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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 - Sunrise and Sunset
References: [Adams, et. al, 2003, Adams, et. al., 2002]

In the previous section the tilt of the Earth on its axis of rotation was shown to affect the height of the Sun in the sky which in turn determines how directly the Sunís rays heat the Earth and how long the Sun remains above the horizon. In the summer months the Sun reaches its maximum height producing the warmest temperatures and the longest days, while in the wintertime the Sun rises to its minimum height all year leading to colder temperatures and shorter days. Knowing this one should expect the Sunís position to transition between its maximum and minimum heights as the seasons change from summer to autumn to winter and then again from winter to spring to summer. These transition days are recognized as ďthe first day of summerĒ, ďthe first day of winterĒ, etc., but what really determines the changing of the seasons, and how are they defined by the Sunís position in the sky?

Answering these questions requires a closer look at the path that the Sun follows in the sky over the course of a day and through the four seasons of the year.

The Sun's Path in the Sky

The figure below shows the Earth titled on its axis of rotation, this time with an arrow indicating that the Earth rotates counterclockwise as viewed from above the North Pole. This counterclockwise rotation allows Tokyo (Japan is known as ďThe Land of the Rising SunĒ) to see the sun rise before London, London before Washington DC, and Washington DC before Honolulu, with the positions of the cities and their respective longitudes indicated on the globes. Hence the counterclockwise rotation of the Earth explains why the Sun appears to rise in the East and set in the West.

Counterclockwise Rotation Means Sun Rises in the East
Images of globe rotated counterclockwise
Globe images courtesy of GraphicMaps.com

But what path does the Sun follow through the course of the day as it moves from East to West? How is this path affected by the seasons, and why does the Sun not set at the North Pole in the summer, or rise there in the winter?

U.S. Naval Observatory (USNO) Sun Path Website

To answer questions about the Sunís position during the day we consult an astronomy website maintained by the U.S. Naval Observatory (USNO). The USNO is the preeminent source for celestial position information in the United States, and its Astronomical Applications Department maintains a website tool called the Sun or Moon Altitude/Azimuth Table that tabulates the Sunís position as viewed from any latitude and longitude on Earth. The Sunís altitude or height in the sky is measured in degrees above the horizon. (Recall that this information was used the previous discussion to compare the height of the Sun in Washington DC and Bahia Blanca, Argentina.) The Sunís azimuth is measured in degrees along the horizon with 0 degrees corresponding to North, 90 degrees to East, 180 degrees to South and 270 degrees to West as shown in the figure below.

Azimuthal Direction as Measured Along the Horizon
Horizonal Directions

The tables below indicate the Sun's height and direction for Washington DC during four different times of the year as obtained from the USNO website. Pay particular attention to the direction the Sun's path follows during the course of the day, as well as the length of time between sunrise and sunset on each day.

Sun Altitude and Azimuth from USNO Website
Sun's Path on March 21
Sun's Path on June 21
Sun's Path on September 21
Sun's Path on December 21

These data indicate that in the Northern Hemisphere that the Sun rises in the East and then follows a southernly path in the sky until it sets in the West. The Sun's path in the sky is illustrated below for the four days tabulated. Notice that that as the Sun's height increases or decreases so does its "span in azimuth" across the horizon. This increased "span" along the horizon translates into longer times above the horizon and therefore longer hours of daylight. A decreased span indicates shorter amounts of time above the horizon and therefore fewer hours of daylight. Notice also that the Sun rises and sets exactly in the East and West, respectively, on the March and September dates, with almost exactly 12 hours of daylight.

Northern Hemisphere: Sun's Path in Washington, DC
Sun's Path on December 21

The Sun's Path in the Southern Hemisphere

The figure below shows similar Sun path information for the Southern Hemisphere city of Bahia Blanca, Argentina for the same four dates. In addition to the reversal of paths for June and December notice now that the Sun moves through the sky from East to West in a northernly direction! Pay particular attention to the way in which the direction along the horizon is delineated: North now sits at 0 degrees which is equivalent to 360 degrees, Northeast to Southeast run from 0 degrees to 135 degrees, and Northwest to Southwest run from 360 degrees to 225 degrees.

Southern Hemisphere: Sun's Path in Bahia Blanca, Argentina
Sun's Path on December 21

In the next section the discussion continues with a closer examination of the Sun's path during the seasonal "markers" known as the solstices and the equinoxes.

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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