A sense of scale

The observable universe at atom scale

Make the Sun the size of a hydrogen atom and the model distance to the observable universe's horizon becomes about 31,588 kilometers. That is shorter than one trip around Earth's equator, about 40,075 km.

It is an enormous change of perspective. At tennis-ball scale, the nearest star is a journey across a continent. At atom scale, that same star is only a few millimeters away, while a cosmological horizon finally becomes a distance we can compare with our planet.

What is the edge of the observable universe?

The observable universe is the part of the universe from which information has had time to reach us. Its commonly quoted present-day, or comoving, radius is about 46.5 billion light-years. Double that for an approximate diameter of 93 billion light-years.

This horizon is not a wall, a star, or the known edge of everything that exists. It surrounds the observer in every direction. There may be regions beyond it that we cannot observe. The map includes it as a distance reference, rather than a point with a unique constellation.

How an atom-sized Sun produces 31,588 km

The app uses 0.1 nanometers for the model Sun's diameter, a representative hydrogen-atom size. The real Sun's model diameter is 1,392,700 km. Reducing the Sun to that atom gives a scale factor of about 13.9 quintillion to one.

At this scale, one light-year becomes approximately 0.679 micrometers. Multiply that by the 46.5-billion-light-year horizon radius and the result is approximately 31,588 km. The arithmetic uses a selected cosmological reference; it is not a new measurement of the universe.

Does the entire universe fit inside Earth?

No. “Fits around Earth” compares the one-way model distance to the horizon with Earth's circumference. It does not say that a sphere with that radius would fit inside Earth. The model's observable diameter is about 63,176 km, already longer than a single trip around the equator.

The map's “Around Earth once” readout asks a different question: how many real light-years would one Earth circumference represent at the current scale? With an atom-sized Sun, the answer is about 59 billion light-years. With a tennis-ball Sun, it is only about 88 light-years.

On a spherical Earth, a distance longer than half the circumference can wrap past the antipodal point. A flat map also repeats and distorts geography. The circle is a visual scale reference around the origin, not a literal astronomical boundary drawn onto Earth.

Why is the radius larger than the age of the universe in light-years?

The universe has expanded while ancient light traveled toward us. The current distance to the region that emitted that light can therefore be much larger than the light-travel time multiplied by the speed of light.

That distinction also matters for Earendel. This app uses the roughly 12.9-billion-year light-travel reference for that object, whereas its current cosmological distance is larger. The horizon uses a present-day comoving radius. These are different distance conventions, included here to build intuition rather than replace a cosmology calculator.

Start with a simpler comparison in the nearest-star distance guide, or open the atom-scale map.