Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne 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.



Click here to return to the Astronomy Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory