why does tilt of earth not change relative to sun?

During the summer, the Northern hemisphere tilts towards the sun, winter;
away. But why? Like any object rotating about another object, I would have
thought it's orientation rotates as it's position rotates. So that it should
be fixed in it's orientation wrt the sun as it orbits it.

Thnaks for any replies.


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"ah" wrote in message
...
DarkMotion wrote:
During the summer, the Northern hemisphere tilts towards the sun,
winter;
away. But why? Like any object rotating about another object, I would
have
thought it's orientation rotates as it's position rotates. So that it
should
be fixed in it's orientation wrt the sun as it orbits it.

You'll need an assistant for this procedure.

Materials list:

1 orange (California preferred)
1 sharpened pencil (Derwent preferred)
10 sticky-notes (3M preferred)
1 cup of jelly beans (any variety will do)
2m of string (cotton butcher's twine preferred)
1 150m^2 room (empty, or mostly devoid, preferred)

Use the pencil to draw a dark "X" on each of the sticky-notes. Stick one
(at about eye-level) to the North wall of the room; stick another on the
floor at the center of the room

Hold one end of the string at the center of the sticky-note attached to
the
floor, and have your assistant hold the other at the other end. Slowly,
place a sticky-note at each position of the compass (N,S,E, and W), the
same distance from the center by using the string. Do the same, but
rotated 45deg around the compass (NE,NW,SW, and SE).

Poke the pencil entirely through the orange, staying as close to the
center
as possible.

Now, stand at the center of the room, and have your assistant hold the
orange at about eye-level and 2m away from you (on, or around, one of the
'orbital' sticky-notes). Ask he/she to tilt the orange slightly so that
the pencil angles from the 11-o'clock to the 5-o'clock position

Now, ask the assistant face the sticky-note attached to the wall. While
they are facing that wall, have them walk around you, holding the orange
still (in it's angled position), stepping from note-to-note. Make sure
you
turn yourself to follow the circumnavigation, and make sure they are
always
facing said wall (hence the request for an empty room).

Notice that the pencil juts from the orange at the same angle no matter
where they are in relation to you.

That's the idea: the Earth is, basically, a spinning top, but the top is
at an angle. As it travels around the Sun, no one side faces the Sun for
the entire trip.


http://www.enchantedlearning.com/sub.../Seasons.shtml

You can now eat your jelly beeans

Woops! The heading of the post should be "Why does tilt of earth change
relative to sun?"
During the summer, the Northern hemisphere tilts towards the sun, winter;
away.

Your explanation makes sense to me, but it doesn't correspond to the
orientation of the earth relataive to the sun as it rotates around it in
that link you have given above.

Thanks.








ah


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DarkMotion wrote:
During the summer, the Northern hemisphere tilts towards the sun, winter;
away. But why? Like any object rotating about another object, I would have
thought it's orientation rotates as it's position rotates. So that it should
be fixed in it's orientation wrt the sun as it orbits it.

You'll need an assistant for this procedure.

Materials list:

1 orange (California preferred)
1 sharpened pencil (Derwent preferred)
10 sticky-notes (3M preferred)
1 cup of jelly beans (any variety will do)
2m of string (cotton butcher's twine preferred)
1 150m^2 room (empty, or mostly devoid, preferred)

Use the pencil to draw a dark "X" on each of the sticky-notes. Stick one
(at about eye-level) to the North wall of the room; stick another on the
floor at the center of the room

Hold one end of the string at the center of the sticky-note attached to the
floor, and have your assistant hold the other at the other end. Slowly,
place a sticky-note at each position of the compass (N,S,E, and W), the
same distance from the center by using the string. Do the same, but
rotated 45deg around the compass (NE,NW,SW, and SE).

Poke the pencil entirely through the orange, staying as close to the center
as possible.

Now, stand at the center of the room, and have your assistant hold the
orange at about eye-level and 2m away from you (on, or around, one of the
'orbital' sticky-notes). Ask he/she to tilt the orange slightly so that
the pencil angles from the 11-o'clock to the 5-o'clock position

Now, ask the assistant face the sticky-note attached to the wall. While
they are facing that wall, have them walk around you, holding the orange
still (in it's angled position), stepping from note-to-note. Make sure you
turn yourself to follow the circumnavigation, and make sure they are always
facing said wall (hence the request for an empty room).

Notice that the pencil juts from the orange at the same angle no matter
where they are in relation to you.

