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The Earth and the Sun

The Earth's Orbit -
The First Great Misconception
The Seasons -
A Popular Misconception
The Seasons - Sunrise and Sunset The Seasons - Solstices and Equinoxes The Seasons - Understood!
The Sun Tracker & Other Activities

The Seasons - A Popular Misconception
References: [Bailey, et. al., 2003, Sadler 1992]

The Four Seasons
The Four Seasons

The most “popular” misconception in astronomy may be the reason for the seasons. Many people believe that the seasons are caused by the Earth’s changing distance from the Sun. Since the Earth follows an elliptical path these people know that the Earth will be closest to the Sun at one point in its orbit (the perihelion) and farthest at another (the aphelion). Taking a cue from everyday experience, they reason that an object will be warmest when it is closest to a source of light and heat, and coldest when it is furthest from the source, and consequently the Earth must be closest to the Sun during the summer and furthest from the Sun during the winter.

However there are three problems with this explanation of the seasons:

  • The first problem is almost immediately apparent: when it is wintertime and cold in the Northern Hemisphere, it is summertime and warm in the Southern Hemisphere. For example, in December the average high temperature is 39.4 degrees Fahrenheit in Washington, DC (located at 39 degrees North latitude), while at a comparable location in the Southern Hemisphere (Bahia Blanca, Argentina at 38.75 degrees South latitude) the average high temperature in December is 84 degrees Fahrenheit. So how can two different parts of the Earth experience different seasons when they are the same distance from the Sun?

  • The second problem with the distance explanation may actually surprise those of us who live in the Northern Hemisphere: the Earth is actually at its closest distance to the Sun in early January, and furthest from the Sun in early July of each year! So, how can it be so cold in January when the Earth is closest to the Sun, and so warm in July when the Earth at its furthest point?

  • The third problem has to do with the shape of the Earth's orbit: while the Earth’s orbit around the Sun is elliptical, it is really not all that elliptical. At perihelion the Earth is 147.5 million kilometers from the Sun, and at aphelion the Earth is 152.5 million kilometers from the Sun. The difference between perihelion and aphelion is so minor at these great distances that even if the Earth’s orbit around the Sun were perfectly circular the seasons would look and feel pretty much the same as they do now.

So what is the reason for the seasons?

Northern and Southern Hemispheres

Before proceeding with the discussion of the seasons, here's a quick review of how the Northern Hemisphere (NH) and the Southern Hemisphere (SH) are designated. The world atlas image below depicts the Earth's land masses on a two-dimensional grid with the equator and prime meridian, respectively, bisecting the globe horizontally and vertically. The Northern and Southern hemispheres, as well as degrees of latitude are defined with respect to the equator at 0 degree, with the North and South Poles at the respective 90 degree latitudinal limits. Washington, DC is in the Northern Hemisphere, located at 39 degrees North latitude, while Bahia Blanca, Argentina is located somewhat symmetrically in the Southern Hemisphere at 38.75 degrees South latitude. With respect to longitude the prime meridian, running through Greenwich, England is historically designated at 0 degree longitude, with the "West" running 180 degrees to the left and the "East" running 180 degrees to the right and meeting at the International Date Line which separates the Western and Eastern Hemispheres.

The Globe Designated by Latitude and Longitude
Latitude and Longitude
Image courtesy of GraphicMaps.com

The Seasons - Summer and Winter

An explanation for the seasons and their respective differences in temperature can be found, not in the Earth’s distance from the Sun, but in the Earth’s tilted orientation as it orbits the Sun. As the Earth follows its path around the Sun it also spins about its axis of rotation, an imaginary line running through the center of the Earth from the North to South poles. This axis of rotation is not perpendicular or “straight up and down”, but is actually titled 23.5 degrees with respect to the plane of the Earth’s orbit around the Sun, known as the ecliptic plane.

The figure below illustrates how the Earth is oriented in its orbit around the Sun during the months of June and December. Notice that because of the Earth’s tilted axis of rotation the Northern hemisphere is tilted toward the Sun in June and is tilted away from the Sun in December, while the reverse is true in the Southern hemisphere.

The Earth's Tilt during December and June
Earth-Sun position in June and December
Globe images courtesy of GraphicMaps.com

The Sun's Height in the Sky

As a consequence of each hemisphere’s orientation, the maximum height the sun reaches at mid-day (also known as "high noon") is higher in the sky during the summer months, but lower in the sky during the winter. For example an observer in Washington D.C. will see the sun rise 74.6 degrees above the horizon at noon on the first day of NH summer (June 21), but only 27.7 degrees above the horizon on the first day of NH winter (December 21). In the southern hemisphere an observer in Bahia Blanca, Argentina will see the sun rise only 27.8 degrees above the horizon at noon on June 21, but a full 74.7 degrees above the horizon on December 21.

The Earth's titled rotation affects the Sun’s height in the sky to produce two familiar seasonal effects:

  • First the Sun’s height determines how directly the Sun’s light and heat fall on the Earth: the higher the Sun the more direct its rays.
  • Secondly the Sun’s height determines the length of daylight hours: the higher the Sun the longer it remains above the horizon as the Earth rotates.

With sunlight falling more directly on the Earth during the long summer days, the Earth’s atmosphere is heated more substantially producing the warm summer temperatures. During the winter months the low Sun altitude results in less direct sunlight and shorter daylight hours during which to heat the atmosphere producing colder temperatures and longer nights. It is also important to point out that, due to the cumulative effect of the Sun's heat on the Earth's atmosphere, the hottest days of the year are actually experienced a month or so after the beginning of summer (July in the NH and January in the SH), while the coldest days similarly occur a month or so after the beginning of winter (January in the NH and July in the SH).

The discussion of the seasons continues in the next section with a closer look at the Sun's path from sunrise to sunset during the four seasons.

The Earth's Orbit -
The First Great Misconception
The Seasons -
A Popular Misconception
The Seasons - Sunrise and Sunset The Seasons - Solstices and Equinoxes The Seasons - Understood!
The Sun Tracker & Other Activities


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