In 1672, the known universe spanned millions of miles. By 1838, it was trillions. Every time astronomers invent a new way to measure distance, the cosmos shatters its own boundaries.
The first rung of what is now called the cosmic distance ladder was forged in the 3rd century BCE, when Aristarchus of Samos used geometry during a half-moon to calculate that the Sun was much further away—and therefore much larger—than the Moon. While his exact numbers were off, he established that Earth was not the largest object in the cosmos. In 1672, Giovanni Cassini and Jean Richer accurately measured the distance to Mars using parallax, observing the planet simultaneously from Paris and French Guiana to triangulate its position. This established the Astronomical Unit (the distance from Earth to the Sun) at roughly 87 million miles, expanding the known solar system to previously unimaginable proportions.
The stars remained a mystery. For centuries, astronomers assumed they were other suns, but had no proof of their distance. That changed in 1838 when Friedrich Bessel measured the parallax of a star called 61 Cygni. By tracking how the star appeared to shift against background stars as Earth orbited the Sun, Bessel proved it was 60 trillion miles away. The universe suddenly expanded from millions of miles to light-years across.
By the early 20th century, the Milky Way was thought to be the entire universe. Henrietta Swan Leavitt discovered that Cepheid variable stars pulse at a rate directly tied to their true brightness, giving astronomers a standard candle to measure extreme distances. In 1923, Edwin Hubble spotted a Cepheid in the Andromeda nebula.
Using Leavitt's law, Hubble calculated that Andromeda was at least 900,000 light-years away. Andromeda was not a cloud of gas within the Milky Way, but an entirely separate island universe.
Today, the distance ladder relies on Type Ia supernovae and the cosmological redshift of light to measure galaxies billions of light-years away. The cosmic microwave background radiation places the edge of the observable universe at 46.5 billion light-years in any direction, a radius calculated by systematically overlapping these techniques, using each established rung to calibrate the next.