The distance light travels in one year — about 9.5 trillion kilometres. Light is the fastest signal possible (~300,000 km/s), and even at that speed, getting across the galaxy takes 100,000 years. Distances in space are so large that 'how long does light take to get here' becomes the most useful unit.

How big is the observable universe?

About 93 billion light-years in diameter. That sounds bigger than the universe's age (13.8 billion years) times the speed of light, and it is — because space itself has been expanding while the light has been travelling. The observable universe is just the part we can see; the actual universe is much larger (and possibly infinite).

Are we at the centre of the observable universe?

Yes — but so is every observer everywhere. The observable universe is just a sphere centred on you, defined by 'how far has light had time to reach me since the universe became transparent'. An observer in a distant galaxy would have their own observable universe, partially overlapping ours. No location is privileged.

The Universe at a Glance — Sizes, Scales, and What's Where

The universe is huge, and structured

The cosmos is organised in nested scales, each one dwarfing the last. The mistake people make when picturing space is imagining everything at the same scale — as if planets, stars, and galaxies were comparable kinds of objects. They aren't, by many orders of magnitude.

To see the layout, walk up the ladder of scale. Each rung up is roughly 1,000× larger than the one below.

Rung 1 — Inside the Solar System

Earth. ~13,000 km across. Light circles it in about a tenth of a second.
Earth–Moon. ~380,000 km. Light takes 1.3 seconds to make the trip.
Earth–Sun. ~150 million km, called one astronomical unit (AU). Light takes 8 minutes 20 seconds. When you watch a sunset, you're seeing light that left the Sun eight minutes ago.
Solar System (out to Neptune). ~30 AU, light hours.
Solar System (out to the heliopause, where the Sun's wind stops). ~120 AU, ~17 light-hours.

The Solar System is the world of light-seconds, light-minutes, and at its outermost edge, light-hours. We've sent spacecraft (Voyager 1 and 2) to its edge; the trip took 45+ years at over 60,000 km/h.

Rung 2 — The local stars

Proxima Centauri, the nearest star to the Sun. 4.24 light-years away. Light from it left in the year 2022.
Major nearby stars (Sirius, Alpha Centauri, Barnard's Star, Wolf 359). Within 10 light-years.
Stars you can name (Betelgeuse, Vega, Polaris). Tens to hundreds of light-years.
The Pleiades star cluster. 444 light-years. Light left it during the time of Galileo.

Distances inside our neighbourhood of stars are light-years. Even our nearest stellar neighbour is over 4 years away at light speed. The Apollo missions reached the Moon in 3 days; reaching Proxima at the same speed would take 134,000 years.

Rung 3 — The Galaxy

The Milky Way (our galaxy). 100,000 light-years across, containing ~200 billion stars.
Distance from Earth to the galactic centre. ~26,000 light-years.
Sagittarius A*, the supermassive black hole at the centre of the Milky Way. 4 million times the mass of the Sun. See the black hole article.

Our galaxy is a flat disc with spiral arms. The Sun sits about halfway out from the centre. We're moving around the galactic centre at about 220 km/s — so the Sun completes one orbit every ~225 million years (the dinosaurs we know of all lived during a single such galactic year).

Rung 4 — Galaxies and groups

The Andromeda Galaxy (M31), our nearest large neighbour. 2.5 million light-years away.
The Local Group. ~10 million light-years across, containing the Milky Way, Andromeda, Triangulum, and dozens of dwarf galaxies.
The Virgo Cluster. 65 million light-years away, containing 1,000+ galaxies.
The Laniakea Supercluster. 500 million light-years across, containing 100,000+ galaxies including ours.

Galaxies cluster gravitationally into groups; groups into superclusters; superclusters into vast filaments and walls separated by emptier voids — the so-called cosmic web.

Rung 5 — The observable universe

The observable universe. ~93 billion light-years in diameter. Contains an estimated 200 billion galaxies.
Age of the universe. 13.8 billion years.

Here's where intuition breaks. The observable universe is 93 billion light-years across, but the universe is only 13.8 billion years old. How? Because space itself has been expanding while light has been travelling. Light from distant objects left them when the universe was smaller; the universe has expanded since, stretching out the distances. The light is still arriving from points that are now much farther away.

The observable universe isn't all of the universe — it's just the part close enough that light from it has had time to reach us. The actual universe is larger, possibly infinite. We can't see beyond the observable boundary because the light from there hasn't reached us yet.

Why this matters — time is built in

When you look at distant things, you're looking at the past. The Moon you see is the Moon as it was 1.3 seconds ago. The Sun you see is 8 minutes old. The closest stars are years to centuries old. The Milky Way's centre is 26,000 years old as you observe it. Andromeda is 2.5 million years old as it appears to you tonight.

The most distant galaxies the James Webb Space Telescope has photographed sent their light when the universe was 200-400 million years old, billions of years before Earth existed. We're literally watching the universe assemble itself in slow motion across all distances.

Why we can't see further

At distances of about 13.8 billion light-years, you reach a wall called the cosmic microwave background — light emitted when the universe was 380,000 years old and first became transparent. Before that moment, the universe was a hot opaque plasma; light couldn't travel through it. So we can't see anything older than the CMB by direct light.

Different signals can in principle probe even earlier moments. Gravitational waves and neutrinos passed through the early plasma freely. But we don't yet have detectors sensitive enough to see them from the very early universe.

A sense of scale

To make all of this concrete:

If the Earth were a marble, the Moon would be a small bead 30 cm away. The Sun would be a 13-meter-wide ball 1.4 km away.
Same scale: Pluto would be a poppy seed 60 km away. Proxima Centauri would be another 13-meter ball 14,000 km away — about the diameter of Earth itself.

You cannot visually picture the universe to scale. The mind is not built for it. The yardstick is light travel time, and at every step up the scale, it stretches by a factor of thousands.

If you'd like a guided 5-minute course on cosmic scales with quizzes, NerdSip can generate one on this topic.

The takeaway

The universe is nested: planets around stars (light-minutes), stars in galaxies (thousands of light-years), galaxies in groups and superclusters (millions of light-years), all inside an observable universe 93 billion light-years across. Light travel time is the unit, and every observation of something distant is also an observation of the past. We sit halfway out from the centre of one ordinary galaxy in one ordinary supercluster, watching the universe still unfolding in light that has been travelling for billions of years.

The Universe at a Glance

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