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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. *** Free account sponsored by SecureIX.com *** *** Encrypt your Internet usage with a free VPN account from http://www.SecureIX.com *** |
#2
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why does tilt of earth not change relative to sun?
"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 *** Free account sponsored by SecureIX.com *** *** Encrypt your Internet usage with a free VPN account from http://www.SecureIX.com *** |
#3
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why does tilt of earth not change relative to sun?
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 |
#4
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why does tilt of earth not change relative to sun?
"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 *** The Cookie Has Spoken *** *** Free account sponsored by SecureIX.com *** *** Encrypt your Internet usage with a free VPN account from http://www.SecureIX.com *** |
#5
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why does tilt of earth not change relative to sun?
"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 *** The Cookie Has Spoken *** *** Free account sponsored by SecureIX.com *** *** Encrypt your Internet usage with a free VPN account from http://www.SecureIX.com *** |
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