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  #1  
Old March 31st 10, 02:01 AM posted to sci.astro
F/32 Eurydice
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Default Find the mistake


See if you can find the mistake in this argument. We know that the
sun rises in the east and sets in the west, which means that earth
rotates west to east.

My textbook (Abell) says that the sun moves eastward on successive
days against the stellar background. This means that we are actually
the ones moving westward against the stellar background.

If the earth rotates eastward but orbits westward, that means our
rotation is retrograde to our orbit. They are in opposite directions.
Can you find the mistake in that argument?
  #2  
Old March 31st 10, 02:24 AM posted to sci.astro
Bill Owen
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Default Find the mistake

F/32 Eurydice wrote:
See if you can find the mistake in this argument. We know that the
sun rises in the east and sets in the west, which means that earth
rotates west to east.


Yes.

My textbook (Abell) says that the sun moves eastward on successive
days against the stellar background. This means that we are actually
the ones moving westward against the stellar background.


Yes, but see below.

If the earth rotates eastward but orbits westward, that means our
rotation is retrograde to our orbit. They are in opposite directions.
Can you find the mistake in that argument?


"Westward" was an unfortunate choice of words to describe the direction
of the earth's orbital velocity. If you're standing on the surface of
the earth *at noon*, with the sun on your meridian, the earth is indeed
moving toward the west. But that's only because our velocity vector
points to a spot about 90 degrees west of the sun. At midnight you'd
think the earth was moving east.

It's a question of perspective. If you look at the Solar System "from
above" (that is, from well north of the ecliptic), you'd see that the
earth both rotates and revolves counterclockwise.

-- Bill Owen


  #3  
Old March 31st 10, 06:53 PM posted to sci.astro
F/32 Eurydice
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Posts: 40
Default Find the mistake

On Mar 30, 9:24*pm, Bill Owen wrote:
F/32 Eurydice wrote:
See if you can find the mistake in this argument. *We know that the
sun rises in the east and sets in the west, which means that earth
rotates west to east.


Yes.

My textbook (Abell) says that the sun moves eastward on successive
days against the stellar background. *This means that we are actually
the ones moving westward against the stellar background.


Yes, but see below.

If the earth rotates eastward but orbits westward, that means our
rotation is retrograde to our orbit. *They are in opposite directions..
Can you find the mistake in that argument?


"Westward" was an unfortunate choice of words to describe the direction
of the earth's orbital velocity. *If you're standing on the surface of
the earth *at noon*, with the sun on your meridian, the earth is indeed
moving toward the west. *But that's only because our velocity vector
points to a spot about 90 degrees west of the sun. *At midnight you'd
think the earth was moving east.

It's a question of perspective. *If you look at the Solar System "from
above" (that is, from well north of the ecliptic), you'd see that the
earth both rotates and revolves counterclockwise.


The moon, too, right?
  #4  
Old March 31st 10, 10:02 PM posted to sci.astro
Bill Owen
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Posts: 154
Default Find the mistake

F/32 Eurydice wrote:
On Mar 30, 9:24 pm, Bill Owen wrote:
F/32 Eurydice wrote:
See if you can find the mistake in this argument. We know that the
sun rises in the east and sets in the west, which means that earth
rotates west to east.

Yes.

My textbook (Abell) says that the sun moves eastward on successive
days against the stellar background. This means that we are actually
the ones moving westward against the stellar background.

Yes, but see below.

If the earth rotates eastward but orbits westward, that means our
rotation is retrograde to our orbit. They are in opposite directions.
Can you find the mistake in that argument?

"Westward" was an unfortunate choice of words to describe the direction
of the earth's orbital velocity. If you're standing on the surface of
the earth *at noon*, with the sun on your meridian, the earth is indeed
moving toward the west. But that's only because our velocity vector
points to a spot about 90 degrees west of the sun. At midnight you'd
think the earth was moving east.

It's a question of perspective. If you look at the Solar System "from
above" (that is, from well north of the ecliptic), you'd see that the
earth both rotates and revolves counterclockwise.


The moon, too, right?


Yes, the moon too, all of the planets, and most *but not all* of their
satellites.

-- Bill
  #5  
Old April 3rd 10, 06:24 PM posted to sci.astro
Odysseus[_1_]
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Posts: 534
Default Find the mistake

In article ,
Bill Owen wrote:

F/32 Eurydice wrote:
On Mar 30, 9:24 pm, Bill Owen wrote:


snip

It's a question of perspective. If you look at the Solar System "from
above" (that is, from well north of the ecliptic), you'd see that the
earth both rotates and revolves counterclockwise.


The moon, too, right?


Yes, the moon too, all of the planets, and most *but not all* of their
satellites.


IIANM Uranus rotates clockwise -- nominally, at least, with an axial
inclination of ~98°.

--
Odysseus
  #6  
Old April 6th 10, 02:43 AM posted to sci.astro
Bill Owen
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Posts: 154
Default Find the mistake

Odysseus wrote:
In article ,
Bill Owen wrote:

F/32 Eurydice wrote:
On Mar 30, 9:24 pm, Bill Owen wrote:


snip

It's a question of perspective. If you look at the Solar System "from
above" (that is, from well north of the ecliptic), you'd see that the
earth both rotates and revolves counterclockwise.
The moon, too, right?

Yes, the moon too, all of the planets, and most *but not all* of their
satellites.


IIANM Uranus rotates clockwise -- nominally, at least, with an axial
inclination of ~98°.


Right you are. I was thinking only of orbital motion when I wrote that.
Not only Uranus but also Venus and Pluto have retrograde rotation.
The "regular" inner satellites of Uranus orbit in the same direction
that the planet rotates.

-- Bill

 




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