Transcript Seasonal Motion

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