Download Monday, February 10 - Otterbein University

Seasonal Motion
Daily and yearly motion intertwined
Solar vs Siderial Day
–
–
Earth rotates in 23h56m
also rotates around sun
 needs 4 min. to “catch
up”
Consequence: stars rise 4
minutes earlier each
night (or two hours
per month, or 12
hours in ½ year)
After 1/2 year we see a completely different sky at night!
Seasonal Motion
• Daily Rising and Setting:
– Due to the rotation of the
Earth around its axis
– Period of rotation:
1
siderial day= 23h56m4.1s
– 1 solar day (Noon to Noon) =24h
– Stars rotate around the
North Star – Polaris
• Seasonal Changes:
– Monthly differences caused
by Earth’s orbit around sun
The Zodiac throughout the Year
Example: In Winter sun in Sagittarius, Gemini at night sky;
in summer sun in Gemini, Sagittarius at night sky
Zodiacal signs vs. Constellations
•“Constellation” is a modern,
well-defined term
- Some constellations are big, some
are small on the celestial sphere
•“Zodiacal sign” is the old
way of dividing the year and
the Sun’s path into 12 equal
parts
-
360/12=30, so each zodiacal sign is exactly 30 degrees “long”
0 degrees: Aries, 30 degrees: Taurus, 60 degrees: Gemini, 90
degrees: Cancer, etc.
Another Complication: Axis Tilt!
• The Earth’s rotation axis is tilted 23½ degrees
with respect to the plane of its orbit around
the sun (the ecliptic)
• It is fixed in space  sometimes we look
“down” onto the ecliptic, sometimes “up” to it
Rotation axis
Path around sun
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
Precession of the Equinoxes
Precession period
about 26,000 years
“The dawning of the age of
Aquarius”
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.)
The Seasons
• Change of seasons
is a result of the tilt
of the Earth’s
rotation axis with
respect to the plane
of the ecliptic
• Sun, moon, planets
run along the
ecliptic