01
Diameters, not precise boundaries
"Where the Milky Way ends" or "how big the Local Group is" doesn't have a sharp edge — these are commonly cited approximate diameters.
Universe · Question
Scale of the universe: how big is the observable universe?
The observable universe is roughly 93 billion light-years across; that is the region we can observe, not necessarily the size of all space. Zoom out from human height through planets, stars, and galaxies to compare each scale jump without confusing size with separation.
Visual model
Eight zoom-outs.
Source: NASA (Apollo 17) · Original source and image context
Step 1 of 8
You
~1.7 m tall
Everything else on this ladder is measured relative to something you already understand: your own height.
The precise logarithmic ruler below remains available without this illustration.
The camera transition is an artist's conceptual illustration, not a continuous physically scaled journey. The eight-step montage now uses actual NASA mission photography, ESA/Hubble observations, and one explicitly labeled JPL artist's concept; the images are documentary references, not a shared physical scale. Each chapter compares a different quantity (height, diameter, orbital span or separation). The 27-decade ruler and table provide the numeric comparisons. A Hubble deep field is a tiny observed patch of sky, not a picture of the whole observable universe.
Source-image provenance and limits
Each active chapter shows its full credited source directly below the image. Photos and composites are not measurements of the 1.7 m example human, planetary separations or the observable universe's boundary. Sources include Apollo 17, Apollo 17 Earth, Voyager 2 Neptune, ESA/Hubble Proxima Centauri, NASA/JPL artist concept of our Milky Way, ESA/Hubble Andromeda survey view, and Hubble XDF. Source photographs are cropped and subtly panned to fit a cinematic layout; their camera choreography is not a journey through literal distance. The Earth–Moon explanation remains a numerical ratio; the imagery is illustrative, not the two bodies shown in a true joint photo.
A ruler across 27 powers of tenSize / separation in metres · logarithmic scale
Equal horizontal gaps on this ruler represent equal multipliers of ten, not equal kilometres. The eight marks are positioned by actual approximate values. The measures differ (height, diameter, orbit span, distance): check the description when comparing them.
Starting scale: one human height
This point represents roughly 1.7 metres. Select Earth to see how many tenfold jumps separate these lengths.
Preparing the scale controls. The first step and the eight-step reference table below are ready to read.
Teacher quick use
Make orders of magnitude discussable.
A short predict-and-compare prompt for the eight-step scale ladder.
- Learning objective
- Compare astronomical scales across orders of magnitude and distinguish the observable universe from the whole universe.
- Activity
- Before each zoom-out, ask students to predict whether the next jump is tens, hundreds, thousands, or millions of times larger.
- Misconception to test
- “The edge of the observable universe is a measured physical edge of all space.”
What's simplified
These are round numbers, not exact ones.
02
The universe may be infinite
93 billion light-years is the diameter of the observable universe — the part whose light has had time to reach us. The whole universe could be larger, or infinite.
Scale of the universe · reference
Eight scale steps, spanning about 26 orders of magnitude.
The table mirrors the interactive model above. “Size” means height, diameter, span, or distance depending on what is being compared, so unlike quantities are not mislabeled as diameters.
| Step | Scale | Measure used | Approximate value | Jump from previous step |
|---|---|---|---|---|
| 01 | Human | Height | ~1.7 m | Baseline |
| 02 | Earth | Diameter | 12,742 km | ~7.5 million × |
| 03 | Earth–Moon | Average center-to-center distance | 384,400 km · ~1.28 light-seconds | ~30 Earth diameters |
| 04 | Solar system to Neptune | Approximate orbital span | ~9 billion km · ~60 AU · ~8.3 light-hours | ~23,000 × Earth–Moon distance |
| 05 | Nearest star | Distance to Proxima Centauri | ~4.25 light-years | ~4,500 × solar-system span |
| 06 | Milky Way | Rounded stellar-disk diameter | ~100,000 light-years | ~23,500 × nearest-star distance |
| 07 | Local Group | Approximate group extent | ~10 million light-years | ~100 × Milky Way diameter |
| 08 | Observable universe | Diameter | ~93 billion light-years | ~9,300 × Local Group extent |
Scale questions
What the 93-billion-light-year number actually means.
Q1
How can the universe be 93 billion light-years across if it is about 13.8 billion years old?
Because space expanded while the light traveled. The matter that emitted the oldest light we can observe is now roughly 46 billion light-years away in comoving distance, making the observable region about 93 billion light-years across.
Q2
Is the observable universe the entire universe?
No. “Observable” describes the region from which information has had time to reach us. It is not a measured physical wall, and observations do not tell us the total size of the full universe.
Q3
How much wider is the observable universe than the Milky Way?
Using rounded diameters of about 100,000 light-years for the Milky Way and 93 billion light-years for the observable universe, the observable universe is roughly 930,000 times wider.
Misconception check
Quick check.
Quick check
Go deeper
Keep exploring.
Next in this series →
What is a light-year? ↗
Understand the distance unit used for the final four steps of this scale ladder.
Solar System
Distance scale
Zoom back in and compare planetary distances without compressing them into one diagram.
Galaxy
Milky Way
See where the Sun sits inside the galaxy used in step six.