## Who (and when) discovered that the earth's axis is on a 23 degree tilt? (Intermediate)

The axial tilt of the Earth has been known for a surprisingly long time. The first reasonably accurate measurements were made in China and India. The first (that we know of) was made in 1100 B.C. (over 3000 years ago!) by Chou Li. The next set of measurements came from greek geographers about 750 years later.

All you really need to estimate the tilt of Earth's axis is a carefully positioned pole. Ancient calculations of the axial tilt were made by measuring the length of a shadow cast by a gnomon (a vertical pole) during summer and winter solstice (the longest and shortest days of the year). If the Earth's axis weren't tilted, a rod at the Equator would cast no shadow at noon. Because the axis is tilted, such a rod casts a north-south shadow that varies throughout the year. However, a rod placed on the Tropic of Cancer does not cast a shadow at noon during the summer solstice, and a rod placed on the Tropic of Capricorn does not cast a shadow at noon during the winter solstice. Ancient geographers were very clever and curious people. With vertical rods and some ingenuity, they made pretty accurate measurements of the tilt of Earth's axis.

Nowadays we can make more nuanced measurements of the axial tilt because we have more accurate observations of how planets move and a more thorough understanding of why they move the way they do. Astronomers define a coordinate system on the sky that lets them identify the relative positions of stars, planets, the Sun and other celestial phenomena (the coordinates are called "right ascension" and "declination", but we don't need to worry about that here). Because of their great distance from the Earth, the positions of most stars are fixed on the grid. The positions of the planets and the Sun, however, are seen to "travel" through the coordinate grid in systematic (though sometimes complicated) patterns. The paths of the planets was carefully observed by Tycho Brahe in the late 1500s. Following Brahe's death in 1601, Johannes Kepler used the data to develop a set of laws that govern the way the planets move around the Sun (he proposed them in 1609). These laws, (creatively) called Kepler's Laws, still hold today. Combining Brahe's observations and Kepler's laws of planetary motion, we can infer the axial tilt. Brahe's calculations are probably the first modern, western measurements of the tilt of Earth's axis.

#### Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

Kristine's email: