What the stars once meant to ordinary people

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.

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.

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.

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.

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.

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.

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.

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.

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.

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...

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.

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.

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 medieval meaning of your birthstone

Medieval people viewed birthstones very differently from modern jewelry marketing. Rather than simply being decorative, gems were believed to possess extraordinary magical, medical, and mystical powers. This article breaks down some fascinating insights into medieval gemstone beliefs, including that diamonds could fend off insanity and nightmares.

For the medievals, the moon was a versatile religious symbol

Within the distinct traditions of Christianity and Islam, the moon became a central metaphor and sign for religious teaching. For Christians, its light was used to explain Jesus' humanity with the light of the sun (divinity). For Muslims, the moon represented a cosmic longing for god.

Why atoms are used to keep track of time and define the meaning of a second

Small variations in the production of human-made devices, such as mechanical watches and pendulums, make them unreliable for the ultraprecise timekeeping required by modern technology, including financial transaction timestamping and GPS. Because atoms obey fundamental laws of nature and never wear out, their light emissions when energized provide a consistent timekeeping signal.

By separating light with tools such as prisms, scientists can reveal the presence and composition of celestial matter by identifying color fingerprints unique to specific elements and molecules.

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.

What did people do before eyeglasses were invented?

An estimated two-thirds of the world's adults use a form of prescription glasses or contacts, proof spectacles were a godsend when they were invented in 1286 CE. But what did people do before that? Different tools were used for distinct vision problems, but some would use magnifying lenses, clear emeralds, and more. Watch this video for a historical explanation.

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.

Hear Carl Sagan reflect on the smallness of humanity

The cosmic dark ages: before stars existed

The theories behind the Big Bang suggest the process of the universe beginning to the creation of stable, neutral atoms took just half a million years. But the transition from such atoms to star formation took surprisingly much longer, upwards of 100 million years. Why? This deep dive explores the science behind the unique physical composition of the so-called cosmic dark ages.

Within massive clouds of gas and dust called nebulae, many stars often form on short cosmological timescales and collectively orbit one another, creating groups called clusters.

Some stars take longer to form than the current age of the universe

The first stars only burned for millions of years, providing the universe's first heavy elements. Some in the current population will burn for trillions of years, delaying the appearance of black dwarfs and other exotic cold stars.

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.

Scientists validated Albert Einstein's theory of gravity by measuring the apparent positions of stars via their starlight as it traveled near the sun on its way to Earth during a total solar eclipse.

Shooting stars are streaks of light produced by meteoroids

As these rocks enter Earth's atmosphere at high speeds, friction causes them to burn up, briefly leaving streaks of light. The color of these fireballs can be used to determine the composition of the meteoroid, if it does not reach the ground and become a meteorite for analysis.

The origins of our fascination with time travel

Humans have been captivated by the prospect of leaping forward and backward in time for thousands of years. Georgia Institute of Technology Science Fiction Studies Professor Lisa Yaszek recounts how the concept has been utilized by storytellers from 400 BCE to modern-day to explore our relationship with the supernatural, technology, and the past. In a way, this historical recounting is a journey through time.

Some 19th century astronomers thought Mars was inhabited

As telescope technology advanced, astronomers became able to observe the surfaces of objects in our solar system with greater detail. The dips, valleys, and lines crisscrossing Mars, particularly, captured onlookers' imagination, leading some to speculate that the seemingly organized features could only have been created by an intelligent civilization.

Even in an infinite universe, its limited age prevents a sky full of starlight

Olbers' paradox asks why the sky would ever appear dark if the universe is full of stars in all directions emitting light everywhere. Light from the farthest stars requires more time to reach our skies than the universe's age.

Dark stars are hypothetical stars that would have existed in the early universe

Primarily composed of visible matter, this celestial object would also contain a significant amount of dark matter that would annihilate with itself, converting its mass into energy. This energy would prevent the star from collapsing under its own gravity and from triggering nuclear fusion until its dark matter fuel was exhausted.

The masses of celestial objects can be determined by analyzing their orbits, observing how they bend light, or measuring their brightness.

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.

The sun is not as exceptional as once believed, but it remains rarer than most stars

Once believed to be the center of the universe, the sun is a G-class star, the same category to which about 6% of the Milky Way's stars belong. However, the sun is younger than 85%, more massive than 95%, and more metal-rich than about 93% of all stars. Unlike the sun, half of all stars also exist in multi-star systems.

The lifestyles of early humans

For 2.5 million years, humans lived on Earth without leaving a written record of their lives—but they left behind other kinds of remains and artifacts.

A 2,000-year history of alarm clocks

Clocks have revolutionized the way we live, structuring our days into abstract, regular units and providing quantification for those who are chronically late. The alarm clock was the next obvious step: clocks which not only tell you the time but...

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.

When humans discovered fire

Fire has proven crucial for the development of human civilization, but there was a time when our hominin ancestors could not make effective use of it. This article traces the major milestones in humanity's journey with fire based on evidence from...

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.

Explore an interactive solar system with 3D visualizations of meteor showers—streams of dust-like debris left behind from comets or asteroids that burn up in Earth's atmosphere.

Light echoes are expanding shells of reflected starlight

Just as sound waves reveal the location of obstacles by reflecting off them, pulses of light emitted by stars illuminate rings of interstellar dust, giving the appearance of ejected material as the light travels outward.

By analyzing the color of starlight, astronomers can determine how fast stars within a galaxy are moving toward or away from Earth, revealing the expansion rate of the universe.

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.

A data visualization exploring six decades of spacewalks

Astronauts' initial small step for mankind may standout, but how many steps have we taken so far? Space and moonwalks further our understanding of spaceflight, celestial bodies, and space itself. Created to resemble a constellation map, this chart...

Spikes of light around stars are artifacts of light bending around the mirror supports

Through diffraction, electromagnetic waves can bend around obstacles, as observed in the silver linings at the edges of clouds, where light should be blocked. Within reflecting telescopes, starlight diffracts around struts—beams holding a secondary mirror in place—and spreads perpendicular to them, creating pairs of spikes.

The faint triangle of light occasionally seen above the horizon, known as zodiacal light, is due to sunlight reflecting off ice and dust scattered throughout the inner solar system.

Space travel sparked inventions we now use every day

Technologies developed for space exploration produced lasting spinoff innovations—from camera phones to memory foam—displaying how the space race reshaped everyday life here on Earth

Take a virtual tour inside Stonehenge, the stone monument built roughly 5,000 years ago.

Prehistoric visitors approached Stonehenge along a processional route known as the Avenue, where the sunrise of the summer solstice aligns with the sarsen stones. This interactive 3D tour recreates that experience with explainer videos and detailed sidebars.

An interactive timeline of history, from beginning to middle

The history of humanity is a long, winding road, and the history of everything else is even more complex and interwoven. This interactive timeline takes users through the history of the world and includes links to related videos, blurbs, and events. Each event in history is represented by a dot, providing users with a glittering horizon filled with moments of intrigue and impact.

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.