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The Celestial Sphere • • • • Model describing the sky. (Glass bowl over the Earth) Pretend that the stars are attached to it Celestial Meridian: Line North to South. Zenith: Spot straight up. Celestial Sphere • The sun and stars rise from East to West. • What causes this? • Earth’s Rotation • What star is that? • North Star (Polaris) Finding Your Latitude • Find the Big Dipper. • Use Pointing stars. • Measure angle from ground to the North Star. • That angle is your latitude. North Star from different latitudes • At the North Pole, where is the North Star in the sky? • Answer: Straight up! • At the Equator, where is the North Star in the sky? • Answer: On the Horizon! Right Ascension (RA) • Corresponds to longitude, but different units are used. • Instead of 360°, a circle is broken into 24 hours of right ascension. • So, 360° = 24 h R.A., 15° = 1 h R.A., and 1° = 4 min R.A. Right ascension increases from west to east (note that we are looking at the exterior of the celestial sphere in the above picture). Declination (Dec) • Corresponds to latitude - relative to the celestial equator (0° dec). • Circles of constant declination are all parallel to the celestial equator • With the two numbers of declination and right ascension, the position of any object in the sky can be precisely described. Ecliptic • Stars are fixed relative to each other. • The Sun traces out a circle on the celestial sphere called the ecliptic. • Ecliptic is tilted at an angle of 23.5° with respect to the celestial equator. The Ecliptic & Zodiac First Point of Aries Cardinal Directions • Why is East & West switched? Equatorial Starmaps Reading a Map • • • • • Key – Different objects Magnitude Identify Zodiac Plotting RA & Dec. of stars Plot a moving or wandering object over time.