Transcript Seasonal Motion

Seasonal Motion
The Zodiac throughout the Year
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
Note: Three Coordinate Systems!
Two rotations about differently oriented
axes, plus an observer that is oriented in a
third direction!
Homework: Starmaps 1& 2,
March 1, 2006 & April 1, 2006
Homework: Star Maps 3 & 4,
March 2, 2006 & March 1, 2007
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”