Ancient Greek astronomer Hipparchus created accurate maps of the night sky way ahead of his time, with coordinates sometimes within a single degree of modern values.

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.

From the Greek for 'star taker,' the astrolabe allowed individuals to tell time and determine their latitude based on the position of the sun or other stars, as well as calculate the height of objects.

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.

View depictions of world maps throughout history

The first map to use latitude and longitude positions was created by Ptolemy in 150 CE, over 700 years after the oldest known world map, the Imago Mundi, was made with a Babylonian clay tablet. The Spanish crown created a secret master map from hundreds of sailors’ reports.

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.

Early timekeeping tools included sundials, water clocks, and astrolabes

Astrolabes also provided a person's latitude based on the height of the stars in the sky, thus aiding in navigation. Incense and candle clocks relied on the constant burning rate of fire to track the passage of time based on premarked locations with equal-interval spacing.

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.

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.

No map of the Earth is perfect because each introduces inaccuracies in the shape, size, or orientation of landmasses and oceans by trying to flatten the planet's curved surface.

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.

Gaia mapped two billion stars in the Milky Way

Using geometric parallax, the European Space Agency mission made position measurements accurate to 2.78 × 10⁻¹⁰ degrees, revealing large-scale galactic structure and dynamics. Remnants of ancient galactic mergers were also identified within the Milky Way.

What the stars once meant to ordinary people

The stars were a crucial aid for ordinary human beings in premodern societies, acting as regular timekeeping phenomena and celestial navigators. Today, light pollution and modern technology have rendered the stars a mere aesthetic delight for most of us, but their significance in history remains. Watch this brief video to learn how premodern humans used the stars for navigation, mathematics, and more.

Asteroids were discovered by 18th-century planet hunters

While looking for a planet between Mars and Jupiter to coincide with an apparent mathematical pattern in the distances of known planets from the sun, a group calling themselves the "Celestial Police" discovered multiple objects that fell into a new classification.

The Antikythera mechanism: The oldest-known analog computer

A device now considered the world's oldest computer was pulled from a shipwreck more than 120 years ago. Known as the "Antikythera Mechanism," it is now believed to have allowed precise tracking of astronomical positions, and its sophistication overturned understanding of Ancient Greek capabilities.

Explore a detailed map of Odysseus' fictional journey from 'The Odyssey.'

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.

Galileo's Jupiter discovery shattered Earth-centered universe beliefs

Using a telescope inspired by Dutch designs, Galileo spotted four moons orbiting Jupiter in 1610, providing undeniable evidence against geocentrism and fueled support for the heliocentric model. Galileo’s original telescope design stretched 20 meters long—a feat of optical engineering for its time.

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.

Astronomers inferred Neptune's existence through mathematics while attempting to explain irregularities in Uranus' orbit, but Neptune had been directly observed 234 years earlier by Galileo.

Halley's comet is named after the astronomer who predicted its next appearance

Edmund Halley realized that historical accounts of a celestial object passing in the night sky in 1531 and 1607 referred to the same object he observed in 1682. By applying Newtonian mechanics to his observational data, he calculated its orbit and accurately predicted its return in 1758.

The history of the world map

What did people used to think the world looked like? From the beginning of human history to right before the age of exploration, learn how the world maps were created.

Galileo’s discovery of Jupiter’s moons led to multiple scientific advancements

Beyond challenging Earth-centric views upon their discovery in 1610, the Galilean moons were also later analyzed to contribute to early astrometry and astronomical interferometry. Measurements of eclipses were also used to estimate the speed of light.

The 'Celestial Police' accidentally discovered asteroids while planet hunting

The group was formed in 1800 to find a planet some astronomers believed existed between Mars and Jupiter. Prior to their disbanding in 1815, the group instead discovered Vesta, Juno, and other objects they would call asteroids.

The scientific name for northern lights came from two Greco-Roman gods

Although cave paintings may have depicted them millennia earlier, one of the oldest known references to auroras is a Babylonian cuneiform tablet from about 600 BCE. In 1619, Galileo Galilei named the phenomenon in the northern hemisphere "aurora borealis" after the Roman goddess of dawn—Aurora—and the Greek god of the north wind—Boreas.

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.

The new history of the Milky Way

New data garnered from European spacecraft Gaia in 2018 allowed astronomers to rewrite the Milky Way Galaxy's biography. Charting the movements of more than 1 billion stars, Gaia made astronomers reassess how our galaxy formed. Evidence of long-ago collisions and proof of unexpected and unusual movements were just the beginning of the beginning.

Astronomers use a set of scaffolding methods to measure distances across the universe

Different methods are used depending on how far away an object is—from radar for nearby planets, to parallax for stars, to redshift and supernovae for distant galaxies. These tools allow scientists to chart the universe across unimaginable scales using astronomical units (AU), light-years, and parsecs.

