Astronomy 10: Discussion 1
Reading Assignment: Arny pp. 1-40, Cosmos Chpt. 1
I. The World Around You (continued)
- The Horizon
- Where Earth meets Sky
- How far to it?
- How much sky can you see at different altitudes?
- Local Coordinates
- Zenith - point directly above your head
- Nadir - point opposite in the celestial sphere from the Zenith
- Meridian - line joining North and South through the Zenith
- Azimuth - angle along the horizon starting from North.
- Altitude - anlgle above the horizon
II. The Shape and Size of Earth
- What is the Shape of the Earth?
- Looks Flat
- As we saw in the horizon discussion the closer to Earth's Surface you get the more and more you cannot discern the curvature and it looks flat.
- Evidence for Curvature
- As ships head out to sea they don't disappear all at once, but rather their masts disappear last.
- The Sun's position in the sky is different from different locations on the same day.
- As you move north or south you can see different stars that were previously hidden by the horizon. Not possible on a flat Earth.
- Evidence for Spherical
- The Moon and the Sun are circular in projected appearance (the Moon's Phases give away its spherical shape), Why not Earth?
- Earth's shadow on the Moon during a lunar eclipse is circular. Eclipses happen at all altitudes so Earth must be Spherical (Aristotle ~300 B.C.)
- What is the size of Earth?
- Eratosthenes figures it out.
III. Motions of the Stars, Sun, and Planets
- Daily or Diurnal Motion
- Everything in the sky rises in the East makes a great arc across the sky and sets in the West
- This Motion is Due to the rotation of Earth
- The stars are all so far away that we have no sense of their distance from us. So they appear fixed on a sphere, named the Celestial Sphere.
- Earth rotates West to East (San Francisco heads toward Washington D.C.). If we imagine ourselves as motionless then the Celestial Sphere appears to rotate East to West.
- The angle that the paths of the stars make with the horizon is equal to the latitude of the observer.
- Imagine what the star paths looks like from the extremes
- The Equator
- The Poles
- At 45° latitude
- Annual Motion
- On any given night at any given time you can see thousands of stars (or maybe only hundreds to dozens if you are looking from a bright city location), their collective patterns (constellations) stay the same throughout the night and from night to night all year long.
- However, as the year passes different constellations appear in the sky.
- This changing of constellations throughout the year is caused by Earth's orbit around the Sun.
- These sky rhythms changed with the seasons and helped ancient people predict the changing of the seasons.
- The Ecliptic
- The Earth orbits the Sun in a plane, and this plane is called the Ecliptic
- The ecliptic is titled with respect to the Celestial Equator. This is because Earth's rotation axis is tilted by an angle of 23.5° with respect to its orbital plane.
- This tilt causes the Sun's position in the sky at any given time to change throughout the year.
- The tilt causes the length of the days to change and the location of the rising and setting Sun to change also.
- The extreme rising and setting places occur on the Solstices.
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