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Hi Patrick,
Thanks. There obviously are times when I do need someone to do my
thinking for me !
Not having a grasp on things discovered 300 years ago, by quill pen
and candlelight, helps to keep one humble. And supposing, for
unaccountable reasons, that the earth would return to the starting point
in 6 months helps too. Duh.
Yt, DW
Patrick Kelly wrote:
>
>
>
> One passage in this account has me baffled and perhaps someone can
> explain. The passage being "The changes in the atmosphere could
> also answer the old persuasive objection to Milankovitch's theory
> -- if the timing of ice ages was set by variations in the sunlight
> falling on a given hemisphere, why didn't the Southern Hemisphere
> get warmer as the Northern Hemisphere cooled, and vice-versa? The
> answer was that changes in atmospheric CO2 and methane physically
> linked the two hemispheres, warming or cooling the planet as a
> whole.(52*) "
>
> What configuration of tilt and orbital distortion could lead to
> one hemisphere receiving more insolation in e.g. summer than the
> other hemisphere would receive 6 months later ? Tilt would have to
> be symmetrical and even if the sun was at times not exactly at the
> intersection of the major and minor axis of the orbital ellipse,
> surely this assymetry would not flip in the space of 6 months.
>
>
>
> Hi everyone:
>
> We already have that exact situation because the Earth's orbit about
> the sun is an ellipse, not a circle. Earth's makes its closest
> approach in early January (typically about 147,000,000 km) and is
> farthest from the Sun in early July (typically about 152,000,000).
> While the difference is only several percentage points, the amount of
> heat the Earth receives depends on the inverse square of its distance,
> so Earth receives about 7% more heat in January that it does in July.
> It also turns out that at the present, the Earth's axial tilt is such
> that the southern hemisphere has its maximum tip towards the Sun in
> late December (the December solstice) at the time it receives more heat
>
> Thus the southern hemisphere's summer is somewhat warmer than ours. It
> is rather difficult to compare the hemispheres directly since the
> atmosphere does move the heat around, and the southern hemisphere is
> mostly water while the northern one is mostly land.
>
> A more extreme example is Mars, which has a much more elliptical orbit
> than the Earth. Its distance varies from 205,000,000 km to 249,000,000
> km for a difference of just over 20% giving about a 40% difference in
> the amount of heat received. Its orbital tilt is almost the same as
> that of Earth, and, like Earth, its southern hemisphere is tipped
> towards the Sun when it is nearest to the Sun. While summer is the
> north is not all that impressive, the huge amount of extra heating in
> Mars' southern summer puts enough energy into its thin atmosphere
> (about 1% of Earth's) to drive huge dust storms that can envelop the
> planets for months.
>
> At the other extreme is Venus, which has an atmosphere composed almost
> entirely of carbon dioxide, and which contains 90 times as much gas as
> Earth's. Despite the fact that in rotates so slowly once every 200+
> days, the greenhouse effect on Venus means that the temperature is a
> balmy 450°C uniformly over the entire planet; day side and night side,
> equator and poles.
>
> [As a side note, if you take all of the CO2 that is tied up on Earth
> as fossil fuels, dissolved in the oceans (which release CO2 when
> heated) and carbonate rocks (ditto) and put it back into the
> atmosphere, ours would look very much like that of Venus. There is
> still debate as to whether Venus has always been like this (too close
> to SUn for liquid water to form and start dissolving CO2) or may have
> been more Earth-like in the past and experienced a runaway greenhouse
> effect as the Sun slowly increased its energy output over the last 4.5
> billion years.]
>
>
> ==========================================================================
>
> Patrick Kelly
> Director of Computer Facilities
> ==========================================================================
>
> Faculty of Architecture and Planning
> Dalhousie University
> ==========================================================================
>
> PO Box 1000 Stn Central 5410 Spring Garden Road
> Halifax, Nova Scotia B3J 2X4 Halifax, Nova Scotia B3J 2X4
> Canada Canada
> ==========================================================================
>
> Phone:(902) 494-3294 FAX:(902) 423-6672 E-mail:patrick.kelly@dal.ca
> ==========================================================================
>
>
>
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