Why do we have different seasons? This lecture looks at the

consequences of the tilt of the Earth's rotation axis relative to its

orbital plane (the Obliquity of the Ecliptic) combined with the apparent

annual motions of the Sun around the Ecliptic. The important factor

determining whether it is hot or cold at a given location at different

times in the year is "insolation": how much sunlight is spread out on

the ground. This, combined with the different length of the day when

the Sun as at different declinations, determines to total amount of

solar heating per day, and drives the general weather. It has nothing,

however, to do with how far away we are from the Sun at different times

of the year. Finally, the direction of the Earth's rotation axis slowly

drifts westward, taking 26,000 years to go around the sky. This

"Precession of the Equinoxes" represents a tiny change that is still

measureable by pre-telescopic observations, and means that at different

epochs in human history there is a different north pole star, or none at

all! Recorded 2006 Sep 28 in 100 Stillman Hall on the Columbus campus

of The Ohio State University.