Key Concepts:

1. How are time and calendar defined by the apparent motions of the Sun, Moon, and stars ? ("day", "month", "year")
2. How do the annual motions of the Sun (through the zodiac) affect our lives?
3. Why do we have seasons on Earth?

PRS Question 3A: Take two identical quarters. Glue one quarter to a table top and rotating the second quarter around the other in perfect contact. How many revolutions does the second quarter experience as it goes around the first quarter once?

Daily Motions and Time

• Sidereal day (23 hrs 56 min) vs. solar day (24 hrs)
• Sidereal month (27.3 days) vs. synodic month (29.5 days)
• Sidereal year (365.25 days) vs. tropical year (20 minutes less, because of Earth precession)
• lunar calendar with 29 day months (and extra months)
• Julian calendar with leap year
• depends on your planet: 88 days on Mercury, 225 days on Venus, about 2 Earth years on Mars, and about 250 Earth years on Pluto!

Annual Motions and Zodiac

• Ecliptic: path of Sun on the celestial sphere


• Fall and Spring Equinox


• Winter and Summer Solstice


• Tropics of Cancer (north) and Capricorn (south)




• PRS Quiz3B: Walking with no shadows:



• Zodiac: the constellations on the celestial sphere through which the ecliptic passes

Seasons on Earth

Quiz 3b: Myth Busting: Seasons

• The tilt of the Earth rotation axis by about 23.5 degrees changes the angle of the sunbeam and thus the intensity of the sunlight at different seasons.
• The change in the distance between the Sun and the Earth has little to do with the seasons. The Earth is actually slightly closer to the Sun during the winter in the northern hemisphere.

Reading assignment for next lecture: Units 8, 10, & 11