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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 - Solstices and Equinoxes
References: [Adams, et. al, 2003, Adams, et. al., 2002, Wikipedia: Solstice, Wikipedia: Equinox]

The Solstices

There is another interesting phenomenon in the Sun’s path to be observed in the USNO tables in the way in which the Sun's rising and setting positions change during the year. Note that on the 21st day of March and 21st day of September the Sun rises almost directly in the East with azimuth positions near 90 degrees, while in the summer the Sun rises more than 30 degrees North of East at 58 degrees azimuth, but in the winter the Sun rises about 30 degrees South of East at 119.5 degrees azimuth. The following figures illustrate how the Sun would rise as viewed from an eastern Washington DC neighborhood from the months of March through August. (Note: the Sun's height is not indicated in these illustrations.)


Path of the Rising Sun (NH): March to May
Sun's position from March to May
Summer Solstice (NH): Sun's Path June to August
Sun's position from June to August

In these pictures the Sun rises from its directly Eastward position around March 21 and gradually its rising position moves northward until June 21 when the Sun appears to “stop” and then gradually moves southward again. In the Northern Hemisphere this point in time when the Sun has reached its northern most rising position and begins to move southward is called the summer solstice where “solstice” is derived from the Latin words “sol” meaning “sun” and “sistere” meaning “stand still”. The summer solstice generally falls around June 21st and is also known as the first day of summer or the longest day of the year, since it is on this day that the Sun reaches its maximum height in the sky and therefore remains above the horizon for the longest period of time.

The figures below illustrate the path of the Sun’s rising position around September 21 when it again rises directly East, and from which point its rising position gradually moves southward until the winter solstice or the shortest day of the year. The winter solstice is also known as the first day of winter in the Northern Hemisphere, which falls around December 21st, when the Sun again begins to move northward again.

Path of the Rising Sun (NH): September to November
Sun's position from September to November
Winter Solstice (NH): Sun's Path December to February
Sun's position from December to February

The Equinoxes

Most should recognize March 21 and September 21 as falling around the first days of spring and autumn, respectively also called the spring (or vernal) and autumnal equinoxes. (Note: September 23rd is actually closer to the first day of autumn.) The equinoxes are popularly believed to be the two days of every year on which there is an equal amount of daytime and nighttime. Although the word equinox is derived from the Latin words "aequus" meaning "equal" and "nox" meaning "night", in actuality the days of equal light and darkness fall a few days after the equinoxes. In astronomical terms the equinoxes correspond to the two days in the year on which every location on the Earth sees exactly the same amount of day and night as every other location on the Earth, such that the Sun's path from rising to setting is the same everywhere.

Returning to the depiction of the Earth’s orbit around the Sun, consider the Earth as seen without its tilt but as illuminated by the Sun during each of the four seasons. The March or spring equinox corresponds to one point in the Earth's orbit where the Sun is directly over the equator with its rays hitting the Northern and Southern hemispheres evenly. In June at the summer solstice the Sun illuminates the NH more directly and never sets at the North Pole. At the September equinox the Sun has again moved directly over the equator illuminating all points on Earth equally. In December at the winter solstice the Sun illuminates the SH more directly and never rises at the North Pole. The cycle is repeated again as the Earth moves back toward the March equinox.

Seasonal Cycle of Solar Illumination
Cycle of Equinoxes and Solstices
Globe images courtesy of GraphicMaps.com

In the final section the reasons for the seasons are summarized and a number of additional online activities are suggested for investigating how the Sun's path is observed at the equator and the poles, as well as researching how the seasons were experienced by ancient cultures.

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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