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
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.
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.)
Precession: Pattern in the Sky
• We notice that position of the NCP changes wrt
stars
• We notice that the position of the vernal
equinox, i.e. the intersection point of celestial
equator and ecliptic changes wrt stars
• These are very slow & subtle changes!
• Hipparchus discovered them 130 BC by making
an accurate, naked-eye star catalogue
Precession of the Equinoxes
Precession period
about 26,000 years
“The dawning of the age of
Aquarius”
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