1. Chapter 2b: The Sky

2. The Celestial Sphere (II)
From geographic latitude l (northern hemisphere), you see the celestial north pole l degrees above the horizon;
From geographic latitude –l (southern hemisphere), you see the celestial south pole l degrees above the horizon.
90o - l l
Celestial equator culminates 90o – l above the horizon.

3. The Celestial South Pole is not visible from the northern hemisphere.
Example:
New York City: l ≈ 40.70
Celestial North Pole
Celestial Equator
49.30
40.70
Horizon
Horizon
North
South
The Celestial South Pole is not visible from the northern hemisphere.

4. The Celestial Sphere (III)

5. Apparent Motion of the Celestial Sphere
Looking north, you see stars circling counterclockwise around the celestial north pole.

6. Apparent Motion of the Celestial Sphere II
Over the course of the night, stars rise on the east, travel from east to west (left to right if viewed from the northern hemisphere) through the southern sky, and set in the west.

7. The Sun’s gravity is doing the same to the Earth.
Precession (I)
Gravity is pulling on a slanted top. => Wobbling around the vertical.
The Sun’s gravity is doing the same to the Earth.
The resulting “wobbling” of the Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession.

8. It will be closest to Polaris ~ A.D. 2100.
Precession (II)
As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years.
It will be closest to Polaris ~ A.D
~ 12,000 years from now, it will be close to Vega in the constellation Lyra.
There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.)

9. Knowledge Check
Why does the sky look strange to someone from the northern hemisphere?

10. Coming Attraction: Milankovitch Hypothesis
Hypothesis proposed in 1920, that small changes in Earth’s orbit, precession, inclination and other effects may affect the Earth’s climate and trigger ice ages. Controversial at first but becoming more mainstream. There will be a paper in the resources section of the course web site to read regarding this idea which goes beyond the material in the book. You should read this before next Wednesday.

11. The Annual Motion of the Sun
Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations.
The Sun’s apparent path on the sky is called the Ecliptic.
Equivalent: The Ecliptic is the projection of Earth’s orbit onto the celestial sphere.