subject
Astronomy
How astronomers learn about the universe — what we measure, how we measure it, and what counts as evidence at scales no instrument can directly probe.
13 lessons in astronomy
Asteroids, Comets, and Meteors: Three Kinds of Sky Object
Look up at a clear night sky and you might see a streak of light flash across it in less than a second.
3 min · comparison
How a Solar Eclipse Actually Works
Hold a quarter at arm's length and aim it at a streetlight a block away.
3 min · foundation
How a Star Becomes a Black Hole
A star spends almost its entire life in a stalemate. Gravity pulls every atom toward the center; the heat of nuclear fusion in the core pushes back. As long as fusion keeps producing energy, the star …
4 min · deepening
How Stars Make Their Own Light
Hold your hand up to the Sun on a clear afternoon and you are catching photons that began their journey not eight minutes ago, as the textbooks sometimes suggest, but tens of thousands of years ago — possibly longer.
4 min · foundation
How We Found Planets Around Other Stars
Look at the night sky and you see stars. The planets orbiting those stars are completely invisible from Earth, even through our best telescopes. A star like the Sun is roughly a billion times brighter…
3 min · deepening
How We Know the Universe Is Expanding
In 1929, Edwin Hubble plotted the distances of galaxies against the speeds at which they appeared to be moving and saw something strange: the farther a galaxy was, the faster it was receding.
4 min · synthesis
Refractors and Reflectors: Two Telescope Designs
Point a long brass tube at Jupiter on a clear night and you are using, in essence, the instrument Galileo turned skyward in 1609.
4 min · comparison
What a Light-Year Actually Measures
The phrase sounds like a measure of time. A light-year — the very word ends in "year" — invites the ear to hear something temporal, as if it counted moments rather than miles. This is the first thing …
4 min · foundation
Why Galaxies Have Spiral Arms
If you photograph a spiral galaxy like M51, the arms look so solid you could imagine reaching out and touching them.
4 min · deepening
Why the Moon Always Shows Us the Same Face
Go outside tonight, look up at the Moon, and remember this: every human who has ever lived has seen the same side of it.
3 min · foundation
Why the Moon Always Shows Us the Same Face
Look up at a full Moon tonight, and then again next month, and again next year.
4 min · foundation
Why the Sky Is Dark at Night
Step outside on a clear night, away from city lights, and look up.
3 min · foundation
Why the Stars Twinkle but Planets Don't
On a clear night, pick out a bright star and stare at it. Within a few seconds you will see it shiver — brightening, dimming, sometimes flickering between colors. Now find a planet. Venus, Jupiter, an…
3 min · foundation