Transcript Integrative Studies 410 Our Place in the Universe

Quadrants, Ecliptic & Starmaps
“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
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.
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
To Sun
iSkylab: Sun Option
Gnommon
Shadow
• 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.
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.
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.
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
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
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
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.)
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!
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!
Star
Maps
Celestial
North Pole –
everything
turns around
this point
Zenith – the
point right
above you &
the middle of
the map
40º
90º