2. Earth’s Revolution Earth revolves around the sun in a slightly eccentric elliptical path once a year  Ellipse ~ Oval  It takes Earth 365.25 days to revolve 360 o in its orbit around the sun. It moves approximately 1 o per day (360 o /365.26 days = 1 o per day)

3. Evidence of Earth’s Revolution around the Sun  Seasonal Constellations: Because of the Earth’s annual motion, some constellations are visible at night only during certain seasons  Constellations are visible when the dark side of the Earth (away from the sun) faces toward the constellation

4. The 13 constellations the Sun appears to travel through during the year

5. Different constellations at different times of the year

6. Evidence of Revolution: Parallax Shift  Because of Earth’s revolution about the Sun, near stars seem to shift their position against the farther stars (the closer the star, the greater the shift)  If you observe the same star while Earth is at 2 different points during its orbit to the more distant background stars will appear to change

10. Apparent Motion of Sun due to Rotation and Revolution  Rotation: Sun’s apparent daily path has the shape of an arc moving 15 o per hour  Revolution: However, the Sun’s arc changes both its position and its length with the seasons.

11. Apparent Motion of Sun due to Rotation and Revolution Earth’s Revolution around the Sun – the Earth’s axis does not change producing a change of seasons.

12. Because of the tilt of the Earth’s axis, the Sun’s rays are only perpendicular (directly overhead) at noon between 23.5 o N and 23.5 o S during the year.

14. Perpendicular Rays of the Sun: The Zenith Position  The noon perpendicular rays of the Sun travel southward from the Tropic of Cancer (23.5 o N) on June 21.  Passes the equator on September 21 st and hits the Tropic of Capricorn on December 21 st.

15. Zenith Position of the Sun  Because of the Earth’s spherical shape, on any particular date, there is just 1 place where insolation (incoming solar radiation) is at an angle of 90 o. All other places are less than 90 o.  Does the 90 o ever reach the United States?  An observer in New York State will never see the Sun directly overhead

20. The apparent rising and setting of the Sun is due to rotation  Changes in the altitude of the path and horizon position of sunrise and sunset are due to revolution SeasonSunriseSunset SummerNorth of EastNorth of West Equinox Fall/SpringDue EastDue West WinterSouth of EastSouth of West

25. Shape of Ellipses  Eccentricity = 0 = circle  Eccentricity = 0.5  Eccentricity = 1 = line