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A day on earth is decievingly simple concept
Today we'll examine
that day on earth, in detail
perhaps uncovering a few surprises
A day is the length of time takes the earth to spin three hundred sixty degrees
on its axis
OR is it three hundred sixty one ???
Here is a simple model showing the earth,
the sun and some background stars
the earth travels around the sun in ellipse
with the sun on one focus
the model of course,
is not to scale
Let's begin our examination at noon one day
the sun is directly above the red line
that will be our reference point
watching from a high vantage point
we see the earth complete
three hundred sixty degrees of rotation
but during that time
the earth has also moved a bit in its orbit
so even after a three hundred sixty dergree turn
the sun is not directly above the same point on earth
that it was at the beginning of the spin IT IS NOT NOON of the next day
the red reference line needs to spin a little more than three hundred and sixty degrees
to get us to noon
The three hundred and sixty degree rotation
is called a sidereal day
while the noon to noon rotation is called a solar day
Earth orbits the sun
once for about every three hundred and sixty six point twenty six sidereal days
and once for every three hundred and sixty five point twenty six solar days
Not only that...
the length of a solar day varies throughout the year
and for two different reasons
first, because of its orbit is an ellipse and not a circle
the earth moves faster when it is near the sun and slower when it is further from the sun
so the little extra amount of rotation
that the earth needs to do
to get from noon to noon
changes throughout the year
second, because the earth is tilted on its axis
the little extra rotation to get from noon to noon
is largest at the solstices
and smallest at the equinoxes
so solar days grow progressively longer as we move from equinox
March and September
to solstice
June and December
and did we mentioned the earth's spin is slowing down?
and the length of the solar day is increasing
due to gravitational tides between earth and the moon?
The length of the mean solar day is increasing at a rate of approximately
one point four milliseconds (1,4 msec) every century
two billion years ago there were about seven hundred and fifty days in a year
Now let's talk about day-time
that period out of twenty four hours when it is light outside, DAY...
versus NIGHT
Due to refraction and scattering of light by the atmosphere
there can be daylight even when the sun is slightly below the horizon
but day length is usually about the sun's disc being on or above the horizon
so the day begins the moment the sun's disc appears during sunrise
and ends the moment the sun's disc disappears during sunset
At the equator
daytime and nighttime are equal to within a few minutes
but at distances north and south of the equator
the length of the day varies with the season
with the longest and shortest days
being on the solstices
At the poles
once the sun has risen it stays up for six months before it sets again
and during the course of each day
it travels in a complete circle around the edge of the sky
Because the earth travels at different speeds in its orbit the sun is north of the equator
for almost four days more than half the year
and the link for the average day in the northern hemisphere
exceeds the length of the average day in the southern hemisphere by a few minutes
in the northern hemisphere the arctic circle is the southernmost latitude
where twenty four hour day-light can occur at least one day in a year
in the southern hemisphere the antarctic circle is the northernmost latitude
where twenty four hour day-light can occur at least one day in a year
and daylight savings time
is like the old man who cut off one end of the blanket
and saw it on the other end
to make it longer