Evening star., 1932-08-25
Only 100 Seconds to Be
Available for Observers to
Complete Their Studies.
Students From All Parts of
World Prepared in Path
of TotaIity August 81.
BY PROF. PHTLI? FOX,

Director of Adler Planetarium and Astro-
nomical Museum, Chicago.
Special Dispatch TO The Star.
! CHICAGO, August 25 (N.A.NA) "A
total eclipse of the sun provides very
delicate test of the eXactness of oul
/ knowledge of the mechanics of the
! solar system. Centuries of observation
have refined this knowledge so all cir
cumstances can now be predicted with
high accuracy.
! An eclipse can occur only when cer-
tain geometrical relations are fulfilled
To understand what these are it is nee
essary to understand the general plar
of the solar system at least SO far as
concerns the sun, earth and moon.
The earth goes about the sun in a
great orbit, taking a year for the cir
cult, spinning swiftly on its axis once
day as it goes. The moon, meanwhile
is revolving busily about the earth
making more than II revolutions in the
year. going through its phases, a thir
crescent when near the sun. near con-
junction. and full when on the opposite
side of the earth from the sun, oppoSi-
ton. The orbits are ellipses,S but OF
such low eccentricity as to be nearly
! circular
The mean distance of the earth from
the sun is 92,900,000 miles, but the dis
tance varies by one-sixtieth, or 1,55000C
miles, on either side of this mean. The
earth moves SwifteSt in its orbit when
nearest the sun, at perihelion. As the
earth goes about the sun, the sun ap-
parently moves about the earth, mov-
ing among the stars in a circle called
the ecliptiC.
Appear of Same Size.
The mean distance of the moon from
! the earth is 238.840 miles, but because
of the ellipticity of the orbit the dis
tance varies about $4,000 miles on either
side of this value. The suns diameter
is 866 400 miles and that of the moon
2,163 miles. The sun. therefore. has
about 400 times the moons diameter,
but its distance also is greater by nearly
the same ratio, so the two bodies appear
of about the same size
An eclipse of the sun can occur only
when the moon in its orbital motion
comes directly between the sun and the
earth, interposing her body, cutting off
the light of the sun, throwing her
shadow upon the earth. 1t might ap-
pear that there would be an eclipse OF
the sun at each conjunction, or new
moon, but the moons orbit is inclined
at an angle of somewhat more than five
degrees to the ecliptic, so that in gen.
eral the moon at conjunction will be
above or below the plane Of the earth's
orbit and the moons shadow will drift
by harmlessly and unobserved.
But occasionally conjunction will
occur with the new moon near one OF
| the points of intersection of the moons
! ~ ~
and then the earth will be near enough
| to the plane in which the moons
shadow moves for the shadow to strike
the earth and move swiftly across the
continents and seas of its surface. The
ecIiptic is so named because eclipSeS
occur only when the moon is on or neal
that circle.
Because the angular diameters Of the
sun and moon are so nearly equal. the
moon. when placed centrally, at times
lust covers the sun, at other times moi
entirely, in which case there IS thin
rim of the sun about the centrally
placed moon, which gives the name an
nular eclipse, from annulus, ring. The
shadow does not then reach the earth
Even under most favorable conditions
the moon has only slightly greater ap-
parent diameter than the sun. The
VI CAIC ~ AOoo oSSeAOA4
of the shadow on the earth's surface is
1853 miles.
Determine ton Complex.
The moon travels its own diameter in
its orbit in about an hour. The shadow
keeping pace, moves eastward through
space at speed of about 2,100 miles
an hour. The rotation of the earth
also eastward, carries the observer for
ward, partly compensating for the
swift motion of the shadow. The least
speed of the shadow across the earth's
surface is about 1000 miles an hour. In
high attitudes, or where the shadow
strikes at grazing incidence, the speed