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Perihelion, Aphelion, and Seasons
Name: Paul
Status: other
Grade: 9-12
Location: N/A
Country: N/A
Date: 4/1/2005
Question:
Hi, We could use your help on this one. Somewhere I heard
that the Sun is closer to the Earth in the Winter and farther in the
Summer! If this is true, Why is it cold in the Winter?
Replies:
Thanks for your question, Paul. It is true that Earth is closer to the sun
than average during Northern Hemisphere winter and farther from the sun
than average during Northern Hemisphere summer. The specific proximal
(close) and distal (far) points along Earth's orbit around the sun are
called "perihelion" and "aphelion," respectively, and will occur roughly
on January 4 and July 4 of this year (these dates progressively change
over time as a consequence of the planet's orbital path constantly
changing shape.) The distances to the sun at perihelion and aphelion are
approximately 147.5 million kilometers and 152.6 million kilometers,
respectively. This change in distance can be explained by the ellipsoidal
(non-circular or oval) shape of Earth's orbital path around the sun.
Instead of the sun occupying a singular, unique center of that path, it
occupies one of two points called "foci" which can be found slightly and
equally offset to either side of what would be considered the true center
of Earth's orbital path. Imagine a horizontally oriented oval-shaped face
with two eyes at the center and one of those eyes winking at you, and you
will have a
reasonably accurate depiction of what the Earth-Sun orbital system looks
like when viewed from above or below the orbital plane.
Although it may seem that Earth's relative distance to the sun would be a
prominent factor in determining seasonal changes in temperature, it pales
in comparison to the prominence of the planet's axial tilt with respect to
controlling temperature, or more accurately, distribution of solar energy.
It is the axial tilt (presently 23.45 degrees), in fact, which determines
seasonality. During Northern Hemisphere summers, the north pole is
oriented in such a way as to be pointing in the general direction of (not
directly at) the sun; during Northern Hemisphere winters, the north pole
is pointing away from the sun. I should clarify this by saying that the
north pole points in essentially the same direction throughout the year
(i.e. Polaris, the North Star), regardless of where it is in its orbital path.
Incidentally, it is for just this reason that the polar regions are
immersed in full to partial darkness during certain parts of the year
(i.e. the north pole during northern winters.) When a region of the
surface of Earth is more straightly facing the sun (like northern
latitudes do during northern summers), the amount of sunlight falling on
that surface is greater than when that surface is more obliquely facing
the sun (like northern latitudes do during northern winters.) This
seasonal change in the amount of incident solar radiation per unit area on
the surface of Earth is ultimately what determines the seasonal changes in
temperature. Any introductory
astronomy text will have illustrations that depict what I have described
here, if the mental visualization is difficult (which it certainly can
be.) I hope this explanation helps.
Scott J. Badham
Department of Geology and Geophysics
University of Wyoming
Remember that it is only called "winter months" (December-March) - in the
Northern Hemisphere. In the Southern Hemisphere, these months are actually
"summer months". This particular datum should suggest to you that it is
not the relative closeness to the Sun that controls winter/summer, but
rather the tilt of the Earth's axis away or towards the Sun and the
relative exposures to the Sun of each hemisphere.
The other way to think about this (although requiring a bit more Math) is
to try to calculate how much more energy does the Earth get during
perihelion and aphelion. You could, for example try to factor in how many
more degrees of sunlight would be captured by the Earth as it gets closer
(assuming that there is not much loss in sunlight energy over the
different distances).
Greg (Roberto Gregorius)
Paul,
What you say is true. The earth is closer to the sun in the northern winter.
The seasons are caused more by the tilt of the earth's axis with respect
to the sun. In summer, the northern hemisphere is tilted toward the
sun. It gets more direct solar radiation and the days are longer.
Bob Hartwell
The Earth's orbit about the Sun is almost circular -- but not quite. It is
a bit closer to the Sun in winter (in the northern hemisphere), but that
is not the major cause of seasons. The major seasonal changes are caused
by the tipping of the Earth's axis of rotation that alternately exposes
the northern and southern hemispheres toward the Sun. The following site
provides details:
http://aa.usno.navy.mil/faq/docs/seasons_orbit.html
Vince Calder
The reason for it getting cold in the northern winter is to do
with the Earth's tilt on its axis of rotation (23½°) when the North
Pole is tilted away from the Sun; and likewise it is cold in the
Southern Hemisphere 6 months later, when the South Pole is tilted away
from the Sun.
The coldness of winter (or the warmth of summer) is governed by
the amount of time the Sun is above the horizon. This helps seasonal
winds etc. to dominate in one season rather than the other.
The difference of 5 000 000 km (3 000 000 miles) between the
closest point (perihelion) and furthest point (aphelion) from the Sun
is so small (approximately 3%) that it has VERY LITTLE bearing on the
matter, whereas, the amount of time (mentioned above) has a great
bearing.
Howard Barnes.
Astronomer.
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