Observations of the universe are limited by the physical properties of light. Terrestrial observations are limited by the fact that light propagates in straight lines, while the surface of Earth is curved. The distance one can see can be increased by observing from a higher vantage point, but there is a limit: an observer on an infinitely tall platform could only see one quarter of the way around Earth's circumference. Astronomical observations are limited by the fact that light propagates with a finite velocity, c. The only objects that can be seen are those whose emitted light has had enough time, since the beginning of the Universe, to reach Earth today. In both cases, the physical limit to vision is called the horizon.

The mathematics of the terrestrial horizon is straightforward; knowing the observer's height above the surface, and the radius of Earth, simple trigonometry will...