The Sun's Path and Why Planets Create Problems
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The Sun's Path and Why Planets Create
Problems
The Sun's Path
Each day, because of the Earth's rotation, the Sun moves along an imaginary path on the celestial sphere. Over the course of a year, however, it also follows an imaginary path on the celestial sphere. As you can see in the figure below, the apparent position of the Sun with respect to the background stars changes continuously as Earth orbits the Sun.
During Earth's annual journey around the Sun, we view stars from a slightly different position from day to day. Thus, the Sun appears to travel around the celestial sphere during the course of a year along a path called the ecliptic.The part of the sky through which the Sun passes is known as the zodiac,and the Sun crosses the celestial equator at the vernal and autumnal equinoxes.
When Earth has made one complete revolution in its orbit, the Sun will return to its starting point against the background stars. in other words, the Sun traces out a closed path on the celestial sphere once a year. The apparent path of the Sun through the stars on the celestial sphere over the course of the year is called the ecliptic.Since Earth's axis of rotation is tilted 23.5oto the plane of its orbit, the ecliptic is tilted 23.5owith respect to the celestial equator.
The ecliptic is important because the Sun, the Moon, and the planets are always found near it. As we will see later, this occurs because all of these objects in our solar system lie nearly in the same plane.
The Problem of Planets
There were, however, some problems with the geocentric model. Early astronomers also observed that certain points of light changed position with respect to the background of stars in the sky. They called these points of light planets,from the Greek word for "wanderer." Astronomers working before the invention of the telescope and before anyone understood the present structure of the solar system counted seven such wanderers or planets: Mercury, Venus, Mars, Jupiter, Saturn, the Moon and the Sun. This list differs from our modern list of planets in several ways:
● Earth is missing, because no one realized that the points of light wandering in the sky and the Earth on which these observers stood were in any way alike.
● The Sun and the Moon were classified as planets because they wandered on the celestial sphere, just like Mars and Jupiter and the other planets.
● Uranus and Neptune are missing because they were not discovered until the telescope made them easily visible. Uranus, which is barely visible to the naked eye, was discovered in 1781. Neptune, which can't be seen at all without a telescope, was discovered in 1846.
Planets differ from stars in a number of ways. As already mentioned, the relative positions of stars on the celestial sphere are fixed, while planets move relative to the stars. Stars can be seen anywhere on the celestial sphere; planets are always found near the ecliptic (that imaginary yearly path of the Sun on the celestial sphere). Stars appear to "twinkle," but the brighter planets do not. Even through a telescope, stars appear as points of light, while the larger and nearer planets appear as disks.
These observed differences between planets and stars, particularly the "wandering" of planets on the celestial sphere, attracted a lot of attention from early astronomers. Their attempts to explain these differences ultimately led to the development of a new model of the universe.
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