Physics 101 Astronomy Day 2 notes Ch. 1 - second half The Zodiac and the seasons Precession Motion of the Moon and eclipses of Moon and Sun

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Physics 101 AstronomyDay 2 notesCh. 1
-second halfThe Zodiac and the
seasonsPrecessionMotion of the Moon and
eclipsesof Moon and Sun
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The Celestial Sphere appears to rotate around us
at night. But you know that it is the Earth that
is rotating. (link)
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To observers who think the earth is stationary,
The celestial sphere appears to be rotating.
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Stars are (almost) fixed on the celestial sphere
and the Earth rotates inside this sphere. It
only looks like the celestial sphere is rotating
from our perspective if we are on the Earth.
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Question ?

• To us, which way does the celestial sphere appear
  to be rotating?
• Hint, think about the motion of the moon and the
  sun during the day or night.
• Now, if the celestial sphere is actually not
  moving, and the Earth is rotating, which way does
  the Earth rotate?

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Apparent rotation of the celestial sphere is due
to the rotation of the Earth.

• The Earth is rotating around an axis that goes
  from pole to pole through a center.
• Eventually, each day, the Sun sets in the west.
  If we suppose the Sun is the center of the solar
  system, it is fixed, so
• Each point on the surface of the Earth is going
  east all the time (except the poles).

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View of sunset from 100,000 km above the Earth,
as it rotates.

• I got the latitude and longitude from a web site
  www.lat-long.com
• Then I put them into the Earth-Moon viewer
  http//www.fourmilab.ch/earthview/vplanet.html
• Start the view 100,000 km above Macomb, IL at
  midnight UTC, June 7 (in local time, 700 p.m.,
  June 6) and get a picture each hour after that as
  the Earth rotates. See the result in the
  following slides

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Seen from far above the North Pole, the Earth
appears to be rotating counterclockwise (CCW).
Sun
If the Sun is directly above point A, then it is
local noon there, and in 24 hours it will again
be noon at that location on the Earth.
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The Earth is also in orbit around the Sun,
taking 365.25 days to revolve once around.
This orbital motion is also CCW if viewed from
above the north pole.
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In 24 hours, which is called the solar day, the
Earth must rotate more than 360 degrees!
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Solar vs. Sidereal Day

• The solar day is 24 hours long, by definition,
  but Earth actually rotates through an angle of
  360.986o in order to be aligned with the Sun.
  This is due to the orbital motion of the Earth,
  which means that the Earth has to rotate an
  additional 360o/365 or 0.986o per solar day.

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Solar vs. Sidereal Day

• The sidereal day is, by definition, the times it
  takes the Earth to rotate around and come back
  into alignment with the stars. This is a
  rotation of exactly 360o and this takes 3.9
  minutes less than 24 hours.
• 1 sidereal day 0.9973 solar days.

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Why is this relevant ?

• The difference between the solar day and the
  sidereal day means that the Sun and the stars
  appear to be going around the Earth at different
  rates.
• The Sun goes around in 24 hours.
• Stars go around in 23 hours, 56.1 min.
• So the Sun is not in the same place on the
  celestial sphere day after day.

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The Zodiac is the set of constellations that the
Sun appears to go through during the course of
one year.
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• Right now, Jan. 21, 2015, at 210 p.m. the Sun is
  in Capricornus, but we cant see the stars in
  daylight.
• This can be seen on a sky chart if we set the
  time to sometime in the day, say 210 p.m. using
• http//www.heavens-above.com/skychart.aspx?lat40.
  459lng-90.672locMacombalt215tzCST
• Note you have to set the date and time to Year
  2016 Month 1 Day 21 Hour 14 Minute 10 and click
  Update. You can also print black on white to save
  printer ink if you want to take a printed chart
  outdoors, having set the time to the evening hour
  when you plan to go out.
• Compare with the zodiac chart in the previous
  slide.
• At midnight, we could go outdoors and see Cancer
  high in the sky, opposite the Sun from Earth.
  (next slide)
• You can try different months to see that the
  celestial sphere is in different orientations
  during the year.

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• Tonight at 1155 p.m. the sky should look like
  this
• http//www.heavens-above.com/skychart.aspx?lat40.
  459lng-90.672locMacombalt215tzCST
• Set the time for 2355 and notice that we see
  Cancer and Gemini high in the sky.
• The sky chart also show us that Orion is high in
  the sky at this time. Orion is easily seen during
  winter.
• Also notice that if we go forward until about 6
  a.m. we will see four planets along the ecliptic.
• All five of the classical planets (the ones
  visible without a telescope), Mercury, Venus,
  Mars, Jupiter, and Saturn will be visible in the
  morning sky in another week or so. See this
  article in Sky and Telescope magazine
• http//www.skyandtelescope.com/astronomy-news/obse
  rving-news/get-up-early-see-five-planets-at-once-0
  1182015/

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The Ecliptic is the path of the Sun on the
celestial sphere, which is tilted with respect
to the celestial equator, due to the tilt of the
Earths axis with respect to our orbit.
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The axis of the Earth is not perpendicular to
the plane of the orbit of the Earth around the
Sun. The Earth is tilted by 23.5o.
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Seasons are due to the changing orientation of
the Earth and Sun, not because the Earth is
orbiting closer or further from the Sun.
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Precession of a top

• We can demonstrate a type of motion called
  precession by recalling the motion of a toy top
  (a wobbling motion).
• A bicycle wheel can be used to demonstrate
  precession.
• The Earth precesses because it is not a perfect
  sphere and because there are some forces on it
  from the Moon and planets.

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Precession of the Earth
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Precession of the Earth takes 26,000 years.
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The North Celestial Pole moves around a circle on
the celestial sphere over long periods of time.
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Motion of the Moon and Phases of the Moon

• The development of the current model of the solar
  system began with careful measurement of the
  movement of the Sun and the Moon.
• To understand this, lets review the motion and
  the phases of the Moon, as we currently
  understand them.
• When we watch the Moon, its shape changes from
  one night to the next

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From the astronomy picture of the day web site
( link )
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Earth and Moon, separately
From Apollo 17 spacecraft
From Earth
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Earth and Moon, in one picture from the Galileo
space probe as it traveled bythe Earth on its
way to Jupiter.
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Lunar Phases
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To do

• Get the textbook (access cards not needed).
• Finish Ch. 1. Start Ch. 2 !
• Look at the syllabus and the dates.
• Mark your calendars for exam dates.
• First exam is exactly 2 weeks away!
• Welcome back to WIU.
• Enjoy the weekend.