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
Activity: The Ecliptic
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Get out your activities book
Form a group of 3-4 people
Work on the questions
Hand in a sheet of paper with the title of the
activity and the names of the group
members
• I’ll come around to help out !
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
Position of Ecliptic on the Celestial Sphere
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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
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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.
Constellation Quiz Workshop
• Learn the data on the Constellation Quiz
Data Sheet:
- constellation shapes and names
- star names and position in constellation
- deep sky objects’ names and position
• Quiz: You will be asked to find these
objects on a star map.
Constellations of Stars
• About 5000 stars visible with naked eye
• About 3500 of them from the northern hemisphere
• Stars that appear to be close are grouped together
into constellations since antiquity
• Officially 88 constellations
(with strict boundaries for classification of objects)
• Names range from mythological (Perseus,
Cassiopeia) to technical (Air Pump, Compass)
Constellation 1: Orion
Orion as seen at night
Orion as imagined by men
Orion “from the side”
Stars in a constellation are not connected in
any real way; they aren’t even close
together!
Constellation 1: Orion
• “the Hunter”
• Bright Stars:
D) Betelgeuze
E) Rigel
• Deep Sky Object:
i) Orion Nebula
Constellation 2: Gemini
• “the Twins”
• zodiacal sign
• Brightest Stars:
I) Castor
J=K) Pollux
Constellation 3: Taurus
• “the Bull”
• zodiacal sign
• Brightest Star:
F) Aldebaran
• Deep Sky Object:
iii) Plejades
Constellation 4: Ursa Major
• Other name:
Big Dipper
• Stars:
B) Dubhe
C) Merak
• Navigation: go 5 times
the distance from
Merak to Dubhe and
you are at Polaris.
Constellation 5: Ursa Minor
• Other name:
Little Dipper
• α Ursa Minoris is
Polaris [A], the
pole star
Constellation 6: Canis Major
• “Big Dog”
• Stars:
H) Sirius
(brightest fixed star)
Constellation 7:
Cancer
• “Crab”
• No bright Stars
Constellation 8: Leo
• “the Lion”
• zodiacal sign
• Brightest Star:
G) Regulus
Constellation 9: Cassiopeia
• Greek mythological
figure: mother of
Andromeda
• the big “W” in the
sky
• No bright stars
Constellation 10: Pisces
• “the Fishes”
• Zodiacal sign
• No bright stars
Constellation 11: Pegasus
• Greek
mythological
figure: the
winged horse
• big rectangle in
the sky
• No bright stars
Constellation 12: Andromeda
• Greek mythological
figure: Daughter of
Queen Cassiopeia and
King Cepheus rescued
from Cetus by Perseus
• Deep Sky Object:
Andromeda
Galaxy