A brief history of the solar system

I’m somewhat fascinated by the timeline of humans discovering something about the size of the solar system. A lot of people assume that Copernicus was the first to propose a heliocentric model to explain the apparent orbits of the planets relative to Earth.

But the real story goes back a bit further.

The foundation was laid by Euclid in 300 BCE, creating much of the modern understanding of geometry recognizable to students today. For the next 100 years, his new understanding had a profound impact on astronomy as astronomers and mathemeticians pieced together the geometry of the solar system.

Between 300-200 BCE, Greek astronomers and mathemeticians came to several conclusions that provided estimates of the relative position, motion, and size of the Earth, Sun, Moon, and naked-eye visible planets. The shape of the Earth’s shadow on the moon is obvious, if you’ve done enough astronomical observations. You know when the Moon and Sun are on exact opposite sides of the earth, and that corresponds with a lunar eclipse. That informed the acceptance that the Earth, Moon, and Sun are all spheres flying through space in roughly circles. The Greeks knew enough geometry to understand this was simplest and most accurate when a heliocentric model was used. It was practically an observation. Beyond that, it made sense and was easiest to use a model with the planets orbiting the sun in their correct order.

People had been predicting eclipses for some time, already. The saw the shape

Aristarchus discovered the ratio of the Earth’s radius to the Moon’s, and to the distance between the Earth and the Moon. It also gave a distance to the Sun and its size in Earth radii. The numbers are the correct order of magnitude, meaning that they had a sound working model showing the moon was a good fraction of the Earth’s size and 100s of thousands of kilometers away. Notable, the distance to the Sun, and consequently its size, were calculated at 19-20 radii, when the real number is closer to 390. Basically, it was hard to measure whether an angle to the sun was 87 vs. 89+ degrees. Separately, Erastosthenes derived the size of the Earth based on the angle of shadows at two points on the globe at noon on the equinox. In doing so, he measured the curvature of the Earth and learned its circumfrence and radius. He was within 16%, possibly within 1% of the actual size of the Earth.

All of that happened 280-200 BCE. They were briefly controversial, but probably never heretical ideas to the Greeks. Euclid developed a very new, much more accurate understanding of geometry that students today would recognize, around 300 BCE.

By 150 BCE, the moon’s distance was estimated as 380,000 km. The actual number is 384,399 km, with a range of 362,000-405,000 km. This simultaneously pulled the other estimates of the solar system’s measurements into something similiarly accurate.