The tilt (obliquity) of the earth relative to its plane of travel about the sun is what causes seasons. The hemisphere "pointing toward" the sun is in summer, while the opposite hemisphere is in winter. The earth makes one full orbit around the sun each year. If the earth's axis were "straight up and down" relative to the orbital plane, as in the right-hand image, there would be no seasons, since any given point at the top of the atmosphere would receive the same amount of sun each day of the year. Changes in the "tilt" of the earth can change the severity of the seasons - more "tilt" means more severe seasons - warmer summers and colder winters; less "tilt" means less severe seasons - cooler summers and milder winters. The earth wobbles in space so that its tilt changes between about 22 and 25 degrees on a cycle of about 41,000 years. It is the cool summers which are thought to allow snow and ice to last from year to year in high latitudes, eventually building up into massive ice sheets. There are positive feedbacks in the climate system as well, because an earth covered with more snow reflects more of the sun's energy into space, causing additional cooling. In addition, it appears that the amount of Carbon Dioxide in the atmosphere falls after ice sheets grow. Today's "CO2 Climate Change" theorists believe this also adds to the cooling of the climate. More significantly, the shape of the earth is not round like a ball, it is more oblate or egg shaped. This is the basis of the fact that a higher degree of tilt and wobble increases the surface area of the earth exposed directly to the sun.
In addition to the tilt of the earth there is axial precession. Precession is the trend in the direction of the Earth's axis of rotation relative to the fixed stars, with a period of roughly 26,000 years. This gyroscopic motion is due to the tidal forces exerted by the sun and the moon on the solid Earth, associated with the fact that the Earth is an
oblate spheroid shape and not a perfect sphere. The sun and moon contribute roughly equally to this effect.
When the axis is aligned so it points toward the Sun during perihelion, one polar hemisphere will have a greater difference between the seasons while the other hemisphere will have milder seasons. The hemisphere which is in summer at perihelion will receive much of the corresponding increase in solar radiation, but that same hemisphere will be in winter at aphelion and have a colder winter. The other hemisphere will have a relatively warmer winter and cooler summer.
At present, perihelion occurs during the southern hemisphere's summer, and aphelion is reached during the southern winter. Thus the southern hemisphere seasons are somewhat more extreme than the northern hemisphere seasons, when other factors are equal.
The earth's orbit around the sun is not quite circular, which means that the earth is slightly closer to the sun at some times of the year than others. The closest approach of the earth to the sun is called perihelion, and it now occurs in January, making northern hemisphere winters slightly milder. This change in timing of perihelion is known as the precession of the equinoxes, and occurs on a period of 22,000 years. 11,000 years ago, perihelion occurred in July, making the seasons more severe than today. The "roundness", or eccentricity, of the earth's orbit varies on cycles of 100,000 and 400,000 years, and this affects how important the timing of perihelion is to the strength of the seasons. The combination of the 41,000 year tilt cycle and the 22,000 year precession cycles, plus the smaller eccentricity signal, affect the relative severity of summer and winter, and are thought to control the growth and retreat of ice sheets. Cool summers in the northern hemisphere, where most of the earth's land mass is located, appear to allow snow and ice to persist to the next winter, allowing the development of large ice sheets over hundreds to thousands of years. Conversely, warmer summers shrink ice sheets by melting more ice than the amount accumulating during the winter.
To be more accurate on the subject of orbital cycles, there are three distinct and measured cycles.
Orbital shape (eccentricity) is what we have focused upon. Apsidal precession is a 25,000 year cycle by which the earth's orbit shifts (precesses) in space around the sun. Orbital inclination is angle by which the earth's orbit travels around the sun. Generally we overlook orbital inclination and visualize the orbit as if it were on a two-dimensional plane.
Short Term Astronomical Factors
The moon is slowly drifting away from the earth at a rate of about 1.5 inches per year. This slightly increases the variability in earth's tilt or "wobble" and slightly affects earth's rotation speed. The moon also affects tides which play a role on the gravity and shape of the earth.
