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Seasonal Motion Axis Tilt Ecliptic • The Earth’s rotation axis is tilted 23½° with respect to the plane of its orbit around the sun • This means the path of the sun among the stars (called ecliptic) is a circle tilted 23½° wrt the celestial equator Rotation axis pointing to NCP, not SCP Path around sun Is the sun rising in the East? • Typically NOT! See for yourself! – Study variation of the rising/setting points of the sun over time – Need at least 10 sunrises or sunsets; more is better – Measure time and azimuth (angle relative to North) – Note position of sunrise/sunset on horizon – Measure angle to that position relative to some fixed landmark (mountain, etc.) Understanding and using Star Maps • The night sky appears to us as the inside of a sphere which rotates • Problem: find a map of this curved surface onto a plane sheet of paper • Let’s explore our turning star map! Fixed and unfixed Stuff • The stars are “fixed” to the rotating sky globe They move from East to West and also from near to the horizon to higher up in the sky • The Solar System bodies (Sun, Moon, Planets, Asteroids, Comets) move with respect to the fixed stars • SSB’s have complicated paths: their own motion is added to the overall motion of the celestial sphere they cannot be printed on a star map! Star Maps Celestial North Pole – everything turns around this point Zenith – the point right above you & the middle of the map 40º 90º How do we “see” that the earth is moving around the sun or v.v.? • Small discrepancy between sun’s motion and motion of stars • Sidereal vs solar day • At noon, say, the sun is not exactly in front of the same stars on the next day. – It is exactly in the south – The stars are faster, so a little west of south Position of Ecliptic on the Celestial Sphere • • • Earth axis is tilted w.r.t. ecliptic by 23 ½ degrees Equivalent: ecliptic is tilted by 23 ½ degrees w.r.t. equator! Sun appears to be sometime above (e.g. summer solstice), sometimes below, and sometimes on the celestial equator Note: Three Coordinate Systems! Two rotations about differently oriented axes, plus an observer that is oriented in a third direction!