Darrell Heath | Astronomy Column

On December 25th Christians
from all around the
world celebrate the birth of
Christ and for them it is one
of the holiest days of the
year. While we may honor
that date as the birth of
Jesus it is almost certainly
wrong and it is all due to
the bad calculations of a
6th century monk known
as Dionysius Exiguus. The
Bible doesn’t tell us the birth
date of Christ and Dionysius,
wanting a Christian calendar
rather than a Roman one,
took it upon himself to come
up with a system where Year
1 would be that of Jesus’
birth. Unfortunately he
made some serious goofs
with his calculations in trying
to determine the day
and year of Christ’s birth
and, long story short, we
are stuck with his mistakes.
Whatever the origins of
this particular holiday we
know that people have been
celebrating the first day of
winter, better known as the
Winter Solstice, centuries
before the emergence of
the Church. The northern
hemisphere Winter Solstice
occurs every year around
December 20th, 21st, 22nd,
or 23rd. However, if you
lived in the southern hemisphere
you’d be celebrating
the first day of summer. This
year the Winter Solstice
occurs on December 21st
at 10:48pm CST for those
of us here in the northern
The astute sky watcher
knows that the Sun’s path
across the sky is not the
same from month to month.
What’s more, there are only
two days out of the year in
which the Sun will rise exactly
due east and set exactly
due west. We call those
days the equinoxes and
there is one in the month of
March called the Spring, or
Vernal Equinox and one in
September that marks the
Fall, or Autumnal Equinox.
After the Fall Equinox the
Sun starts to rise south of
east and set south of west.
Finally, on the day of the
Winter Equinox, it reaches
its furthest southerly declination;
from here on out
the Sun starts to head north
until finally reaching its most
northerly declination on the
day of the Summer Solstice
in June.
Why do we see the Sun
behaving in such a manner?
Well it’s not the Sun, it’s
us. To be more specific it
is the 23.5-degree tilt of the
Earth upon its axis relative
to its orbital path around
the Sun and it is this tilt
that gives us our seasons.
During the summer months
the northern hemisphere
is tilted more towards the
Sun, meaning we get more
energy per square inch of
surface area and, consequently,
we also get those
hot, muggy days of summer.
Six months later the Earth is
now on the opposite side of
the Sun with the northern
hemisphere tilted away from
the Sun. With the Sun’s light
coming in at a low angle we
get less heating per square
inch of surface area and, so,
we experience more cold,
wintry-type days. Keep
in mind that things like El
Nina events and changes in
atmospheric winds can have
an effect on local weather
where we might see a season
with unusually warmer or
colder temperatures.
We also experience shorter
day length during the
winter as opposed to the
summer and on the day
of the Winter Solstice (the
shortest day of the entire
year!) we only get about 9-10
hours of daylight with 14-15
hours of darkness.
Another cool thing to
notice on the day of the
Winter Solstice: your noontime
shadow is the longest
one of the entire year! This
of course is due to the Sun
being at its lowest angle in
the sky for the entire year.
Many pre-Christian cultures
celebrated the Winter
Solstice this time of the
year with much feasting and
merry making with some of
those traditions (a tree in
the home, gift giving etc.)
making it into our modern
celebrations of Christmas.
Whatever your own reasons
for celebrating the season
I wish you and your loved
ones a most joyous and
happy one.

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