A Preliminary Dissertation on the Mechanisms of the HeavensSomerville, Mary
Science
A Preliminary Dissertation on the Mechanisms of the Heavens
Somerville, Mary
Celestial mechanics
Far as the earth seems to be from the sun, it is near to him when
compared with Uranus; that planet is no less than 1843 millions of
miles from the luminary that warms and enlivens the world; to it,
situate on the verge of the system, the sun must appear not much
larger than Venus does to us. The earth cannot even be visible as a
telescopic object to a body so remote; yet man, the inhabitant of the
earth, soars beyond the vast dimensions of the system to which his
planet belongs, and assumes the diameter of its orbit as the base
of a triangle, whose apex extends to the stars.
Sublime as the idea is, this assumption proves ineffectual, for
the apparent places of the fixed stars are not sensibly changed by
the earth's annual revolution; and with the aid derived from the
refinements of modern astronomy and the most perfect instruments,
it is still a matter of doubt whether a sensible parallax has been
detected, even in the nearest of these remote suns. If a fixed star
had the parallax of one second, its distance from the sun would be
20500000 millions of miles. At such a distance not only the terrestrial
orbit shrinks to a point, but, where the whole solar system, when
seen in the focus of the most powerful telescope, might be covered
by the thickness of a spider's thread. Light, flying at the rate
of 200000 miles in a second, would take three years and seven days
to travel over that space; one of the nearest stars may therefore
have been kindled or extinguished more than three years before we
could have been aware of so mighty an event. But this distance must
be small when compared with that of the most remote of the bodies
which are visible in the heavens. The fixed stars are undoubtedly
luminous like the sun; it is therefore probable that they are not
nearer to one another than the sun is to the nearest of them. In
the milky way and the other starry nebulæ, some of the stars that
seem to us to be close to others, may be far behind them in the
boundless depth of space; nay, may rationally be supposed to be
situated many thousand times further off: light would therefore
require thousands of years to come to the earth from those myriads
of suns, of which our own is but 'the dim and remote companion.'
Public-domain text, read in full here on John Shaqi.
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