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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
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
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
• TeacherTube video
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
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.
Example
The vernal equinox
happens when the
sun enters the
zodiacal sign of
Aries, but is actually
located in the
constellation of
Pisces.