Star gazing has been a part of nearly every culture since the earliest known civilizations. These early men saw almost the same configurations that can be seen in the sky without the powerful telescopes. The ancient star gazers thought that they recognized certain patterns and configurations of stars. They used these patterns and configurations to generate myths and associate them with mystical events. They named these configurations of the stars constellations.
Different civilizations had different interests in astronomy. In the earliest agricultural societies tracking the celestial movements had implications for generating an accurate calendar to keep track of the seasons. The sailors and nomadic tribes used stars to help them navigate large bodies of water and vast areas of desert regions. Knowledge of the skies and its content was used in religion by the early priests to make predications and therefore gave them some control over their congregations.
The Greeks were the first to develop a mechanical model to describe the structure and the operations of the universe. In the fourth century B.C., Plato contributed two attributes to the celestial bodies. He stated that they are spheres attached to spherical shells and that their motions are circular.
Aristotle's view of the universe was that it consists of a set of nested, transparent, crystalline spheres centered on the Earth. There spheres rotates at different speeds and on different axes, and carried the Sun, Moon, stars, and the planets around the Earth producing the spectacular sights seen in the sky. His model consisted of a total of 55 crystal spheres. This model was improved and refined by other astronomers. This model was a complex system. It provided a reasonable approximation of the celestial motions. Its thesis was that the earth was fixed at the center of the universe, and all bodies circled the Earth. The Greeks constructed a model to stimulate what they saw in the sky and used mathematics to explain it. From observational data they concluded that the Earth was round and not flat, and they were able to give a nearly accurate estimate of the size of the Earth. The observation was made mathematically by Eratosthenes when he observed the shadow cast by the sun at the same time in two different cities. This generated the curved Earth model.
Contribution of the Arabs
The geography of the region in which the Arabs lived was suitable for observing the stars. The clear desert sky provided them the opportunity to watch the movement of the stars. After the decline of the civilization of the Greeks the Arabs under Islam had become very enlightened. They preserved and translated the writings of Aristotle, Ptolemy and other Greek philosophers. These works were then stored in their libraries.
In the 9th century the Persian astronomer al Farghani wrote about the motions of the celestial bodies. In the 10th century an observatory was built in Iran, by the astronomer al- Khujandi. Using his data from his observations of the Sun he was able to calculate the obliquity of the ecliptic i.e., the tilt of the Earth's axis relative to the sun. Omar Khayyam revised the existing calendar, and then developed a calendar that was almost as accurate as the one used presently. He used his data to calculate the number of days in a year. He produced a year that had 365.242198558156 days.
The Copernican Revolution
In 1543 the published works of Copernicus "
On the Revolution of the Celestial Spheres
" challenged the concept that the Earth was at the center of the universe. He proposed a new model for the arrangement of the universe. In this system it was proposed that the Sun was at the center of the universe, and that the Earth was a planet and was at the center of the Moon's orbit. He produced a model of this concept. This model assumed circular orbits of each planet as they travel around the sun.
Copernicus realized that Mercury and Venus are always observed near the Sun, he therefore suggested that their orbits must be smaller than the Earth's. These planets were called the
planets. However Mars, Jupiter and Saturn are seen opposite the Sun, he concluded that these planets must have orbits larger than the Earth's and these planets were called
Copernicus used the heliocentric model to determine the time that the planets take to complete one orbit. This is called the period. He was able to distinguish between two different periods of each planet. The
is the time that elapses between two successive identical sights seen from the Earth. The
is the true orbital period of a planet, that is the time takes the planet to complete one full orbit of the sun.
is found from observations of the skies where as
can only be found by calculations. A combination of triangles and geometry and the observation from the
Copernicus generated the following formula 360/S = 360/Pe-360/Pm, where
= the planet's
= the Earth's sidereal period and
= Mars's sidereal period
. This data was generated from the data of Mars and was then to generalized to 1/S = 1/P1-1/Pe, where
= the sidereal period of the Earth = 1 year
= the sidereal period of the inferior planet
= the planet's synodic period
The sidereal periods of the superior planets were used to determine the distance from the sun.
