Posted Jan. 30, 2009 – Our grasp of the structure and immensity of the cosmos is hand-me-down knowledge that started 5,000 years ago, with the **Babylonians and Egyptians**, who accurately noted the cycles of the sun and moon. But through it all, only the **ancient Greeks** went beyond merely observing celestial patterns. They were the first to come up with correct original explanations. Let’s pay some of them a tribute now, in chronological order.

**Anaxagoras** (450 BC) correctly believed that moon reflects light from the sun, and therefore understood why the moon darkens during an eclipse.

**Heracleides** (350 BC) was the first to propose that, since **Mercury** and **Venus** stay so close to the sun, they might orbit it and that the **Earth** might rotate on an axis.

**Eudoxus** (in 375 BC) originated a geometric method of calculating the distance from the Earth to the Sun and Moon.

**Aristotle** (340 BC) is famous, but he set back science for 2000 years with his *geocentric model of the universe*, which went unquestioned until the time of **Galileo**. But some of his other writings were correct, like when he said that the *Earth was not flat, but spherical*.

**Aristarchus** (265 BC) was among the greatest of the great, the first to correctly determine the **relative sizes** of the Earth, Sun, and Moon. And once he realized that the sun is far larger than the Earth, he proposed that the Sun, and not Earth, was the center of the solar system. Aristarchus wins the cigar for the *heliocentric model*.

**Eratosthenes** (200 BC) was another genius – the first person to correctly measure the size of the Earth. He used the angle of the sun’s shadow at noon in two different towns, and the distance between the towns to set up a proportion with the 360 degrees in a circle and the unknown size of our planet.

**Ptolemy** (170 AD) is famous, despite being wrong about nearly everything. Ptolemy supported a geocentric universe, which unfortunately became a religious principle for 1,700 years.

**Hipparchus** (130 BC) discovered the 26-century wobble of Earth’s axis (now called precession) and created the first accurate star catalog, using a system of dividing stars into 6 magnitudes of brightness, which is still used today. He also determined the length of a year to within 6 minutes.

In other words, astronomy may be Greek to you. But it wasn’t to those guys.