Sun spots are a much larger short term variable that cycles over 11 years based upon the sun's fluctuating magnetosphere. Variations in the number of sun spots impacts the wavelengths of energy output by the sun as solar radiation is affected by magnetic fields in the sun's heliosphere. Higher\shorter wavelengths are more efficiently absorbed or reflected by the earth's magnetosphere. Shorter\longer wavelengths penetrate the earth's magnetosphere deeper and transfer more energy to the earth's surface.
Earth's magnetosphere is an important factor and it is fluctuating...
Scientists studying the magma on the ocean floor on either side of the ridges or rifts found that periodically, the iron components were magnetized in an opposite direction and that since the magnetizing relied upon the magnetic properties of the north and south magnetic poles, it was concluded that also periodically, in a time frame that has been measured at being about ten millennium, the poles reverse, just as they do on the sun but in a much shorter time frame. These same scientists claim that the last reversal occurred about 11,500 years ago.
The Earth’s geomagnetic field is in a constant state of change, according to researchers like scientist Larry Newitt of the Geological Survey of Canada who regularly goes hunting with a compass for his quarry, which is Earth's north magnetic pole, which is then usually found in an increasingly distant new location from the last record taken. Scientists have known for some time that the magnetic poles move. James Ross first located the north pole in 1831 after an epic arctic journey during which his ship became icebound for four years. No one returned to the north pole again until the next century when in 1904, Roald Amundsen found the current north pole and discovered that it had moved at least 50 kilometres from the original site moving then at an average of less than 1 kilometre per year.
The pole kept moving slowly northwards during the 20th century, at an increasing average speed of 10 km per year, but lately in the 21st century it has been accelerating and relocating at a new average of more than 50 kilometres per year. At this rate it will soon exit North America and reach
Siberia if it doesn’t 'flip' beforehand.
There is hypothetical evidence that we are already in the transition phase that precedes a polar reversal as shown in the growing areas of magnetic anomaly, field lines that are moving the wrong way and signaling an ever weaker and chaotic state for our protective magnetosphere shield. NASA has verified that there is evidence of positive magnetism energy at the South Pole, which is normally supposed to exist in the North Pole only. The South Pole is supposed to only have a negative magnetic charge. In the past 150 years, it has also been claimed that there has been a migration between the north and south poles and their respective magnetic charges of positive and negative magnetic reversal.
According to a report issued by the British Geological Survey (BGS), there is evidence to suggest that the magnetic reversal has begun. The BGS has taken this position after careful analysis of a region of the Earth known as the South Atlantic Anomaly (SAA). It's an area where the magnetic field is in a state of flux and has weakened noticeably. The data shows the anomaly is growing rapidly and spreading west from
South Africa, indicating that the Earth's liquid core is changing. "This may be early evidence of a forthcoming reversal in the direction of the Earth’s internal magnetic field," states the article on the BGS website.
Additional information
http://www.astronomycafe.net/qadir/q1282.html
http://www.eoearth.org/article/James_Croll_and_the_astronomical_theory_of_climate_change
http://www.ncdc.noaa.gov/paleo/milankovitch.html
http://earthobservatory.nasa.gov/Newsroom/view.php?old=2002120610970
http://aerospace.wcc.hawaii.edu/shape.html
http://curious.astro.cornell.edu/question.php?number=124
http://en.wikipedia.org/wiki/Axial_tilt
http://en.wikipedia.org/wiki/Milankovitch_cycles
(It should be noted that wikipedia is edited by people who do not completely understand the subject or are pursuing political goals. Notably this article has been edited to make CO2 climate change sound more important and completely overlooks the shape of the earth under the false assumption that the earth is completely round. If the earth were round then the total amount of solar radiation would not change as the earth tilted and wobbled because the surface area exposed would not change. It is this fundamental misunderstanding of science that is exploited by people who think the general public is ignorant or pushes man made global warming upon children who have not yet learned scientific fundamentals and do not yet understand the scientific process. The people who are pushing these beliefs upon children in schools fiercely promote "the separation of church and state," but fail to understand that science without
falsifiability is religion. )
(I need the backing to make an entire book out of this subject)
