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Name: Keith J.
Status: Other
Age: Old
Location: N/A
Country: N/A
Date: October 16, 2004

A ten year-old asked me why the globe sits on the table at an angle. Discussion of Earth's axis led to a question I couldn't handle: What was observed way back when that led to determining the Earth's cant to its orbit? Can we see it in our back yard?


I don't know specifically, but ancient astronomers noticed that the plane of Earth's rotation (or perhaps in their way of thinking, the movement of the Sun and other stars around the Earth) is not the same as the plane of the solar system (you could see the Milky Way better in those days without so many lights). The constellations did not stay at the same angle above the horizon throughout the year, but varied seasonally. It therefore became obvious to them that the Earth tilted to the plane of the stars, although they may not have understood why or that this is what causes our seasons.

Ursa Major, which "rotates" about the North pole is a much better example. In the Spring, the bowl of the Big Dipper is inverted, with the constellation high in the sky and in the Autumn the bowl is right side up, with the constellation down towards the horizon. By looking at the middle of the five stars in the handle, you can see the effect of the tilt, with the difference in angle above the horizon being 46.9 degrees between Spring and Autumn (the Earth's tilt is 23.45 degrees).

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory

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