Omaha daily bee., 1914-12-08


nshes the light, and
It is reflected from the nonself- luminous
surfaces of the planets thus making
them visible TO one another. as the faces
Of persons sitting in a dark room are
rendered visible by the turning on Of an
electric lamp.
The relative amount of light reflected
to us from any planet depends upon the
size OF the planets second. its distance
from the sun; third, it's distance from
the earth; fourth, the character Of the
planets surface. which may be more or
less reflective for light
Among these causes the most striking
In its effect is distance For instance,
the planet Jupiter has a surface about
12 times greater than that of the planet
Venus; but Venus, when nearest, appears
much brighter to us than Jupiter when
nearest because Venus is then fifteen
times nearer than Jupiter. and what IL
lacks in relative size 1t more than gains
through relative nearness. the light in
creasing as the square of the decrease
OF distance. Venus also gains brilliance
relative to Jupiter in consequence of her
greater nearness to the source Of the
light, which results in its surface being,
area for area, more brightly illuminated
than that of Jupiter.
As to your second question, you appear
to be confused by the effects of night
upon the inhabitants of the earth. AS
our planet turns rapidly on its axis we
are about half the time on the side that
is toward the sun and half the time on
the side away from the sun. but the cone
of shadow that rests upon the night side
of the earth is only about 100000 miles
long, coming to a point and vanishing a.
that distance. To an eye situated any
where outside that cone some part Of the
earth would be seen illuminated by the
sunshine, unless the line of sight coin
cided with the axis of the shadow cone,
in which case Tho earth would appear as
a dark circle, either covering the sun or
forming a black dot on its disk. accord
ing as the observer was nearby las he
would be IL on the NOON or far off las
he would be IL on any one OF the planets).
Consequently, when we upon the earth
are buried in night our planet may still
be perfectly visible to the Inhabitants OF
the other planets because they are out
side the cone of the earth's shadow, and
light is reflected to their eyes from parts
Of the earth's surface which to them are
in the sunshine. But the brightness. or
just as the brightness or apparent size of
their planets varies to us. and for the
same reasons, viz. variation of distance
and change of place resulting in differ
ence in the amount of planetary surface
illuminated by the sun.
An Illustration of what has Just been
said may be seen in the case of the planet
Venus at the present time. That planet
is very brilliant in the western evening
sky but IL you look at her with tele-
scope you will find that only about half
her surface appears illuminated by the
sun. and she presents the figure of a
half-moon. When she is on the far side
of the sun tom the earth she looks like
full moonsh tho telescope, but then her
total brightness is greatly reduced be
cause OF the greater distance.
The earth looked at from Venus APP
pears now as morning star OF exceed
Ing brilliance, and next winter. when
Venus gets between the earth and the
sun and Is temporarily lost to our view
in the solor glare, the earth Will glow
at midnight on the meridian OF the sky
of Venus with a spleudor far greater
than any planet ever shows TO us, be-
cause il will then turn its fully illum-
mated face toward Venus at the moment
when the two planets are at their points
Of nearest approach.
II II seems strange to you that the sun
light reflected from the surface of the
earth should be bright enough to make
our planet visible millions OF miles away
you may reflect upon an experiment OF
Sir John Herschel at the Cape of Good
Hope He observed that when the face
of Table mountain was illuminated by
the sunshine its brightness exceeded in
intensity that of the face of the full
moon.