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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
Path around sun
Rotation axis pointing to NCP, not SCP
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!
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