Download Introduction to Astronomy

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…
The Sun and the Seasons
6 September 2006
Today:
• Measuring angles in the sky
• How the sun moves through our sky
• Seasons
Measuring Angles
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.
Motion of the Sun
Facing south
Motion of the Sun
Sun’s apparent path among the stars is called the ecliptic
Seasons
Summer
Winter
(A one-square-meter surface, directly facing the sun,
receives about 1000 watts of power.)