Earth Wobble

[Matheous-2]

Can any ABrs give me the proper term for Earth's 26.000 year "wobble" around the axis?

[Jonathan-Joe]

Precession - http://en.wikipedia.org/wiki/Precession

[excelsior-1]

http://en.wikipedia.org/wiki/Chandler_wobble

this?

[mibn2cweus]

Axial precession, in regards to the Earth (aka 'precession of the equinoxes'), corresponds closely to the specified period of 26,000 years.

http://en.wikipedia.org/wiki/Precession_of_the_equinoxes

[Matheous-2]

Thanks all! Shows how poor my maths has always been!!

[Matheous-2]

Further to this question....At which angle position are we at in this particular moment in time? I hope that makes sense. I remember that we should be tilted at 23.5 degrees. What I'm trying to establish is if we are in a favourable position toward the sun?

[Matheous-2]

Or getting there! Should I say!

[mibn2cweus]

The angle of the Earth's axis of rotation to the plane of its orbit varies by about one degree more or less from the ~23.4° it is at now over a period of thousands and millions of years -

http://en.wikipedia.org/wiki/Axial_tilt

The position of the Earth in its yearly orbit around the Sun determines the angle at which it presents itself to the Sun which in turn determines the seasons. On 21 December the Earth had arrived at the point in its orbit at which the Northern Hemisphere was most tilted away from the daytime Sun which marks the onset of Winter in that hemisphere.

http://www.dinosaurtheory.com/seasons.jpg

[Mojo-Jo-Jo]

We are also closer to the sun in Northern Hemisphere winter than we are in Summer.

[Jonathan-Joe]

As mentioned above, the earth's tilt remains fairly constant at 23.5 degree, no point in the 26,000 year cycle is much more favourable than any other.
The tilt of the earth produces seasons and the phenomena of equinox (length of day = length of night) and solstice (shortest / longest day).
Axial (equinoctal) precession will affect these as follows.
For the northern hemisphere, spring starts with the vernal equinox, in June, when the North Pole is towards the sun. But, because of the effect of precession, the point of the earth's orbit where this happens gradually changes. In 6,500 years this point will be about 90 degrees away from the current point and so occur three months earlier. Therefore (unless our calendar is amended) the Spring Equinox will occur in March. After another 6,500 years the Vernal Equinox will have slipped a further 90 dgrees or three months to December. And after another 6,500 years, to September. After the complete cycle of 26,000 years, the Northern Hemisphere Vernal Equinox will be back in June where it started.
Obviously this applies to all equinoxes, solstices and seasons for both hemispheres.
So eventually each month will be spring, then summer, then autumn, winter and spring again. But, ignoring other effects on weather, for any place on earth each season will be much as it is now.
The distance of the earth from the sun has very little effect on climate, seasonal changes result almost entirely from the different angles at which sunlight strikes the earth's surface.
(I know that some claim different ways of defining the start of seasons but pointing that out on this thread won't help anybody)

[mibn2cweus]

Jonathan-Joe
//For the northern hemisphere, spring starts with the vernal equinox, in June, when the North Pole is towards the sun.//
22:02 Fri 04th Jan 2013

Good explanation J-J, apart from the sentence noted above. The equinoxes occur when the Sun crosses directly above the Equator as it changes hemispheres, currently in March and September. The solstices mark the time of maximum tilt of the Earth's axis with respect to the Sun, now in June and December.

http://www.wiley.com/college/sc/arbogast/resources/animations/sunangle/index.html

The Earth's north axis at time of posting is tilted ~22.7° away from the Sun.

[Jonathan-Joe]

Yes, well spotted, I carelessly confused equinox with solstice. So that sentence should read - For the northern hemisphere, summer starts with the solstice in June, when the North Pole is towards the sun.