The Trouvelot astronomical drawings manualTrouvelot, E. L. (Etienne Leopold)
Science
The Trouvelot astronomical drawings manual
Trouvelot, E. L. (Etienne Leopold)
Astronomy -- Pictorial works
The internal condition of the planet Saturn, whether solid, liquid or
gaseous, cannot be discovered from the examination of its surface, as
its globe is enwrapped in a dense opaque layer of vapors and cloud-like
forms, through which the sight fails to penetrate. The appearance of
this vapory envelope is like that of _cumulus_ clouds, and one of its
characteristics is to arrange itself into alternate bright and dark
parallel belts, broader than those seen on Jupiter, and also more
regular and dark. These belts, which are parallel to the equator of the
planet, vary in curvature with the inclination of its axis of rotation
to the line of sight.
The belts of Saturn, like those of Jupiter, are not permanent, but keep
changing more or less rapidly. Sometimes they have been observed to be
quite numerous; while at other times they are few. Occasionally
conspicuous white or dark spots are seen on the surface, although the
phenomenon is quite rare. It is from the observation of such spots that
Saturn's period of rotation has been determined, as stated above. The
equatorial zone of Saturn always appears more white and brilliant than
the other parts, as it also appears more mottled and cloud-like. In late
years the globe has been characterized, and much adorned, by a pale
pinkish tint on its equatorial belt, resembling that of Jupiter, but
somewhat fainter. On either side of the equatorial belt there is a
narrower band, upon which the mottled appearance is visible. Below
these, one or two dark belts, separated by narrow white bands, are
usually seen; but, of late, the bands have been less numerous, being
replaced in high latitudes by a dark segment, which forms a polar cap to
Saturn. The globe of Saturn does not anywhere appear perfectly white,
and when compared with its ring, it looks of a smoky yellowish tint,
which becomes an ashy gray on its shaded parts. It usually appears
darker near the limb than in its central portions; although on some
occasions I have seen portions of the limb appear brighter, as if some
white spots were traversing it.
Some observers have seen the limb deformed and flattened at different
places, and W. Herschel even thought such a deformation to be a
permanent feature of this globe, which he termed diamond-shaped, or
"square shouldered." But this was evidently an illusion, since the
planet's limb usually appears perfectly elliptical, although it
occasionally appears as if flattened at some points, especially where it
comes in apparent contact with the shadow cast by the globe on the ring,
as observed by myself many times. But with some attention, it is
generally found that this deformation is apparent rather than real, and
is caused by the passage of some large dark spots over the limb, which
is thus rendered indistinguishable from the dark background upon which
it is projected.
Public-domain text, read in full here on John Shaqi.
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