In contrast, the natural rhythm of the seasons and the sun, moon and stars that governs much of our lives is comfortingly predictable and seems unchanging.
In fact, these rhythms vary on a time-scale of tens of thousands of years - variations that go unnoticed over a human lifetime, but that nevertheless have a profound impact on our planet.
A different pole star?
For example, over 2000 years ago the Greek astronomer Hipparcos realised that the stars of the night sky had shifted across the sky compared to the positions observed by Babylonians observed centuries earlier. In modern times they are in a different position again. In particular, the position of the pole star, Polaris has changed – today it is almost directly above the North Pole but this has not always been so. It turns out that the Earth is actually wobbling very slowly on its axis as it spins, just like a spinning top, taking around 26,000 years to come back to the same point, a process known as 'precession'. In 12,000 years the pole star will be the bright star Vega!
Earth has other long-term 'wobbles' too, such as changes in the tilt angle of its axis, and the eccentricity of its orbit around the Sun. The axial tilt that causes the seasons varies between 21.5 and 24.5 degrees over a period of 41,000 years. The Earth's orbit is not quite circular, meaning that it is closer to the sun at some times of the year, but on top of this, the actual shape of the orbit changes over a period of around 100,000 years, from almost circular to much more elliptical.
These cycles of movements are known as the 'Milankovich cycles'.
The cumulative effect of all these wobbles, as the Milankovich cycles' is rather complicated, but in essence the poles get alternatively warmer and colder, giving rise to repeated ice ages interspersed with warmer periods.
Other wobbly worlds
Interestingly, Earth is not the only Solar System object that wobbles. On Mars, sedimentary rocks near the poles are layered in an extremely regular and repeating pattern, evidence that its axial tilt has fluctuated back and forth over the ages, much more radically than Earth. It's estimated that its tilt has varied between 0 and 60 degrees in the past!
Obviously a pattern of such extreme variations will lead to extreme fluctuations in climate, all now recorded in layers of rock. Why the huge variation compared with Earth? The axial tilt of both Earth and Mars is affected by the gravitational pull of the other planets in the solar system, but the difference is that Earth has a large moon to counteract this pull, whereas Mars does not. So it looks as if Mars is unlikely to be hospitable to human life in the long term!,
There is evidence that Saturn's moon Titan is also affected by wobbling. The Cassini mission spotted a number of methane lakes on the moon, but they are almost all concentrated in the northern hemisphere; and there are also many more partially or totally dried up lakes in the north. Why such an assymetrric distribution?
It has been suggested that Saturn's eccentric orbit around the sun causes long-term climate variations that have a knock on effect on the movement of methane around Titan, causing lakes to form at one pole in preference to the other
Other planets and satellites too may be affected similarly (but we havent yet had the chance to observed them all in as much detail as Mars and Titan) – after all, they all affect each other gravitationally. In astronomy, as with everything else, it truly seems that 'the only constant is change'.
But given the fast pace of climate change that Earth is now experiencing, it remains an open question whether our species will survive to experience the change to the next ice age.
http://earthobservatory.nasa.gov/Features/Milankovitch/milankovitch_2.php
http://www.space.com/6197-mars-wobbles-created-climate-swings.html
http://www.universetoday.com/46308/lake-asymmetry-on-titan-explained/
1.Topography: Caltech; HiRISE Images: NASA/JPL/Univ. of Arizona
2.http://www.epm.ethz.ch/research/experimental/experimental-precession.html.html
3.The mosaic includes Cassini SAR, ISS, and VIS images (NASA/JPL/Caltech/University of Arizona/Cassini Imaging Team).