of the civil solar calendar and its leap year. The Julian calendar, named after the Roman emperor Julius Caesar, cut the calendar year into 365.25 days, gifting the public an extra day every four years. Learn more of the history here.

Differences in the tilts of planetary orbits make perfect alignment impossible

Mercury deviates within 7 degrees of the ecliptic that marks the path of the sun, with the other planets varying from 3 degrees to approximately flat. Even an alignment within 1 degree between all eight planets is estimated to occur only once every 13.4 trillion years.

More than 2,000 years ago, a hand-powered model of the solar system called the Antikythera mechanism—the oldest example of an analog computer—could predict solar eclipses.

Scientists keep track of time via light absorbed by cesium and strontium atoms

These absorptions happen in cycles, where one second is defined as 9,192,631,770 cycles of cesium atom radiation. Strontium atoms can be used to make clocks that do not gain or lose a second in 15 billion years.

Changes in Earth's orbital cycles influence the timing of ice ages

These cycles include changes in Earth's orbit shape, tilt, and wobble, all of which alter how much solar energy different parts of Earth receive over time. Due to orbital eccentricity, summers in the Northern Hemisphere last about 4.5 days longer than winters.

Aristotle discovered that Earth was spherical by observing the round shadow it cast on the moon during lunar eclipses and by noting that different constellations were visible from different latitudes.

View historical records of solar observations and discoveries, from telescopes in the early 1600s revealing sunspots to NASA's Parker Solar Probe photographing the sun from within its outer corona in 2025.

Learn how the Egyptians, Romans, and other ancient civilizations worshipped the sun

In Egypt, the pharaoh was seen as the living embodiment of the son of Ra, the sun god. In Mesopotamia, the sun god Shamash was a divine judge whose light could reveal the truth. Huitzilopochtli, the Aztec god of sun and war, was thought to require sacrifices to prevent the universe's destruction.

Because the half of Earth that faces the sun is larger than the area of the shadow cast by the moon during a solar eclipse, observers may experience a total, annular, or partial eclipse.

How its uneven rotation produces the sun's 11-year cycle and flips its magnetic poles

While charged material rises and falls in the sun's convection zone, which induces magnetic fields, different latitudes rotate at different rates, making these fields increasingly turbulent. This builds to a critical point—the solar maximum—where magnetic field-driven events, such as solar flares, become more common before the field collapses and flips, restarting the cycle.

Blood moons only occur during total lunar eclipses, when the Earth blocks the sun, but its atmosphere lets redder parts of sunlight through to reflect off the lunar surface.

Researchers once thought Stonehenge functioned as a precise astronomical computer

In the 1960s, scientists used early computers to analyze the stone layout at Stonehenge, arguing it could predict eclipses and other celestial events. This BBC Sky at Night Magazine piece explains how the theory arose—and why scholars now doubt it.

Visualizing the history of Saturn and its rings

Every 13 to 16 years, Saturn's rings become far less visible because its plane becomes exactly even with Earth's. This visual-rich essay explores the history of humanity's perception of Saturn, from Galileo's first sighting (and dubbing the rings "ears") to the idea that the rings won't last more than a few hundred millions years more. Explore this fascinating planet here.