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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
Path around sun
Rotation axis
Activity: The Ecliptic
• 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• 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
- 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” 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
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)
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 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 9: Cassiopeia
• Greek mythological figure: mother of Andromeda
• the big “W” in the sky
• No bright stars
Constellation 11: Pegasus• Greek
mythological figure: the winged horse
• big rectangle in the sky
• No bright stars