Signs of the Zodiac
Even though the planets move on the celestial sphere, they do not wander all over it but are confined to a narrow strip, dividing it in half. Stars along that strip are traditionally divided into the 12 constellations of the zodiac. The name, related to "zoo," comes because most of these constellations are named for animals--Leo the lion, Aries the ram, Scorpio the scorpion, Cancer the crab, Pisces the fish, Capricorn the goat and Taurus the bull.
The apparent path of the Sun across the sky. In summer, the Sun's path is longest, and so are the days. In winter, the Sun's path is shortest, and so are the days.
The apparent path of the Sun across the sky.
In summer, the Sun's path is longest, and so are the days.
In winter, the Sun's path is shortest, and so are the days.
At any time, the Sun is also somewhere on the celestial sphere, and as the Earth turns, it rises and sets the same way as stars do.
Like the planets, the Sun, too, moves around the zodiac, making one complete circuit each year. Every month it covers a different constellation of the zodiac, which is the real reason why those constellations are 12 in number. Of course, during that month, this particular constellation is not seen, because the sky near the Sun is too bright for its stars to be seen (except, very briefly, during a total eclipse of the Sun).
One can however figure out where the sun is on the zodiac (as ancient astronomers have done) by noting which is the last constellation of the zodiac to rise ahead of the Sun, or the first to set after it. Obviously, the Sun is somewhere in between. In this manner each month-long period of the year was given its "sign of the zodiac."
Astrologers, who believe that stars mysteriously direct our lives, claim it makes a great difference "under what sign" a person was born. Be aware, however, that the "sign" assigned to each month in horoscopes is not the constellation where the Sun is in that month, but where it would have been in ancient times. The difference is discussed in the section on the precession of the equinoxes.
The Ecliptic
The path of the Sun across the celestial sphere is very close to that of the planets and the moon. After clocks became available, it was a relatively straightforward job for astronomers to relate the path of the Sun in the daytime to the one of stars at night, and to draw it on their star charts. Because of its relation to eclipses, that path is known as the ecliptic.
The orbit of the Earth around the Sun. This is a perspective view, the shape of the actual orbit is very close to a circle.
The orbit of the Earth around the Sun.
This is a perspective view, the shape of
the actual orbit is very close to a circle.
The significance of the ecliptic is evident if we examine the Earth's orbit around the Sun. That orbit lies in a plane, flat like a tabletop, called the plane of the ecliptic (or sometimes just "the ecliptic"). In one year, as the Earth completes a full circuit around the Sun (drawing above), the Earth-Sun line and its continuation past Earth sweep the entire plane. The far end of that line then traces the ecliptic on the celestial sphere; if you have a star chart handy (it is often included in an atlas), you will find the ecliptic traced there, too.
The Planets and the Moon
Planets seen in the sky are always near the ecliptic, which means that their orbits are never too far from the plane of the ecliptic. In other words, the solar system is rather flat, with all its major parts moving in nearly the same plane.
What about the connection between "ecliptic" and eclipses?
The moon's orbit cuts the ecliptic at a shallow angle, around 5 degrees, which means that on the celestial sphere the Moon, too, follows a path through the zodiac. Half the time the Moon is north of the ecliptic, half the time south of it. If the shadow of the moon hits the Earth, the Sun is eclipsed in the shadow area; if on the other hand the shadow of the Earth covers the moon, the moon goes dark and we have an eclipse of the moon.
Either of these can only happen when the Sun, Earth and Moon are on the same straight line. Since the Sun and Earth are in the plane of the ecliptic, the line is automatically in that plane too; if the moon is also on the same line, it must be in the plane of the ecliptic as well.
It takes close to a month for the Moon to go around the Earth ("month" comes from "Moon") and during that time its orbit crosses the ecliptic twice, as it goes from one side to the other. At the time of crossing, the Sun may be anywhere along the ecliptic; usually it is not on the Earth-Moon line, and therefore an eclipse usually does not take place. Occasionally, however, it is on that line or close to it. If it then happens to occupy exactly the same spot on the celestial sphere, we get an eclipse of the Sun, because the moon is then between us and the Sun. On the other hand, if it occupies the spot exactly opposite from that of the Moon, the Earth's shadow falls on the Moon and we have an eclipse of the Moon.
Preview Image
- From NASA's Remote Sensing Tutorial by Dr. Nicholas Short, "Twelve Zodaical Signs" – The ancients picked certain constellations to relate to the months they had named. These form the "Zodaic" - a word that has 'Zoos' as a root, to denote the animals perceived in most of the constellations chosen. For example, Cancer is actually a Latin word for 'crab'. The image depicts the 12 zodaical signs. As the seasons progress, the Sun has different locations within the constellations on each seaons' starting date at the time of its daily zenith when crossing the ecliptic, as follows: Vernal Equinox (Spring) = Aries; Summer Solstice = Cancer; Autumnal Equinox = Libra; Winter Solstice = Capricorn. (Direct link to Image.)
Disclaimer: This article is taken wholly from, or contains information that was originally published by, David P. Stern - "Educational Web Sites on Astronomy, Physics, Spaceflight and the Earth's Magnetism." Topic editors and authors for the Encyclopedia of the Cosmos may have edited its content or added new information. The use of information from David P. Stern should not be construed as support for, or endorsement by, that David P. Stern for any new information added by EoC personnel, or for any editing of the original content. The EoC has a specific working relationship with David P. Stern, and any changes to any of his content is to be done only with his approval or the approval of those appointed by him to represent his interests in this content.
Citation
Stern, David P. Ds.C. (Contributing Author); Bernard Haisch (Topic Editor). 2008. "Ecliptic." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published March 4, 2008].
<http://www.cosmosportal.org/articles/view/135543/>
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