That's the idea: the Earth is, basically, a spinning top, but the top is
at an angle. As it travels around the Sun, no one side faces the Sun for
the entire trip.

http://www.enchantedlearning.com/sub.../Seasons.shtml

You can now eat your jelly beans.
--
ah

"CeeBee" wrote in message
2.164...
ah wrote in alt.astronomy:

DarkMotion wrote:
During the summer, the Northern hemisphere tilts towards the sun,
winter; away. But why? Like any object rotating about another object, I
would have thought it's orientation rotates as it's position rotates.
So that it should be fixed in it's orientation wrt the sun as it orbits
it
snip

Notice that the pencil juts from the orange at the same angle no matter
where they are in relation to you.

That's the idea: the Earth is, basically, a spinning top, but the top
is at an angle. As it travels around the Sun, no one side faces the Sun
for the entire trip.


This explains to him why the _tilt_ stays the same, not why the tilted
axis
doesn't rotate with the Earth on its way around the sun in exactly one
year.

When I posted the question, I should have made it clear that it was from the
view point of an observer stationary wrt the sun, but not rotaing with it,
so that he sees the earth rotating around the sun, and both the earth and
sun spinning on their axis. But like any object rotating around a point, he
should see the earth rotate around an axis normal to the plane of rotation,
as it rotaes around the sun, so that the Northern hemisphere is always
tilting towards, or away from the sun.

What the OP describes would result in -say- the North pole always in the
dark.

Yeah.

His question is really: why doesn't precession take one year (each
orbit around the Sun?)

Suppose an observer is on the sun always facing the earth as it rotates,
then why does he see the earth tilt change as the year goes by?

Of course the turning axis of the spinning Earth doesn't take one year,
but
approximately 26,000 years.

The tilt itself happened probably during the formation of the solar
system,
precession occurs due to gravitational effects of the Sun and Moon, called
lunisolar precession

http://scienceworld.wolfram.com/phys...Equinoxes.html

--
CeeBee

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"CeeBee" wrote in message
2.164...
"DarkMotion" wrote in alt.astronomy:


When I posted the question, I should have made it clear that it was from
the view point of an observer stationary wrt the sun, but not rotaing
with it, so that he sees the earth rotating around the sun, and both the
earth and sun spinning on their axis. But like any object rotating
around a point, he should see the earth rotate around an axis normal to
the plane of rotation, as it rotaes around the sun,

No, this is an incorrect assumption. This doesn't happen with "any
object". Tilt and precession of any object are not obliged to be
synchronous to a revolution around another object.

Take a cup in an out-stretched hand, with it's handle pointed towards you,
and rotate your whole body about where u are standing. You will see the
handle always pointing towards you, but someone else who is stationary there
will see the handle point towards the opposite end of the room as you rotate
through 180 degrees. It is turning on it's axis in addition to it's centre
of mass rotating about the point you are swivelling on.

Your misconception about how rotating objects behave leads you to believe
that precession of any object is somehow gravitationally locked to one
revolution periode around another object it's bound to.

I'm not talking about precession. I'm talking about an effect which has
nothing to do with the earth spining on it's axis and having a period of 24
hours.

so that the Northern
hemisphere is always tilting towards, or away from the sun.

What the OP describes would result in -say- the North pole always in
the dark.

Yeah.

His question is really: why doesn't precession take one year (each
orbit around the Sun?)

Suppose an observer is on the sun always facing the earth as it rotates,
then why does he see the earth tilt change as the year goes by?

First of all, the tilt doesn't change like you describe, the observer will
see the _orientation_ of the 23 degree tilted axis change.

Second, this is what I explain in the following part of my message, namely
that the orientation of the tilt _does_ change indeed, but only a fraction
during one year you describe. This is called "precession". Precession is
the movement of the tilted Earth axis around the imaginary axis pointing
at the ecliptic pole.

As I explained, what you describe is one rotation around the ecliptic pole
of the tilted Earth axis precisely in one year, giving the impression of
it being locked to one rotation around the Sun. Like one Moon rotation
around its own axis conincides with one orbit around the Erath.

The full rotation ("precession") you're describing doesn't occur in a
year, but in 26,000 years. If one Earth orbit around the sun would last
26,000 years, an observer on the Sun would see what you describe. Now he
only sees 1/26,000th part of what you describe.

Finally, why does precession happen as it does? Well, basically just
"because". The combined gravitational effects of lunisolar precession
results in a precession of about 26,000 years.

Remember that there's no mechanical "logic" or gravitational law
prescribing that precession (the phenomenon you want to be seen sped up)
should be locked to one Earth orbit around the Sun:


The tilt itself happened probably during the formation of the solar
system,
precession occurs due to gravitational effects of the Sun and Moon,
called lunisolar precession

http://scienceworld.wolfram.com/phys...Equinoxes.html



--
CeeBee

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