Why these yardsticks

Wherever it can, this site compares a thing with something you already have a feel for. A moon is put beside a mountain, a telescope beside a bus, an orbit beside the height an aeroplane flies.

The everyday things this site measures against

Every one of them is something you could stand next to, walk the length of, or have already looked up at, because a yardstick you cannot picture is a second number to be baffled by.

Mount Etna

Chosen over Everest as the size yardstick. Everest sits atop a high plateau, so most of its official height never appears above the surrounding land; Etna rises from close to sea level, and its full height is the height you would actually see.

A school bus

The yardstick for a spacecraft. A telescope in orbit has nothing near it to give away its size, and nobody has stood beside one — so the comparison has to be with something the reader has walked past. Hubble is about the length of a bus. The space station spans about nine of them — but that is its width across the solar wings, and most of that is panel and girder. The part you could float through is far smaller than nine buses of room.

An airliner’s cruising height

How high a passenger jet flies — the highest thing most people have any feel for, and the right yardstick for something in low Earth orbit.

The Earth-to-Moon distance

The yardstick for any moon’s distance from its own planet, and for things parked well beyond the Moon, such as JWST at L2.

The Sun-to-Earth distance

One astronomical unit. The yardstick for anything that has left the planets behind.

Why not the obvious ones

A football pitch is the length everybody names, and it is a poor one for a spacecraft: a pitch is flat ground seen from one end, not a solid thing you have stood beside. The right yardstick is not the most famous thing of roughly that size; it is the one whose size you already carry in your head.

Where it stops

It stops at the solar system. Nothing you have walked past is any help with a light-year, so past that point the site changes method and compares one thing in the sky with another — a galaxy against our own, a star against the Sun. When the yardstick becomes something you have also never seen, the page says so rather than pretending the comparison still lands.

🌙 Plan an evening outsideEvery object, one by one