Astronomy

1. Astronomy 3/4/14

2. The celestial sphere • Celestial sphere shares all of the features of Earth: • Celestial equator = Earth’s equator • Celestial poles = Earth’s poles

3. The celestial sphere • Ecliptic: A great circle traced out by the apparent rotation of the sun around the Earth. • The Earth’s axis is tilted away from the ecliptic!!! This likely happened when Earth collided with a planetoid.

4. The celestial sphere • This means that the ecliptic and the equator are separated by an angle (27°). • Equinoxes: Two places where the ecliptic intersects the equator. Vernal: “spring” Autumnal: “fall”

5. The celestial sphere • The starting point (zero) is the vernal equinox. East/West from here is measured by right ascension. • Measured in hours, minutes, seconds from equinox (an hour is 1/24 of a degree). • North/South is measured by declination. Normal degrees. • Example, north star (Polaris) is 2 h, 31m, 48s RA and 89° 15’ 50’’ D.

6. Celestial navigation • How did sailors know where the heck they were on long voyages?

7. Celestial navigation • A trained sailor knew how to make measurements using devices such as sextants, octants, quadrants, etc. • Allowed them to spot a celestial object and measure how far above the horizon they were in degrees.

8. Celestial navigation • A simple mathematical calculation would tell the sailor how far away they were from the zenith point. This would give the sailor a circle with that radius. • The sailor would then sight a second object and do the same thing. • The intersection of these circles gives the sailor two possible points on Earth.