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!