Evening star., 1899-03-25
TWILIGHT IN THE TROPICS
How Long It Really Lasts After the Sun
Has Set.
Shortest mt the Equator and Increases
With the Latftude-Where
1t is Longest.

FUHi tne SiH FruHciScU NDTunlcle
The belief that there is little or no twi
light within the great tropical belt encir-
cling our planet is very Widespread one.
History, however warns US that beliefs
need not necessarily accord with facts. 1t
might indeed almost be said that they
usually do not The toling and moiiing
millions, the humble Units@ that, taken in
the bulk, constitute what is called great
power" have neither the time nor the in
clination to overhaul their opinions. They
are not assailed by doubts as to whether
the Intellectual coffee which they have at
odd moments imbibed may not have been
mostly chicory. Such d state of mind un
questionably makes more happiness. and
although such happiness may bear rather
too much resemblance to that of, say, the
COW. it is none the less real for all that.
Twilight is at it's shortest at the equator
and increases with the latitUde-at the
equinoXes-at about the same rate as that
at which the merdians decrease their dis
tance from one another. The increase Is,
therefore, very slow in low ATTITUDES and
very rapid near the poles.
Practical, or civil. twilight Is the time
which eiapses between the moment of the
suns setting and the moment when he is
seven degrees of great circle below the
horizon. At the equator this time cannot,
of course, be less than twenty-eight min
utes. At the edges of the tropc zone it is
about thirty-one minutes. A person with
good evesight will, should he care to make
the experiment find that when at sea and
on or near the equator he is able to read
average type on deck for at least twenty
eight minutes after the sun has set without
the aid of artificial light and when there
iS no moonlight. This is the common sense
experiment by which the duration of prac-
tical twilight has been determined. The
actual duration is considerably longer than
the practical, and, even at the equator,
something like seventy minutes will elapse
before night holds undsputed sway. This
is the length of the true-the astronomical
twilight.
1t is at or near the equinoxes that the
phenomenon under consideration is at its
shortest all over the globe At those
periods it lasts at Honolulu thirty minutes;
at San Francisco, thirty-ive; at London,
forty-tive; at st. Petersburg. fifty-seven: at
the arctic and antarctic circles, seventy
two, and at the poles. eighteen days.
When. then, We say that the poles have
six months light and six months dark
ness, it is little more than a facon de par
Ter. The north pole has no less than 222
days of practical daylight each year the
south pole 215 days. the equator 197 days.
Nansen, when in high northern ATTITUDES.
was able to read the "VerdenSgang" many
days before the longed-for sunrise follow
ing the long winter night The great
length of the twilight in polar regions is
boon indeed to explorers The popular
imagination usually attaches itself TO the
great cold they have to endure but the
real enemy is the long darkness, which
is trying to the enthusiasm of even Nan
sen, a Peary or a Jackson.
There is practically no difference in the
length of the twilight at the equator at
the equinOXes and solstces. Even at Hon
olulu it will not vary by more than
couple of minutes. But in the higher lat
itudes the varying duration is very marked
In latitude :912 degrees. for instance it
lasts ffty-siX minutes at the equinoxes,
but at the summer solstice it lasts from
sunset to sunri5e-that is to say. from $40
o'clock till $50, or for five hours and forty
minutes.
North or south of SO degrees practical
The sun is. it is true. only twelve hours
above the horizon, but as he can not get
more than seven degrees below it there
Is continual daylight for the twenty-fOur
hours. In such cities as Stockholm and
st. Petersburg there are close on to four
teen hours of daylight at those seasons
when day and night are said to be equal
and at the equator itself the day all the
ear round is made up of thirteen hours
Daylight and eleven hours darkness 1t
must also be not forgotten that in those
la attitudes where the sun remains for months
below the horizon there is during that
period good deal of daylight SO that
along the eightieth parallel north Ga little
north of Which the Jackson II Harmsworth
expedition spent three consecutive years
IN the Franz Josef islands,, although the
sun does not rise between October 10 and
FEBRUARY -, there is a period of three
weeks following the final sunset and an
other of the same length preceding the
hrst sunrise of the year, on each day of
which good type can be read for a few
minutes UP to some hours without arti-
ficial light.