1. Quadrants, Ecliptic & Starmaps

2. “Motion” Debriefing Stars circle NCP counterclockwise
For circumpolar stars: EW if above Polaris, but WE if below Polaris
Stars move E W but also up (rising) and down
The sun AND the stars move around the observer, so the sun stays (approx.) fixed amongst the stars

3. 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!

4. Seasonal Motion Daily Rising and Setting: Seasonal Changes:
Due to the rotation of the Earth around its axis
Period of rotation: 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
Circumpolar – never rise or set

5. The Zodiac throughout the Year
Slow drift across background of fixed stars caused by rotation of earth about sun
Period = 1 year (365 ¼ days)
Example: In Winter sun in Sagittarius, Gemini at night sky; in summer sun in Gemini, Sagittarius at night sky

6. 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
Slow drift across background of fixed stars caused by rotation of earth about sun
Period = 1 year (365 ¼ days)
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.

7. Reminder: iSkylab 1 due in two weeks, Sep 24
Observe!
Ask questions!
Already demonstrated Option 1 measurement (shadow of a stick  altitude of the Sun)
Will construct a quadrant

8. Shadow
Gnommon
To Sun
iSkylab: Sun Option
What: Determine how the height of the sun above the horizon at a specific time is changing as the days pass by measuring the length of the shadow it casts with a gnomon (essentially a stick in the ground).
Time: Once you know how to do it, this only takes a minute per observation.
Commitment: Do this over several, not necessarily consecutive days, at exactly the same time.
Weather: Need to see the shadow for a minute, so can do on partly cloudy, possibly hazy but not overcast days.

9. iSkylab: Moon Option 1
What: Determine the height of the moon above the horizon with the help of a quadrant (essentially a bob dangling from a protractor), and see how it changes as the days go by.
Time: Once you know how to do it, this only takes a minute per observation.
Commitment: Do this over several, not necessarily consecutive days, at exactly the same time.
Weather: Need to see the moon for a minute, so can do on partly cloudy, possibly hazy but not overcast days.

10. iSkylab: Moon Option 2
What: Determine the position of the moon with respect to the stars by sketching the position and the shape of the moon and the bright stars in the sky. Document changes as the days go by.
Time: Once you know how to do it, this takes several minutes per observation.
Commitment: Do this over several, not necessarily consecutive days, exact time does not matter.
Weather: Need to see the moon and the stars for several minutes, so it needs to be a cloudless night with good seeing.

11. Activity: Building a Quadrant from scratch with office supplies
Pick up yardstick, string, tape, push-pin
Make a protractor by dividing angles into two, starting with right angle: 90, 45, 22.5,11.25, etc.
Does not have to be accurate
Measure the alt. angle of a tree from classroom
Write up results and turn in with names of group members

12. 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

13. 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
Skyglobe demo
7 visible “planets” incl. the sun and moon
Planet = “wanderer”
Days of the week named after the planets

14. 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.)

15. 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!

16. 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!

17. Star Maps
40º
90º
Celestial North Pole – everything turns around this point
Zenith – the point right above you & the middle of the map