AST 111 Lecture 4 Motion of Objects in the Sky. The Celestial Sphere Imagine you’re where Earth...
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Transcript of AST 111 Lecture 4 Motion of Objects in the Sky. The Celestial Sphere Imagine you’re where Earth...
![Page 1: AST 111 Lecture 4 Motion of Objects in the Sky. The Celestial Sphere Imagine you’re where Earth is, but there’s no Earth. What do you see? Keep in mind:](https://reader036.fdocuments.in/reader036/viewer/2022070412/5697bf8a1a28abf838c8abc4/html5/thumbnails/1.jpg)
AST 111 Lecture 4
Motion of Objects in the Sky
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The Celestial Sphere
• Imagine you’re where Earth is, but there’s no Earth.
• What do you see?
• Keep in mind:
– Nearest star is 4.4 light years away• Our orbit around the Sun is microscopic compared to this
distance
– Stars are points of light in the night sky• Some are brighter than others, but they’re still points
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The Celestial Sphere
• Now imagine that you go around Earth’s orbit (still no Earth present):
– Do the positions of stars change perceptibly?
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The Celestial Sphere
• What you see is the Celestial Sphere.
• Earth’s orbit around the Sun does not cause the stars to shift.
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Celestial Sphere Terminology
• North Celestial Pole is Earth’s North Pole projected onto the Celestial Sphere– Same for South Celestial
Pole
• Celestial Equator is Earth’s equator projected onto the Celestial Sphere
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Motion of the Night Sky
So we know what the entire sky looks like from Earth. We just can’t always see all of it.
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Motion of the Night Sky
Earth gets in the way!
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The Sun in the sky…
If the Sun comes into the sky, the stars in the sky are still there, but they cannot be seen.
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How does Earth block the Celestial Sphere?
• What does it look like when:– There’s no Earth?– Earth is the size of a basketball?– Earth is the size of a boulder?– Earth is flat and of infinite extent?
• Earth is large enough to where it acts like a “plane of infinite extent”
![Page 10: AST 111 Lecture 4 Motion of Objects in the Sky. The Celestial Sphere Imagine you’re where Earth is, but there’s no Earth. What do you see? Keep in mind:](https://reader036.fdocuments.in/reader036/viewer/2022070412/5697bf8a1a28abf838c8abc4/html5/thumbnails/10.jpg)
How does Earth block the Celestial Sphere?
Earth blocks exactly HALF of the celestial sphere
at a given time.
The half that’s blocked is constantly changing.
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Earth is round and large.
• The sky therefore looks like a dome
• When I refer to “The sky”, that means “the half of the Celestial Sphere that Earth isn’t blocking.”
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Sky Terminology
• Zenith: the point straight above your head
• Meridian: arc that starts at North, passes through zenith, ends on South
• Horizon: where the sky stops and Earth starts
• Altitude: angle of object above horizon
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What’s in the sky?
• In order to visualize what’s in the sky at any given time, you must visualize:
– The Celestial Sphere
– Earth blocking out half of it
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So why do the stars rise in the Eastand arc over to the West?
Hourly Motion of the Stars
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Earth rotates on its axis.
• Looking down on the Solar system (at Earth’s North Pole):
– Earth rotates counterclockwise about its axis
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Day and Night
• At a given time:– Half of Earth is
illuminated (daytime)– Half of Earth is dark
(nighttime)
• Where’s the Sun in this image?
DaytimeNighttime
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Day and Night
• Which side is going from day to night?• Which side is going from night to day?
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The view depends on latitude.
• The motion of the sky changes as your latitude changes
• Objects in the sky also change
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Rotation of the Night Sky
• How does the sky appear to rotate:
– If you’re at the North Pole?
– If you’re at the equator?
– If you’re anywhere in between?
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Rotation of the Night Sky
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Questions
• At the North Pole, what star is always at your zenith?
• At the equator, what star is always on the horizon?
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Day-To-Day Motion of the Stars
So why do the stars shift one degree upward(when looking East) from day to day?
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Earth orbits the sun.
• Goes counter-clockwise (looking at North Pole)
• In January:– Which set of stars is seen at
night?– Can the other set of stars be
seen?– Does the other set of stars
come up?
• And July?
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Planets are not so far away.
• Because the planets are closer, they move relative to the stars.
• The motion of the planets in the sky is due entirely to the orbital motion of the planets around the Sun.
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Sun, planets, moon move along the ecliptic.
• Solar System is more or less planar
• Move along a line in the sky called “The Ecliptic”
• Why is the ecliptic tilted relative to the celestial equator?
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Zodiac Constellations
• The constellations along the ecliptic are Zodiac constellations
• The Sun and planets move along these constellations– 12 Zodiac constellations– The Sun spends roughly equal time in each– Corresponds to astrological sign
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Zodiac Constellations