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.

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Presentation transcript:

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!