.NC STATE UNIVERSITY.

Department of Physics

PY124: Solar System Astronomy

The Seasons

For us in the temperate zone of the northern hemisphere the passing of the seasons is an obvious and stately measure of the passage of time.  Winter becomes spring, which becomes summer, which becomes fall, which becomes winter and the circle repeats.  We all know that summer is warmer than winter, but many people have a complete misunderstanding of WHY that is true.

 

The most common answer, if you ask a group of people why summer is warmer than winter, is that the Earth is closer to the sun in the summer.  But pause a moment and consider those in Australia.   Their summer is our winter and their winter is our summer.  But their summer is warmer than their winter also, just like us.   But the Earth cannot be closest to the sun in our summer AND their summer, because our summer is their winter.  So clearly it is not a simple question of how close the Earth is to the sun.

 

In fact the Earth is actually at the point in its orbit farthest from the sun (called aphelion) about July 3, and closest to the sun (called perihelion) about January 4. 

The seasons are caused by the tilt of the Earth’s axis in relation to the plane of its orbit.  As shown in the figure below the plane of the Earth’s orbit (the ecliptic) is tilted in respect to the plane of the Earth’s equator.   This tilt is 23.5o. (Or we could say that the Earth’s axis is tilted 66.5o, it’s the same thing.)

ecliptic

Thus the amount of sunlight striking any particular point on the Earth varies over the course of the year.   The Earth keeps the same tilt throughout the year, but as shown below, in the summer (northern hemisphere) the northern hemisphere is tilted toward the sun, and in the winter it is tilted away from the sun.

 

seasonalsunlight

The two beams of sunlight shown hitting the northern hemisphere are the same width, but in the summer that sunlight is concentrated into a smaller area on the Earth than it is in the winter.  Since it is the same amount of sunlight, but it is spread out over a greater area, the amount of sunlight per square meter is less in the winter.  Thus winter is colder than summer because in the winter we don’t get as much sunlight as we do in the summer.

The Four Seasons

We divide our year into four seasons, each season being defined by the position of the sun during that time period.

 

At any moment there is one spot on the Earth where the sun is directly overhead.  This spot, called the subsolar point, moves around the Earth in one solar day, 24 hours, thus the subsolar point moves 15o of longitude for each hour of the day  (360o / 24h).  Where this subsolar point is located defines our seasons.

The points we use to define our seasons are when the subsolar point is on the equator, and is moving north, called the vernal equinox, when the subsolar point is as far north as it gets, called the summer solstice, when the subsolar point is on the equator moving south, called the autumnal equinox, and when the subsolar point is as far south as it gets, called the winter solstice.

The sun reaches these points on approximately these dates:

Vernal equinox – March 21 (the first day of spring)

Summer solstice – June 21 (the first day of summer, longest daylight of the year (northern hemisphere).)

Autumnal equinox – September 22 (the first day of autumn)

Winter solstice – December 21 (the first day of winter, shortest daylight of the year (northern hemisphere).)

 

© Copyright 2008, Robert A. Egler. All Rights Reserved.