The aegis & intelligencer. [volume] (Bel Air, Md.) 1864-1923, 1889-05-17
How and When Does the Sun Cross the
Line?

Mr.. EDIToR-Please say In your next issue
what time in the spring the sun crosses the
line, and what time in the fall, and whether the
time varies or not. lit varies, how much?
SUBSCRIBER.
In reply 10 the above query Prof. G. q.
Curtiss, of Oakland School, Fallston, in COM
pliance with our request, sends the following:
Let me use very homely illustration:
Suppose in large tub of water, a small
globe be fixed in about the centre of the tub,
half above and half below the water. Let
this globe represent the sun. Pierce small
er globe with wire to represent its axis, and
around this globe, at equal distance from each
pole, paint line. This second globe shall
represent the earth, and the circles about
its centre its equator, or "the line" Now
place the earth in the water on the north
side of the sun, and lean its axis from the
perpendicular over to the north 23i degrees,
while it rests in the water like the sun, ex-
actly half submerged. The surface of the
water will represent the plane OF the earth's
orbit passing exactly through the centre of
the earth and centre Of the sun. The season
represented with the earth placed north Of
the sun, with its axis leaning over to the
north, is the winter sostice, about the 22d of
December. At this time the earth leans ex-
actly from the sun, and the sun is overhead
%! degrees south of the equator, or on the
Tropic of Capricorn, or just at the surface of
the water in our tub. Now carry the earth
around to the west always keeping its axis
leaning to the north and parallel with ITSELF
and when one quarter of the way around, or
at the west poiut, the earth will lean exactly
sidewise to the sun, and the sun will be on
the line, or overhead at the equator about
the 2lst of March. This is called the vernal
equinox, as the sun shines exactly to both
poles and so gives equal days and nights all
over the earth's surface.
Now as the earth moves on as before, from
the west towards the east the north pole be-
gins to lean towards the sun and about the
22d of June it leans exactly towards the sun
and the sun is overhead 23t degrees north of
the equator, or on the Tropic of Cancer.
where the surface of the water in our tub
now touches the earth while its equator is
23I degrees below the water on the side
towards the sun. Now after another quarter
OF the circle, the east point is reached, and
the earth again leans sidewise to the sun and
it is now about the god of September and the
sun is crossing 'the line" 10 the south, and
ii is now the autumnal equinox.
The exact interval of time from the vernal
equinox to the same again is 1865 days, 8
hours, AZ minutes, 1852 seconds that is, the
time in which the earth makes complete
revolution, or between eleven and twelve
minutes short of six hours. Suppose it leap
year, and at the hour of noon on the 21st of
March, and that the sun is Crossing the line"
After theaapse of common year of 1865
days. or at noon on the 2lst Of March the
next year, the earth will have reached the
same point in her orbit, by about hours, and
the sun will not cross the line till 6 O'Clock,
Z. NJ, on the 21st, and at the end of the next
year, also common year Of 800 days, it will
be midnight when the sun is crossing the
line, and after another year it will be 6 A. NJ.
of the 22d of March, but the next year will be |
a leap year, one day is added after the end of
February, and the earth, now behind about
"a hours, uses this day to catch up, and the
sun "Crosses the line" again about noon
March 2lst
But it will be seen that in adding whole day
every four years, or allowing hours each |
year, we allow II to 12 minutes too much |
To correct this error the leap year will be |
omitted in 1900 The year 1000 will be
leap year. but 2100 will not be leap year,
nor 2200 nor 1900 but 2400 will be leap
year. The rule is when the first two figures |
expressing the completed century are divisi |
BILE by 4, the leap year is had, but when not
dvisible, it is omitted and this rule observed, |
will give but one day error in about 5000 |
years.
The vernal equinox calls for about the 21st
of March; the autumnal the 22d Of Septem-
Der, with this sccumulating variation of
about Is hours at most between leap years.
But rs we are now approaching the end of
the century the vernal equinox is falling
back to about the Goth of March. We have |
not space here to touch upon another cause
of very slight variation, the procession of the
equinoXes. G. &C.
Oakland School, FdllStOn.