The Earth in the Solar System
ATLAS OF THE UNIVERSE
W
hy do we live on the Earth? The answer must be:
‘Because we are suited to it’. There is no other planet
in the Solar System which could support Earth-type life
except under very artificial conditions. Our world has the
right sort of temperature, the right sort of atmosphere, a
plentiful supply of water, and a climate which is to all
intents and purposes stable – and has been so for a very
long time.
The Earth’s path round the Sun does not depart much
from the circular form, and the seasons are due to the tilt
of the rotational axis, which is 23^1 ⁄ 2 degrees to the perpen-dicular. We are actually closer to the Sun in December,
when it is winter in the northern hemisphere, than in June- but the difference in distance is not really significant,
and the greater amount of water south of the equator tends
to stabilize the temperature.
The axial inclination varies to some extent, because
the Earth is not a perfect sphere; the equatorial diameter
is 12,756 kilometres (7927 miles), the polar diameter only
12,714 kilometres (7901 miles) – in fact, the equator
bulges out slightly. The Sun and Moon pull on this bulge,
and the result is that over a period of 25,800 years the
axis sweeps out a cone of angular radius about 23°26’
around the perpendicular to the plane of the Earth’s orbit.
Because of this effect – termed precession – the positions
of the celestial poles change. At the time when the
Egyptian Pyramids were built, the north pole star was
Thuban in the constellation of Draco; today we have
Polaris in Ursa Minor, and in 12,000 years from now the
pole star of the northern hemisphere will be the brilliant
Vega, in Lyra.
We have found out a great deal about the history of
the Earth. Its original atmosphere was stripped away, and
was replaced by a secondary atmosphere which leaked
out from inside the globe. At first this new atmosphere
contained much more carbon dioxide and much less free
oxygen than it does now, so that we would have been
quite unable to breathe it. Life began in the sea; when
plants spread on to the lands, around 430 million years
ago, they removed much of the carbon dioxide by the pro-
cess known as photosynthesis, replacing it with oxygen.
Life was slow to develop, as we know from studies
of fossils; we can build up a more or less complete
geological record, and it has been found that there were
several great ‘extinctions’, when many life-forms died
out. One of these occurred about 65 million years ago,
when the dinosaurs became extinct – for reasons which
are still not clear, though it has been suggested that the
cause was a major climatic change due to the impact of
a large asteroid. In any case, man is a newcomer to the
terrestrial scene. If we give a time-scale in which the total
age of the Earth is represented by one year, the first true
men will not appear until 11pm on 31 December.
Throughout Earth history there have been various
cold spells or Ice Ages, the last of which ended only
10,000 years ago. In fact, the last Ice Age was not a period
of continuous glaciation; there were several cold spells
interrupted by warmer periods, or ‘interglacials’, and it is
by no means certain that we are not at the moment simply
in the middle of an interglacial. The reasons for the Ice
Ages is not definitely known, and may be somewhat
complex, but we have to remember that even though
the Sun is a steady, well-behaved star its output is not
absolutely constant; in historical times there have been
marked fluctuations – for example, the so-called ‘little ice
age’ between 1645 and 1715, when the Sun was almost
free of spots and Europe, at least, was decidedly colder
than it is at the present moment.
Neither can the Earth exist for ever. Eventually the
Sun will change; it will swell out to become a giant star,
and the Earth will certainly be destroyed. Luckily there is
no immediate cause for alarm. The crisis will not be upon
us for several thousands of millions of years yet, and it is
probably true to say that the main danger to the continued
existence of life on Earth comes from ourselves.
Earth’s history is divided into different ‘eras’, which
are subdivided into ‘periods’. The most recent periods are
themselves subdivided into ‘epochs’. The main divisions
and subdivisions are shown on the table opposite.
▼ Planet Earth, seen from
the command module of the
lunar spacecraft Apollo 10
in May 1969. The Earth is
coming into view as the
spacecraft moves out from
the far side of the Moon.
The lunar horizon is sharp,
as there is no atmosphere
to cause blurring.B Atl of Univ Phil'03stp 31/3/03 4:03 pm Page 36