Summary for Cosmology September 15, 1995
This Friday was for the most part an explanation of the growth in humans' understanding of the Universe's size and structure. Professor Mobius began the discussion by defining cosmology as our description of the Universe as we see it. Cosmology is concerned with the observable Universe, models of the Universe, the Universe's development and fate. Science defines observable as what we see with our eyes and can measure with equipment. Instrumentalists use this argument to define that what is real can be measured. As an interesting note, many of Galileo's contemporaries were skeptical as to whether Galileo was seeing the -real thing' through his telescope or not, they thought that the telescope made things up, and thus his observations were incorrect. However, this thought has not survived.
The observable Universe has expanded tremendously since the ancient Greeks. One's observations are based on the model that one uses, and vice versa. Changing a model, due to a paradigm shift, produced huge expansions in the view of the Universe. The Greek view of the universe was a flat Earth, an underworld below the Earth, and a dome-like sky above the Earth. Beyond the sky was the perfect heaven where the gods lived. This model makes sense in the experience of daily living, as the Earth seems to be flat and the sky looks like a dome overhead. Because of this it would take solid proof to move away from the flat Earth idea.
Several Greek philosophers, based on observations which they heard about began the paradigm shift from flat Earth to spherical Earth. Aristotle noted that one sees the mast of a ship coming towards land first and then as it gets closer one sees more of the ship from top down. He also noted that the stars were at different heights in northern Europe than in Egypt. Eratosthenes measured the diameter of the Earth knowing that at the zenith of the summer solstice the sun shone down to the bottom of a well, and at the same time in Alexandria he could measure a shadow. Using some geometry principles he could use the data to calculate the diameter. Aristarch noted that during the lunar eclipse a circular shadow covers the moon. He knew that the moon and sun were on opposite sides of the Earth, thus the Earth is round. From this, and knowing the diameter of the Earth, Hipparchos determined the moon's distance from Earth by timing how long it took the moon to pass through the Earth's shadow. At half moon he measured the angle that the sun made with the moon, and could then calculate the distance between the sun and the Earth.
These observations led to Polemy's model of an Earth centered Universe with heavenly bodies moving in perfect circles about the Earth, as our experience suggests that the Earth is stationary and the stars, etc. are in motion. Ptolemy had to explain the irregular motion of the planets (Greek for wanderers). He did this by suggesting that the planets had smaller epicycles on their main orbits in which they traveled backwards. During the Renaissance this model got very messy as new observations did not fit the model, and more epicycles were needed.
The next paradigm shift towards a heliocentric Universe began with Copernicus. Copernicus supported his view with the belief that the sun was more important than the Earth and God would have put it in the center of the Universe. Galileo furthered Copernicus' model with observations of Venus' phases and the moons of Jupiter. This sift was the most scary for people up to this point as it meant that the Earth was not the center of the Universe, the center of importance, but rather it was a tiny speck. Little did they realize how tiny the Earth was to become.
Copernicus' model had an extra gift with it, one could now measure the relative distance between the sun and planets. To visualize the size of the outer space, consider a ball with a 14 cm diameter, this is the sun. The Earth is 15 m from the sun and is the about the same size as the head of a pin. Jupiter is the size of a small marble. The nearest star is in San Francisco.
Tycho Brahe found flaws in Copernicus' model, as Copernicus left the orbits as perfect circles and had to use epicycles to make his model work. Kepler, using Tycho's observations of Mars showed that the orbits were ellipses. Tycho also observed that the stars showed no parallax, a difference in position of the object as the observer moves, which should be accounted for in the Earth's orbit about the sun. However, Tycho did not have the aid of a telescope, and one cannot measure the size of a 30 m room in Durham from San Francisco with the unaided eye. In 1883 Bessel measured the parallax of Alpha Centauri, the nearest star, to be .7' of an arcsec. Tycho had no idea of how far the stars were from the Earth. Bessel's work led to a new unit, the Parsec. One Parsec is the distance that 1 AU (the distance between the sun and Earth) is equal to one arcsec, or 3.26 light years. Although parallax is an accurate measure of a star's distance from us, it has severe limitations, as we can only measure a parallax of .02 arcsec. The European Space Agency has a satellite, Hipparchos, to improve science's ability to calculate the parallax of the stars.
However, there is another method for measuring the distances of the stars by assuming that stars are standard candles. If we know a star's luminosity and intensity we can find its distance by f = L / (4/r2), where r is the distance, f the intensity and L the luminosity. This leads to the question of how does one find the luminosity. Objects at certain temperatures give off light, e.g. an electric heating element. One organizes the temperatures from hot to cold with their respectiv colors by -Oh Be A Fine Girl/Guy Kiss Me (Right Now, Smack).' As it turns out, the luminosity of the stars is just a functin of temperature, which can be determined by the color of the star. Once one knows the color one can determine the star's distance. This method has allowed science to calculate distances on the order of several 10's of kPc.
Due to time constraints this where we ended. I found this introduction very
helpful in grasping the vastness of the Universe and how truly small the Earth really is
how infinitesimally (forgive the misused vocabulary) small we humans are. The fact
that the nearest star (using the above mentioned model) is in San Francisco really
drove home the point of how isolated and alone the Earth is, and yet that is a very
small distance, comparatively.
Dan Hussey, September 26, 1995