Table 1 The distances of planets from the sun
Planet | Copernicus(AU) | Modern Value (AU)
Mercury | 0.38 | 0.38
Venus | 0.72 | 0.72
Earth | 1.00 | 1.00
Planet | Copernicus(AU) | Modern Value (AU)
Mars | 1.52 | 1.52
Jupiter | 5.22 | 5.30
Saturn | 9.17 | 9.54
1 AU = 1
astronomical Unit. Copernicus used the sidereal periods of the superior planets to help him determine the distance of these planets from the sun.
Brahe and Kepler
Tycho Brache (1546 - 1601).Tycho Brache's observations solidified the heliocentric theory. He attempted to measure the distance of a new star that was seen in 1572. He discovered that the distance of the star was too far to use parallax to calculate its distance. Again in 1577 he attempted to use parallax to measure the distance of the comet, but he also failed. His observations proved that the heavens were not fixed and unchanging. With the support of the Danish king who made him a new observatory he measured the position of the stars and the planets. Even though he never accepted the Copernican
model of the universe because he could not prove mathematically the stellar parallax that should have accompanied it he achieved the highest accuracy in measurement without the use of the telescope.
The work of Brahe and Kepler dispelled the concept that the orbits of the planets around the sun were circular, and established from observed data that the orbits were elliptical. From Brahe's data observational data Kepler, a keen mathematician was able to deduce from the data of the planets that their orbits were not circles but were ellipses. From the data Kepler was able to extract three generalities. These became known as Kepler's Laws.
After he established that the orbits were elliptical, he attempted to study the shape and the motion of the planets. He needed a law to describe the shape of the orbits and another to specify the speed of the planets as they moved along their orbits.
1. Kepler's First Law states that the path of each planet is an ellipse with the sun at one focus.
Once Kepler knew the shape of the planets' orbits, He described exactly how they move on the orbits. He found that has the planets travel in the elliptical orbits their speeds changes. These changes were dependent on the distance from the sun. The planets move more rapidly when they are closest to the sun. This point on the orbit is called the perihelion. The planets moves more slowly when they are farthest from the sun. This point is called the aphelion. This law he called
the law of equal areas
2. Kepler's second law states that a line from the sun to the planets sweeps out equal areas in equal times.
From Tyco's data he calculated the motion of the planets. He found that there was a relationship between the size of the planets and the time it takes to complete a revolution round the sun. He found that the larger the semimajor axis which determines the distance from the sun, the longer the sidereal period.
3. Kepler's third law states that square of the orbital period of a planet is proportional to the cube of the semi-major axis of the orbit. This law indicates that larger planets are move slower when they revolve around the sun. It quantified the relationship between orbital speed and orbital size and explained the findings of Copernicus. The third law written as an equation is given as
= planet's sidereal period, ( the time it takes the planet to orbit the sun) in years
planet's semimajor axis, in AU.
Kepler's laws simplified the models that existed. They made it easier to calculate the motions of the planets and they also produced more accurate results. The laws are significant today because they can be applied to calculate the orbits of modern space crafts and to calculate the orbits of other relationships of other stars in their orbits.
Galileo's observations: The Italians Contribution
Galileo's contribution was made from the use of the observations he made with his telescopes. He discovered that Jupiter had four moons, and that those moons continually changed their positions. Galileo's observations about the moons circulating Jupiter led him to question whether or not the sun was being circled by the planets. He communicated these observations to Kepler, who concluded that the moons obeyed his harmonic law.
Galileo was also the first to detect the irregularities of the moon's surface. These observations were contradictory to the earlier concepts that the celestial bodies were perfect spheres. His observations of dark spots on the sun led to the belief that the sun also rotated. He also observed the phases of Venus. This finding solidified Copernicus's model that the planets rotated around the sun.
Galileo also made major contribution to the field of physics
. The branch of physics known as mechanic was developed from his work with falling objects. From the observations from falling objects he formulated the law of inertia. This law was in opposition to the concepts of the early Greeks that claimed that an object at rest was in a natural state. Galileo's law of inertia stated that a body at rest is a special condition because that any object once set in motion will continue to move until a force is applied to it.
Kepler's work and the contribution of Galileo laid the foundation for the study of the solar system