1. Miscellaneous announcements… Homework 1 due today by 5:00. Pick up Homework 2; solutions are on the web. Get started on a project soon, if you haven’t already! Astrophotographers: please see me after class. Questions on motion of the stars? Try out Sky View Café and/or Sky Chart III…

2. The Sun and the Seasons 6 September 2006

3. Today: Measuring angles in the sky How the sun moves through our sky Seasons

4. Measuring Angles

5. A sunset photo (July)

6. Equinox sunset

7. Motion of the Sun On any given day, the sun’s motion is essentially the same as that of a star (rises in the east, sets in the west). The sun’s motion doesn’t quite keep up with the stars: It completes a 360º circle in 24 hours. With respect to the stars, the sun appears to move once a year around a great circle, tipped 23.5º with respect to the celestial equator. In late June (summer solstice), the sun is farthest north; in late December (winter solstice), the sun is farthest south. In late March and September (equinoxes), it’s on the celestial equator.

8. Motion of the Sun Facing south

9. Motion of the Sun Sun’s apparent path among the stars is called the ecliptic

10. Seasons SummerWinter (A one-square-meter surface, directly facing the sun, receives about 1000 watts of power.)