by Edna DeVore - Deputy CEO
When we look up at the night sky, we seek planets, stars, and often our own Moon. The understanding of HOW and WHY planets orbit the Sun developed slowly. A thoughtful person who watched the day and night sky could conclude that the Sun, Moon, stars, and planets orbit the Earth which sits still at the center of the universe. Worldwide, most cultures developed just such a model and it is a part of many ancient cosmologies.
Western science inherited this geocentric (Earth-centered) model from Aristotle, and it stood for almost 2000 years. In the early 1500's, Copernicus suggested that it made just as much sense to put the Sun at the center of the universe with the stars and the planets orbiting about the Sun. He only knew of one Moon, and properly suggested that it orbited the Earth. About a century later, a shy mathematician named Johannes Kepler asked if there might be a key to the orbit of planets in the data of Tycho Brahe. Kepler worked with great diligence, and discovered that the planets orbit the Sun on elliptical orbits ("Kepler and Mars--Understanding How Planets Moved"). Although Kepler could not say WHY the planets orbited the Sun, he did develop mathematical descriptions for HOW they orbited. Galileo used his new telescope to observe the heavens, and argued that Copernicus and Kepler must be correct because Galileo discovered evidence that Venus orbits the Sun, and that four tiny moons orbit giant Jupiter. In the later 1600's, Isaac Newton came up with the answer to WHY planets orbit the Sun: gravity. Today, we know that the Sun is simply one of billions of stars in the Milky Way Galaxy. And, in the 20th century, astronomers discovered evidence that the Milky Way is one of billions of galaxies.
To simply understand our place in the universe, we often begin by building scale models. Scale models of the planets reveal that the Earth is one of the small ones and Jupiter is the giant. But all are dwarfed by the Sun. Understanding the way that planets move in their orbits can be modeled as well using simple materials and tools, and helps us to explore the place of the Earth in the cosmos. As Kepler deduced, each planet orbits the Sun at a different speed, and the farther from the Sun, the slower the speed. Mercury whips around in 88 Earth-days, we orbit the Sun in one Earth-year (365.26 days), and lonely Pluto completes one lap in 248 years.
SETI Institute's "Cosmic Wheels: Measuring the Orbits of Planets" models the size of planetary orbits, and the speed of each planet in its journey around the Sun. You can model the solar system with you family, students or friends. To wheel about the solar system, please visit SETI Institute's education site and select the sample lesson from "The Science Detectives," a teaching guide on the solar system in our "Life in the Universe" series.