solstices

What time is it? Telling time is the oldest practical application of astronomy. Today's lecture is the first of a 2-part lecture on the astronomical origins of our methods of keeping time and making calendars. This lecture reviews the divisions of the year into the solstices, equinoxes, and cross-quarter days, the division of the year into months by moon phase cycles, months into weeks, and the division of the day into hours by marking the location of the Sun in the sky Recorded 2007 Oct 2 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Why do celestial objects appear to rise and set every day? How does this depend on where you are on the Earth, or the time of year? In today's lecture we we set the heavens into motion and review the two most basic celestial motions. Apparent Daily Motion reflects the daily rotation of the Earth about its axis. Apparent Annual Motion reflects the Earth's annual orbit around the Sun. We introduce the Ecliptic, the Sun's apparent annual path across the Celestial Sphere, and note four special locations along the Ecliptic: the Solstices and Equinoxes. This sets the stage for many of the topics of the rest of this section. Recorded 2007 Sep 26 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

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.