Seasonal Motion.

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Presentation transcript:

Seasonal Motion

The Zodiac throughout the Year Slow drift across background of fixed stars caused by rotation of earth about sun Period = 1 year (365 ¼ days) Example: In Winter sun in Sagittarius, Gemini at night sky; in summer sun in Gemini, Sagittarius at night sky

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 Skyglobe demo 7 visible “planets” incl. the sun and moon Planet = “wanderer” Days of the week named after the planets

Note: Three Coordinate Systems! Two rotations about differently oriented axes, plus an observer that is oriented in a third direction!

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Homework: Use Proportions Angle traveled compared to full 360 degrees = time elapsed while traveling the angle compared to total time to complete a 360 degree, full rotation angle/360 = time elapsed / x x is the period we are looking for, will come out in same units as we put in “time elapsed”