Evening star., 1927-10-16
What Is a Day?

WHAT is day? In the sense In
w which the word is commonly
| used day" includes the period OF
light as distinguished from. that OF
darkness. This period of light to the
resident Of a city merely means a
certain number of hours out OF the
"a, but it may mean any length
of time under six months, accord.
| ing to the latitude Of the observer
and the season Of the year.
Another popular meaning IL the
space Of "a hours. including a period
of light and a period of darkness
This, again. is not the same the
world over. as the time Of the be-
ginning and the termination vary In
| different countries; some reckon from
sunset to sundown, some from dawn
| to dawn, some from midnight tc
midnight.
Astronomers. also, have their vari.
Gus days; the absolute solar day
| ranging from about half a minute
| under to the same amount over "a
hours at different times of the year;
the mean solar day, being our common
day OF "a hours: the lunar day of
| nearly 25 hours, and the sidereai day
OF about four seconds short Of "a
| hours.
These different answers and the
! phenomena to which reference must
| be made for their explanation would
provide material for a very long and
minutely scientific article. But there
| Is another aspect to the matter which
is not less interesting, though not
so frequently alluded to-namely, the
duration Of each day on the surface
of the earth. the place and time at
which 1t Is first seen and at which
it finally disappears.
The succession Of day and night
depends on the rotation of the earth
} on its axis, and, since the earth Is
OF globular shape, it is evident that
the whole Of the surface cannot be
turned toward the sun at one and
the same moment. In other words,
1t cannot be noon all over the earth
| at precisely the same time. A little
| thought will show that whenever II
| Is noon at any one place it Is mid
| night on the opposite side of the
! earth; and at the different points be,
tween all the. times are at one and
| the same moment to be found.
| Take particular example to make
this clearer. When 1t is noon i'm
| London--and time is usually meas.
ured from Greenwich Observatory
|
| the earth are turned directly away
from the sun and are covered by the
darkness of midnight. Paris, being
n little farther east than London
will have been brought directly Under
the sun little earlier: that IL to say
when 1t Is noon In London It IL a
few minutes past noon in Paris. AT
! Constantinople, still farther east
noon has been gone an hour or two
Farther east again. and India will
be found approaching the evening
hour. while Japan and China have
|. already sunk into darkness.
IT. however, you turn your face
West, cross ths mighty Atlantic Ocean
and land in one OF the Eastern cities
or the United States you will find that
the people have not yet reached their
usual breakfast hour. Energetic Bos
tonians, New Yorkers and Philadel
phians may perhaps be up
and about. but in Chicago and St
Louis people are Just beginning to see
the first faint gleam of dawn, while
San Francisco is still wrapped in q
mantle of darkness. But IL you travel
la little. farther West, strange as II
may seem at the first glance, YOU will
reach. the light again and 1t will be the
dawn OF new dayi And this new
day. beginning. say. in New Zealand
and countries In that longitude, will
travel swiftly round the. world from
East to West as all it's predecessors
have done since time began.
! For sake Of illustration, the day
Just described was the historical