The Earth and the Sun
The Seasons - Solstices and Equinoxes
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| Path of the Rising Sun (NH): March to May |
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| Summer Solstice (NH): Sun's Path June to August |
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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 |
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| Winter Solstice (NH): Sun's Path December to February |
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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 |
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| 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.
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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 |
Basics of Celestial Motion. Copyright 2006 S. E. Scruggs
