THE SOUTHWORTH PLANETARIUM
207-780-4249       www.usm.maine.edu/planet
70 Falmouth Street  Portland, Maine 04103
43.6667° N,                    70.2667° W
Founded January 1970
               "This sentence no verb."



THE DAILY ASTRONOMER
Wednesday, February 3, 2016
The Astronomy of Winter


Yesterday, we marked an important annual milestone here in the far
flung hinterlands.   We passed the "coldest day" and now are
progressing glacier-like toward the warmer season.  Perhaps we should
explain.      Every day has its own high and low temperatures. These
temperatures can vary widely from year to year.    For instance, Jan
4, 2017 could be bitterly cold, while Jan 4, 2018 could be mild and
pleasant.     However, when the temperatures are averaged over many
decades, each day will have its own average high and low temperatures.
A graph of these temperatures over a year describes a beautifully
smooth undulating curve which reaches its maximum around August 2 and
a minimum around February 2.    Both of these dates are about six
weeks after the summer and winter solstices, respectively.     The
coldest day, on average, is February 2 and so now the average will
increase until August 2nd.  Granted, February can still be a frigidly
cold month, just as August can be stiflingly hot.

This momentous occasion gives us the opportunity to discuss the
astronomy of weather: why we experience cold weather now and will, in
theory, experience hot weather six months from now.   Our planet Earth
is tilted on its axis by 23.5 degrees, a value known as its
"obliquity."     As the planet revolves around the Sun, its poles are
constantly changing their orientation relative to it.    Around the
summer solstice, the North Pole is aligned toward the Sun as closely
as possible.  Six months later, during the winter solstice, the North
Pole is aligned away from the Sun.   Consequently, in the winter, the
Sun is low in our sky; in the summer, its altitude is higher.

This changing altitude explains why we have more daylight in June than
we have in December.  However, the seasonal changes are not merely the
result of the daylight duration difference.   If daylight duration
were alone responsible for the heating, the North Pole would be a
roiling furnace between March and September, when the Sun is always
visible there.   We know that even around the solstice, the North Pole
remains quite cool.      What causes our seasonal temperature change
is the atmosphere, or, more precisely, atmospheric absorption.  When
the Sun's angle above the horizon is low, its radiant energy passes
through a great amount of atmospheric gases.  When the Sun is higher,
its heat energy s less impeded.     After all, there is forty times
the amount of atmospheric gases between you and the horizon as exists
between you and the zenith (point directly overhead).   In winter, the
Sun never attains a high altitude and its heat is blocked by the
thicker atmosphere close to the horizon.  In summer, the Sun's angle
is much higher and more of it copious heat flow penetrates to the
ground which, when warmed, then imparts its heat onto the air above
it.

We know, of course, that we can have unseasonably warm periods in deep
winter and despicably cold days in mid summer.    This is because
weather is a complex, sub-chaotic mess.      All weather results from
Earth attempting to reach a state of thermal equilibrium.    Our
planet is doing precisely what a cup of hot tea does:  it moves the
heat energy from excess to deficiency so that everything will be
balanced between it and its surroundings.  The difference is that the
tea will eventually achieve thermal equilibrium once the heat source
is removed from it.   Earth is constantly receiving prodigious amounts
of thermal energy and therefore our world will not achieve a state of
thermal equilibrium until the Sun stops imparting energy onto it
billions of years from now.  Not, of course, that we're complaining.