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    Astronomy and climate
    From: George Huxtable
    Date: 2004 Dec 4, 16:01 +0000

    Apologies, in advance, for the lack of navigational content in this message.
    It's just an example of the subtle way that astronomical factors can affect
    Earth's climate, over historical (well, archaeological) periods of a few
    thousand years. Just in terms of the Earth-Moon-Sun system; nothing to do
    with interplanetary chaos.
    What controls the Earth's weather is mainly radiation from the Sun. The
    Earth's elliptical orbit round the Sun means that when it's closest to the
    Sun (early in January), the heat and light received by the Earth is about
    7% greater than it is 6 months later, when the distance is greatest. That's
    quite a big difference.
    It's also a convenient difference, for those of us that live in the
    northern hemisphere, in that we get our 7% boost just around the coldest
    time of the year. It helps to ameliorate the changes, to make northern
    climates more equable than they would have been otherwise.
    Not so for those in Southern latitudes, however. Their increased Sun energy
    is around midsummer, when it's needed least. As a result, southern climates
    are more extreme than they would have been otherwise.
    However, in 9,000 years time, around 11000 AD, mainly because of the
    precession of the Earth's axis, it will all be the other way round, and
    northern climates will be correspondingly more extreme between winter and
    summer; and in the south, more equable. And that would also have been the
    case around 9000 BC.
    All this is quite inevitable, and has nothing to do with global warming or
    greenhouse gases.
    There's another effect, caused by slow changes in the tilt of the Earth's
    axis (obliquity), by getting on for a degree either way, which can also
    affect the variability of the seasons over a similar timescale.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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