The ancient Babylonians performed methodical observations of the night sky and built great observatories where priests would study the skies, and the celestial bodies that they believed controlled life and events on Earth.
In those times, objects in the sky were believed to be affixed to transparent celestial spheres and their motions were thought to be a result of the motion of these spheres as they revolved around the Earth. Groups of bright stars were observed to form prominent patterns in the night sky called constellations, which have been historically ascribed to mythological figures.
The early astronomers recognized that constellations appeared and disappeared with the change of the seasons throughout the course of a year. In the same way, the Sun, Moon and planets were observed to move in relation to the fixed background of stars, or constellations.
The Earth travels in space as it revolves about the Sun in a planar orbit that is approximately circular. If one drew a line from the center of the Earth through the center of the Sun, that line would "draw" a large plane in the heavens as the Earth orbits the Sun. This large plane is called the ecliptic plane, as is shown in Figure 1. Figure 1 The ecliptic plane and the ecliptical coordinate system.
Figure 2 The equatorial plane, the annual seasons and the equinoxes. Figure 3 The band of the zodiac and the apparent position of the constellations with respect to the Earth-Sun system.
Based on their observations of the night sky, ancient astronomers determined that during the daytime, the Sun would appear to "enter" or pass through different constellations throughout the year. Because of their perspective from Earth, they observed that the Sun, the Moon and all the planets visible with the naked eye seemed to pass in the course of a year through a region in the sky occupied by twelve specific constellations.
Those constellations are the ones that we would intersect if we extend the ecliptic plane out into space. These twelve constellations were called the Zodiac.
Many ancient people believed that a person's behavior, emotions, and fate were heavily influenced by the time of that person's birth i. The zodiac constellations, as envisioned by ancient astronomers, were ascribed specific patterns that resemble the shapes of animals and human beings.
The constellations of the zodiac actually form an imaginary belt in the sky that extends about eight degrees above and below the ecliptic plane as is shown. As we look at the position of the zodiac constellations at any given time of the year, the Sun is between the Earth and one of these constellations, as Figure 3 shows.
Vernal and autumnal equinoxes. We are familiar with the Earth's equator because of our knowledge of geography. If we could extend the earth's equator into space so that it could be viewed against the background of stars, we would be able to see what in astronomy is called the celestial equator. Because the Earth's axis of rotation is tilted with respect to the ecliptic by The two points in the sky where these two planes cross are called the equinoxes.
We call the vernal equinox the intersection point where the Sun, in its apparent motion against the background stars along the ecliptic, crosses the celestial equator from south to north, usually occurring around March 21 st. Similarly, we call the autumnal equinox the intersection point where the Sun, in its apparent motion against the background stars along the ecliptic, crosses the celestial equator from north to south, usually occurring around September 21 st.
The first day of Spring then corresponds to the vernal equinox and the first day of Fall corresponds to the autumnal equinox. During the time of the equinoxes, we on the Earth experience twelve hours of day and twelve hours of night. The precession of the equinox. Read more: How the constellations got their names. From Earth, stars appear to move across the sky on a regular schedule. That appearance of star movement actually has more to do with the rotation of Earth than the movements of stars themselves.
Similar to how our sun looks like it's moving across the sky when it's really our planet rotating as it orbits the sun, constellations appear to be moving when it's really our viewpoint that is changing. So, what we actually see from any given spot on Earth only looks like a rotating collection of constellations. Read more: Space junk is blocking our view of the stars. Although we're looking at stars across vast expanses of three-dimensional space, the arrangement we see looks two-dimensional.
Stars that seem to be right next to each other in a constellation may in fact be hundreds of light-years apart. For example, in one well-known constellation called Orion , the nearest star to us is Bellatrix, a bit more than light-years from Earth. But the farthest star in that constellation is Alnilam, which is about 1, light-years away.
Orion isn't one of the zodiac constellations, though. Those constellations are defined mostly by their position on the ecliptic , an imaginary or projected line in the sky that marks the perceived path of the sun as well as the rough paths of the planets and the moon, which are all more or less on the same plane over the course of a year on Earth.
The path of the ecliptic passes through 13 of the 88 constellations officially recognized by the International Astronomical Union IAU since Just 12 of those make up the Western zodiac. Most of the constellations in the Northern Hemisphere's skies bear Greek and Roman names. According to Encyclopedia Britannica , that is thanks in part to the Almagest, an influential catalog of stars and constellations created by the ancient mathematician Claudius Ptolemy in the second century. However, people were mapping the sky long before these names took hold.
Sun Appears There. Total Days. Apr 19 to May May 14 to Jun Jun 20 to Jul Jul 21 to Aug 9. Aug 10 to Sep Sep 16 to Oct Oct 31 to Nov Nov 23 to Nov The Serpent Holder.
Nov 30 to Dec Dec 18 to Jan Jan 19 to Feb The Water Bringer. Feb 16 to Mar
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