Explore the night's sky from anywhere

Stellarium Web is a planetarium running in your web browser. It shows a realistic star map, just like what you see with the naked eye, binoculars or a telescope. Using its interface, you can click on object's visible at your location on the Earth...

Galileo Galilei—considered the father of modern science—was once president of the Academy of Lynxes and used the name Galileo Galilei Linceo on the title pages of his works in its honor.

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.

Explore how far you've traveled through space while living on Earth

Using the "Cosmic Odometer," users can enter their birthday and learn how far they have moved through space due to the combined effects of Earth's rotation around its axis, the planet's orbit around the sun, the solar system's trajectory through the Milky Way, and the galaxy's motion through the universe.

Railroads and long-distance communication led to the development of time zones, with Greenwich established as the prime meridian—the longitude of 0 degrees—due to British advancements in star mapping and navigation.

The breakthrough that changed our perception of the universe

In the 1920s, the Hooker Telescope helped Edwin Hubble make a discovery that forever changed how we see the universe. Building on the work of astronomers like Henrietta Leavitt and Harlow Shapley, Hubble used the telescope to prove that galaxies exist beyond the Milky Way—revealing a vast and expanding universe.

Collections of stars like the Big Dipper are not constellations because they are not among the 88 groups of stars designated as such by the International Astronomical Union.

How did Polynesian wayfinders navigate the Pacific Ocean?

Ancient Polynesian navigators used the sun, moon, stars, clouds, currents, and more to sail the seas. People of Southeast Asia began their voyages to the region now known as Polynesia in roughly 1500 BCE. They would settle a vast area of 15 million square miles, including hundreds of islands, moving from islands due to overpopulation or war. Such voyages dwindled in 1300 CE as societies became more rooted.

Charting 4,000 years of human history

Care to hang all of human history on your wall? This data visualization charts 4,000 years of humanity. This "histomap" was originally published in 1931 and sold for $1 per copy of the 5-foot-long chart. It was said to detail the course of...

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.

The New Year's ball was based on a navigational tool

Robert Wauchope, a member of the Royal Navy, developed the contraption in 1818 so crews could know the time while out at sea. The ball would descend at the same time every day, allowing ships to set their clocks accordingly.

New ways of seeing the cosmos have redrawn the map of human identity

From Galileo’s crude spyglass to the James Webb Space Telescope, each technological leap has expanded our observational reach and redefined philosophical and moral conceptions of our place in the universe. Identifying signs of life through the study of exoplanet atmospheres would represent the next great revolution.

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.

Space telescope development was driven by a desire to bypass Earth's atmosphere

The most important figure in the effort to create what would become the Hubble Space Telescope was Lyman Spitzer, who first proposed a telescope in orbit in 1946. The nearly 50-year endeavor led to space-based observatories, which are not limited by Earth's atmosphere absorbing or distorting most wavelengths of light.

The Hubble Space Telescope is named after astronomer Edwin Powell Hubble

Using the Hooker Telescope at the Mount Wilson Observatory in California in the 1920s, Hubble identified Cepheid variable stars in what was believed to be the Andromeda nebula. These stars can be used to determine distance in space, allowing Hubble to realize that Andromeda was a galaxy of its own.

Nancy Roman, NASA's first chief of astronomy and the architect of the Hubble Space Telescope, was discouraged from pursuing science throughout her academic career.

A map of the Milky Way

Curious about the layout of the cosmos? This resource presents a detailed map of our galaxy, the Milky Way. Learn the basic structure and identify prominent global clusters, nebulae, main arms, spurs, and constellations. With numerous different maps and mediums, get oriented with the universe’s configuration and where exactly we fit into it.

How do scientists study far away stars?

Astronomy is the only science that is purely observational: meaning, scientists can't study stars in a lab. Here's a breakdown of how modern researchers use new tools to understand objects billions of light years away. As it turns out, the rules of science allow us to identify, categorize, and experiment with phenomena light years away.

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.

The Greek city of Alexandria in Egypt helped form the template for modern cities by considering function, commerce, culture, and politics.

The ancient Greeks and Romans viewed comets and meteors as divine omens

Both considered the appearance of a comet or its fragments as a precursor to significant events, such as the birth of a notable figure or the death of a ruler. One observed in 44 BCE was believed to be a sign of the deification of Julius Caesar after his assassination.

Some of the oldest photographs in the world

Scholar and artist Joseph-Philibert Girault de Prangey ventured around the Mediterranean with some of the earliest photographic equipment in tow. He captured hundreds of images during his tour of Rome, Venice, Cairo, and more, in what would become...

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