The Tacoma times. [volume] (Tacoma, Wash.) 1903-1949, 1942-05-09

I III NhNO1 sNNee eait sAH, ana al 10
the same plane. This greatly SIM
piifies the construction. though
some have been built. notably the
famous one that David Ritten-
house made for the University Of
Pennsylvania in colonial DAYS. in
which the paths are much more
accurate. Actually. the planets
move In ellipses. though most Of
them are nearly circular. and in
planes that are inclined to that
in which the earth travels. OF
the naked eve planets. the orbit
of Mercury Is inclined the most,
degrees. and IL also the most ell
liptical. On the average 3e.o00.-
600 miles from the sun, Mercury
can approach that orb to within
2s.550. 900 miles. and II can re-
cede as far as 43.35o, 900 milesL
In contrast, the range of distance
OF the earth from the sun 10 only
from about 1880. 900 10 e4.5oo,-
900 miles.
This is responsible for a great
variation In the distance of Mer-
curry from the sun at the time of
elongation. II IL occurs when Mer-
curry In nearest TO il, then the
greatest elongation il only IS de-
grees, a. measured against the
sky: but II IL comes when the
planet is farthest from the sun, II
can get as far as rs degrees away
The elongation this month In about
average. ~ degrees IL minutes.
However. there is another factor
determining the visibility of Mer-
curry. In the United States. or any
place not in the tropics. this rs TO
rs degree figure IL never meas
ured in a vertical direction. but al
ways TO the side. However. when,
for the northern hemisphere.
greatest eastern elongation comes
at this time of year. the line from
the sun to Mercury Is most nearly
vertical. and then 1t 10 highest in
the sky at sunset.
There Is still another factor In
the distance the planet 10 north
or south OF the suns path, q line
called the ecliptic. 1t happens that
on May Is. Mercury is about 8 de-
grees, out of a possible Z, north Of
the eciptic and this brings the
line from sun TO planet still more
nearly vertical. with the result
that this month Mercury will set
as much as an hour and q half
after sunset.
Because of its proximity to the
sun, Mercury is very difficult to
observe. and no very distinct
markings have been seen. How
ever, the planet Is so small, about
300 miles diameter. that II does
not have an atmosphere: II 10 not
massive enough TO hold one even
II we could now endow II with
such a layer Of air. And just A.
the moon always keeps the same
face toward the earth. 20 does
Mercury always present the same
hemisphere 70 the sun. This was
demonstrated several years ago at
the Mt. Wilson observatory when
very delicate measurements, with
q device called q thermocoupls.
were made Of the planets surface
temperature. No heat whatever
could be detected from the dark
side. which is close to the absolute
zero of minus $50 degrees Fahren-
heit. even though the side toward
the sun is about $50 degrees Fab
renheit. hot enough 10 melt lead
Even If Mercury turned at such
a speed that many weeks were re-
quired from sunset to sunrise.
the dark side would hold little
heat, which could be detected.
The reason why Mercury should
rotate 50 slowly as always TO keep
the same face 10 the sun u the
same as that for the similar be
naglor OF iDe mDcH 70 INC rnAAh
When Mercury was younger. and
the globe was still in a plastic
condition. tides were caused BY the
suns pull These acted as brakes.
and gradually slowed It's turning,
but finally. when the present con
dition was reached. this orsking
action ceased. Ths same thing
happened xdth the moon. due To
tides caused by the esrth's pull
The effect IL still gong on with
the earth. rdes here, largely
caused BY the moons attraction.
are gradually slowing down our
rotation. and the time y1il CONS,
in the very distant future. When
ore hemisphere q the earth In al
Ways turned 70 the moon IA this
viii Be some 5000. 900 900 years
from nov. we hardly need worry
about Il.
1