The evening times. [volume] (Grand Forks, N.D.) 1906-1914, 1909-12-23

The latitude OF place is It's an
gular distance frtI the equator
measured along meridian. This
meridian is the half circle drawn from
the south pole to the north pole
through that place. Thus, a meridian
would cover 1S0 degrees, and the an
gle from the equator to entire pole
would be 80 degrees. The latitude of
the north pole would be 80 degrees,
that Of any point between the equa-
tor and the pole would. be less than
80 degrees. and at the equator It
would be zero degrees.
One of the simplest methods OF
determining latitude is by. measuring
the angle from the horizon to the
north star called Polaris. This. pro-
cess Is known as observing the alti-
tude of Poiaris. AT the instant this
angle is measured the time Is noted
from the .chronometer. This timeswill
enable one, by referring to tables in
the Nautical Almanac and American
Ephemeris. to apply certain small
corrections, so that the latitude can
be accurately determined.
Necessity Of Corrections. -
The necessity of one of these cor
rections Is due to the fact that ..Po-
larfs moves in small curved path
about the pOle, and only due
north or in line with the earth's axis
at the highest and lowest point of its
path Therefore, it is necessary to
calculate from tables in the Nautical
Almanac and American Ephemeris
the amount this star is out Of line
with the the earth's axis at the re-
corded time of the observation, and
apply this correction to our altitude.
Also we apply correction from other
tables for the amount Of light from
the star is bent upon entering the
earth's atmosphere. s'.' 5-o - 80, .s,ss+;. =.
Now comes a use for the thermome-
ter. We must record the atmosphere
temperature In order to use the ia
BILE and correct for this bending Of
the light from the star.
Another method of determining lat
itlde is to observe the altitude or
angular distance from the horizon Of
the sun at local noon, or at its high
est point. AT the instant Of .oDser-
vation, the time iiffst De @1foted from
q schronometer. The temperature Of
the atmosphere Is also noted, and
corrections made from table for the
bending of the suns rays upon enter
ing the air and for the. fact that the
observer is not at the earth's cen
ter. These corrections are spoken of
as refraction and parailax.
Calculations by Sun.
The angle from the horizon to a
pboint called the zenith, directly
above the observers head is suppos-
ed to be 30 degrees. The altitude is
subtracted from 80 degrees to give
the angle from the sun to the zenith,
called the senith distance.
From the. Nautical Almanac and
American Ephemeris, the angle that
the sun is above or below the equa-
tor is calculated from the date and
time of observation. II the sun is
above the equator, this angle is added
to the zenith distance to give the
angular distance of the observer from
the equator, or the latitude. II the
sun is below the equator, this angle
must be subtracted.
Use Of Artificial Horizon.
To determine altitude by the sex
tant on land, it is necessary to use
the artificial horizon.
The star will appear as far below
q horizontal line as the star is above
It.
Since the horizon cannot be deter
mined, 1t is necessary to measure the
angle from the star to its reflection
One-half this angle will give the al
titude of the star.
Chech on Chronometers.
Tn
communication. where the standard
time from an observatory cannot be
obtained. occasional time observations
upon the sun or stars should be TAX
en- as check on the thermometers.
There are several methods by which
chronometers may be checked from
sun or star observations. But in or
der to give general idea concerning
the data that would be recorded dur
ing explorations one of the simpler
operations may be suggested.
1t will be supposed that one chro-
nOmeter is set to some standard time,
such as that for the meridian at
Greenwich or Washington, and that
the other is set on sidereal time.
.Sidereal time, instead of being reck-
oned by the revolution of the earth
with respect to the sun, is reckoned
by the revolution of the earth with
respect to a point called the vernal
equinox. It is not necessary here to
discuss the position of this point, but
its definition may be had from books
on elementary astronomy.
@ We will suppose that we are to
observe the time from the north star
or Polaris, and that the two chron-
ometers have been compard at some
place of starting where standard time
may be '--d ee-o1n telegraph.
To Calculate Time.
The star POlaris will be at certain
angular distance measured at the
center of the earth from this vernal
equinox, in the direction of the
earth's rotation; and the time reuuir-.
ed at any date for a point on the
earth's surface totravel this distance
may be obtained from tables In fne.
Nautical Almanac and Americas Eph-.
emeris. The point at which the oD-.
Continued ON page 3.