The Washington herald., 1911-06-26

Few OF the residents Of Washington
and its environs know anything OF the
beautiful luminous tip crowning the
Washington Monument, which can now
be seen on the sunlit mornings from
various points in the city, especially
through the Mall. This pheomenon.
which has been observed by few OF the
older Washingtonians during its recur-
renee from year to year, and which has
been appropriately termed THE Kohl
moor of the Washington Monument" is
caused by the reflection of the sun from
the eight.pound aluminium quadrangular
pyramid which tops the shaft
It is a fundamental law Of optics that
the angles of incident and reflection are
equal, that Is, a ray Of light Is re-
fiected at the same angle that 1t strikes
a rejecting surface. When a person
gets in the Me of the reflected light
from the metallic apex OF the Monument,
the "Koh-i-noor'' is seen in all its glory,
shining forth with its dazzling yellow
brlliancy like some stupendous star Of
incalculable magnitude.
Changes in Time.
But the sun is constantly changing its
position from hour to hour during the
day. in addition to its daily change OF
position in the heavens, caused by its
apparent yearly revolution around the
earth, so that the time and place OF OBJ
servation Of the blazing tip change with
It.
Although the rays Of the sun strike
one or the other Of the faces Of the
pyramid as long as they shine. the faces
are inclined, and consequently when the
sun is in certain positions the reelected
- -. I.
horizon, so that a person standing on
the ground cannot see them.
The strange phenomenon Is visible
from about May % to the latter part Of
June, and can also'be seen again in Oc-
tober, although from different view
points. From a selected point Of obser-
vation the luminous tip can be seen for
about thirty minutes, after which IL
gradually fades away.
The brightness Of the sky surrounding
the sun is responsible for the dazzling
light lasting so long, as the full glory
of the tip can be seen for only about
three minutes. The observer must be
within twenty or thirty yards Of the
point OF best observation or the phe-
nomenon cannot be seen at all
Points of Observation.
To determine the points Of observation
get map OF the city which is not fore
shortened and draw a line from the CEN
ter Of the Capitol to a point 100 yards
due north of the Monument The sun
light from the eastern face Of the apex
will be visible from the following places
on the line in the morreng about June 1:
Capitol terrace, SIS a. M; Four-and-a-
half street, SGT a. m; Seventh street,
S:37 a. m; Twelfth street, $40 q. m:
Fourteenth street, 1874 a m. Although
the time changes daily, the above calcp-
lations give a good approximate basis
for the observation Of the ''Koh-i-noor''
as long as IT is visible.
For determining the points Of observa-
tion for the reflection from the northern
face, draw line from the Monument
itself to Ninth and K streets northeast
On June ] the phenomenon can be seen
from the southeast cogner Of Twelfth
and B streets at $40 p. m; northwest
corner OF Seventh street and Pennsyl-
vania avenue at 5:3S p. u-, and Fourth
and II streets northeast at 6:55 p. m.
The calculation OF the place and time
OF observation is a simple problem to
one versed in spherical trigonometry. AS
far as is known, Prof. R W. Prentiss.
who was at one time connected with
the Nautical Almanac Office, was the
first person to make the calculations.
He worked out the problem in April,
1855.