Page 303 - NS-2 Textbook

298 NAUTICAL SCIENCES

-... ..... ....
------------------------ ---- ---..... .............. ..... ............ ..... ....
------------------ -------- -.... ............ .............. .... ...... .... .....
------- ---.... -......... .......... ..... ......
-------------------------- -................ ........ .............. .......... .....,,,"-, ' .... ""
-....... -..... .............. ....' ..... " .... .. "
-.....
.... ..... ~"
---- ------------- ------........,",', \
'\
.. ,. ... ------------------ .............. '........ \ \ \ \
/--(~)~>"'"'' "\ \ ~.'"'~ Y
~ 1
h..,"", I,
OVenus/ 8
I /'" "''oMerc~ry OEarlh\ I \ I I I I PlutoQ
J
I
I
\ /, /1 J f I I I
........ _- _ .... ; I 1 I 0 / J
I
I
.......... _-- --- ... -..-...-'" .... / dMars / I
--------- ............./ Jupiter / ~_~.- - Uranus I /
...... _____ _ ____ ---- ........ ..- .. // __ I I
.... ------- // // /'
--------------- _..... ....././;/ /
.... /",/
-_....... .... ",,/ ,/ // /' // /
~__ __-- ~ ... '" .... ... / ;/ " .I'
------------ --- ....... ~ , /" //
------------- ......... -...... // ///// ///
--------- ------ ... - ",'" .... /
--- ... -- ,..-'" .....
----------------------- ~..--..-"" ............. ...... /
The nine traditional planets of our solar system. The planets orbit the Sun in the same direction and in generally the same plane. The plane
of Earth's orbit is called the Plane of the Ecliptic. Each planet's orbit is in the shape of an ellipse (oval), which varies in most cases only slightly
from a circle.
TV cameras mounted on the Mariner 10 spacecraft grees F and returned pictures of a barren, hmarlike land-
observed Mercury in 1974 and 1975. It has a heavily scape. TIlese observations, plus others by American
cratered, dusty surface like that of the Moon and a large spacecraft, have shown that the planet is a very unlikely
core of iron somewhat like Earth's. Mercury has a series place for life of any kind.
of cliffs, some extending up to 2 miles high, which cut There is no water and no free oxygen on Venus. The
across the surface for hundreds of miles. They probably surface temperahrre of 900 degrees F is hot enough to
formed about 4 billion years ago when cooling of the melt lead and zinc. The atmosphere is even less friendly.
planet's core crumpled the crust. A deep layer of carbon dioxide, 100 times heavier than
Mercury was named for the speedy messenger of the Earth's atmosphere, would bear down on an inhabitant
gods in Greek mythology. It has the shortest period of with the weight of the ocean at a depth of 3,300 feet. The
revolution about the Sun-SS days. Because it is so close great heat on Venus is caused by what is commonly
to the Sun, the planet is difficult to observe. It is best seen called the "greenhouse effect." The heavy layer of car-
just after sunset in March and April and just before sun- bon dioxide traps entering sunlight and prevents the es-
rise in September and October. cape of heat energy. The dense atmosphere keeps the in-
tense heat evenly distributed around the planet, with
little variation between day and night or from pole to
pole. Vision in this atmosphere of carbon dioxide would
VENUS
be limited to a few hundred feet, since only 2 percent of
It was once believed that Venus was almost a twin sister the Sun's light breaks through the cloud layers to the
of Earth, because the two planets are so nearly alike in surface.
size, mass, and density. Astronomers of the seventeenth The thick cloud cover makes it nearly impossible to
and eighteenth centuries believed Venus to be very observe any surface features. However, the areas in
warm, but with plentiful water and lush vegetation, and which the Soviet Venera spacecraft landed appeared to
probably peopled by small, dark-skinned people. be composed of loosely packed granite.
Since 1962 about twenty Soviet and u.s. spacecraft Earth hrrns once on its axis every day, but Venus
have explored Venus. Several of a series of Soviet Venera hrrns only once in 243 Earth days. Because of its orbit
landers launched from the late 1960s to the early 19S0s around the Sun, a solar day on Venus is 117 days from
successfully penetrated the Venusian atmosphere and re- one sunrise to the next, but because of the super-
ported its density and pressures before landing on the refractivity (extreme bending of light rays by the ultra-
surface. They recorded surface temperatures of 900 de- dense ahnosphere), no one on the Venusian surface could