Motions in the Sky

Chapter 2 Motions in the Sky

1. Constellations

a. In ancient times, constellations only referred to the brightest stars that appeared to form groups

b. Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.

c. the 88 semi-rectangular regions that make up the sky

d. Northern constellations have Latinized Greek mythology names:

i. Orion, Cygnus, Leo, Ursa Major, Canis Major, Canis Minor

e. Southern constellations have Latin names:

i. Telescopium, Sextans, Pyxsis

f. Some popular patterns are not constellations – these are called asterisms

i. Big Dipper

ii. The Teapot

2. Earth orbits the sun

a. The sun’s motion on the ecliptic reflects Earth’s orbit around the Sun

b. As earth moves, the Sun is seen against different constellations – the zodiac

3. Precession

a. The Sun’s gravity is doing the same to Earth. The resulting “wobbling” of Earth’s axis of rotation around the vertical with respect to the Ecliptic takes about 26,000 years and is called precession.

b. As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years.

c. It will be closest to Polaris (North Star, which never moves) ~ A.D. 2100.

4. Earth rotates on its axis

a. As viewed from above the North Pole, Earth rotates counterclockwise on its axis.

b. One rotation takes 24 hours.

5. Frames of reference

a. An observer sees things from a frame of reference.

b. Relative motions between objects are important.

c. On Earth, different observers see

i. Different parts of the sky.

ii. Different motions of the stars.

6. Verifying the orbit of earth

a. Earth’s motion also produces the aberration of starlight.

b. Slight change in stars’ positions

c. Analogy: falling rain or snow as seen from a moving car. (slanted, but when still vertical)

7. At the earth’s north pole

a. Would see half the celestial sphere.

b. The north celestial pole is directly overhead....