Astronomy 101 WA1

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    Written Assignment 1 AST-101 1

    David Spencer

    Written Assignment 1

    1. Measurement and Distance

    A. Why do scientists use the metric system of measurement instead of the English system ofmeasurement? What is scientific notation and why is it useful? Include math examples for eachpart.

    Scientists use the metric system of measurement instead of the English, Imperial,

    Standard system of measurement or other traditional system because of the simplicity of

    calculation. Units in the metric system are segmented into base 10 divisions that align with the

    base 10 decimal number system commonly used in the modern world. An example of the

    additional complexity introduced by the English system is calculating the difference in height

    between two men. In the English system, the height of these men is expressed as six feet and

    one inch, and five feet and nine and a half inches. Finding this difference requires conversion

    between feet and inches, and in the end leaves a fraction customarily expressed in multiples of 2

    and 4. Instead, in the metric system, these two mens heights are 1.85 meters and 1.76 meters

    respectively. Finding the exact difference is only a matter of subtraction, equaling 0.09 meters.

    The utility of the metric system is extended by using scientific notation, which is a way of

    expressing very large numbers without writing a large number of zeros. In scientific notation,

    380000 becomes 3.8x10^5 (Seeds & Backman, 2010).

    B. Why are some distances measured in light-years and some in astronomical units? Include a

    definition of each of these distance measurements.

    Different units of measure are used for different scales of objects because of the

    substantial difference in size of different structures of the universe. For measurements of

    distance within the solar system, astronomers use the astronomical unit (AU). An AU is the

    average distance between the Earth and the Sun, which is 1.5x10^8 km (Seeds & Backman,

    2010). For measurements between the stars, galaxies, and large-scale structures of the universe,

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    astronomers use the light-year (ly) as the standard unit of measure. A light-year is approximately

    10^13 km or 63000 AU (Seeds & Backman, 2010). Using the different units makes it easier to

    think about and talk about these large scales. For instance, the average distance from Mars to

    the Sun is 227,900,000 km, which is not an easily comprehensible number; however, that

    distance is also expressed as 1.52 AU, or about one and a half times the distance between the

    Earth and the Sun. This is conceptually much easier to think about. We can also look at this

    distance in light-years, which is worse than meters at 2.4x10^-5 light-years between Mars and

    the Sun.

    C. Answer Review Questions 2 and 4, and Problems 2 and 4 on page 21a (Chapter 2) of theSeeds textbook. Include all computations for the problems.

    RQ 2. What is the difference between an asterism and a constellation? Give someexamples.

    Historically, constellations were groups of stars named to celebrate heroes, gods, and

    mythical beasts (Seeds & Backman, 2010). Ursa Major is an example of a constellation. In the

    modern era, constellations represent a sector of the sky and there are 88 constellations officially

    recognized that do not overlap and cover the sky. An asterism is a less formally defined

    grouping of stars, and many asterisms are part of classical constellations (Seeds & Backman,

    2010). An example is the Big Dipper that is part of the Ursa Major constellation.

    Do people from other cultures on Earth see the same stars, constellations, and asterisms that you

    see?

    People from other cultures on Earth in the Northern Hemisphere see the same stars that I

    see. Cultures in the Southern Hemisphere see different stars as they are facing a different part of

    the sky. And this will vary with latitude as cultures near the equator will set some but not all of

    the stars from both the southern and northern skies. In terms of classical constellations and

    asterisms, different cultures see different constellations, or sometimes the same constellations by

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    different names originating from a different mythology and heritage. By the definition of

    modern constellations, people from all cultures see the same constellations visible in their part of

    the world. This is because of the 1928 standardization of constellations.

    If two stars differ by 8.6 magnitudes, what is their intensity ratio?

    The intensity ratio of two stars with a difference of 8.6 magnitudes = 2.5128.6

    = 2755.

    By what factor is sunlight more intense than moonlight? (Hint: See Figure 2-6)

    Sunlight has an apparent magnitude of -26.74, and the mean magnitude of the full moon

    is -12.74. The absolute value of the difference in magnitude is 14. 2.51214

    = 398359. Therefore,

    sunlight is 398359 times more intense than moonlight

    2. Magnitude

    A. Discuss stellar magnitude. Include in your answer the definition of the term and the difference

    between absolute and apparent magnitudes.

    There are two primary ways in which to discuss stellar magnitude, they are in absolute

    and apparent terms. The absolute magnitude of a star is the measure of its intrinsic brightness as

    defined by measuring the apparent magnitude from a standard distance. The standard luminosity

    distance is 10 parsecs without obstruction. Apparent magnitude is the measure of the brightness

    of an object as seen from an observer on the surface of the Earth. This differs from absolute

    magnitude in that a very bright object far away may appear to have the same magnitude of a dim

    object very close to the observer on Earth. Apparent magnitude is useful for describing the

    observability of an object from the Earth while absolute magnitude is useful for comparing stars

    magnitude to one another.

    B. Relate how the magnitude scale was originally organized by Hipparchus and how today'sastronomers have modified it.

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    The original magnitude scales used by the ancient astronomers were categorical. Starting

    with Hipparchus, the ancient astronomers classified stars into orders of magnitude. They used

    six classes of magnitude with the brightest stars in the first class and dimmer stars falling into

    successive classes of brightness. With modern techniques and instrumentation, todays

    astronomers have made adjustments to the magnitude scale so that it is a high precision standard

    of brightness (p. 13). For example, in the class system the star Theta Leonis is third magnitude,

    but in the modern scale its magnitude is 3.34 (p. 13).

    3. Models of the Universe

    A. Compare the Ptolemaic and Copernican models of the universe. State the main tenet of each

    theory; how they are alike or different, what evidence each used to support the ideas, and how

    each explained the retrograde motion of the inner planets.

    Ptolemy was the greatest of the ancient astronomers, he used mathematics to try and

    predict the positions of celestial objects. Ptolemaic models used mathematics to describe the

    Aristotelian universe (p. 43). Copernicus was a pivotal figure in history of astronomy. He was

    a student of Ptolemy and believed in Aristotles philosophy, particularly that of the spherical

    elegance of the heavens (p. 47). Where the two astronomers differed was in the main tenet of

    their theories on planetary motion. The Ptolemaic model is geocentric, meaning that the Earth is

    the center of the universe and all celestial bodies rotate around it. The Copernican model is

    heliocentric, meaning that the Sun is the center of the universe and the Earth, the Planets, and all

    celestial bodies rotate around it. Ptolemy and the ancient astronomers used the lack of

    observable parallax to support their idea that the Earth did not move. But more than parallax, the

    ancient astronomers based their astronomical models, not on evidence important to modern

    science, but on reason and accepted first principles. The first principle of a geocentric universe

    was the primary basis for the ancient model of the universe. Within this geocentric construct,

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    Ptolemy used a complex system of epicycles, deferent, and the offset of the Earth and the Equant

    to describe and predict the retrograde motion of the planets. In contrast to the complexity of the

    Ptolemaic model, The Copernican model of the universe explained the retrograde movements of

    the planets very easily by describing the retrograde movements as apparent movements as an

    observer from Earth sees the planet revolve around the Sun. Copernicus used his observations of

    the movement of the planets over the years to support his ideas; however, the Copernican model

    was no better at prediction of the position of the planets than the Ptolemaic. This was because of

    Copernicus use of perfect circular orbits derived from his belief in Aristotles philosophy (p.

    47).

    B. How did Tycho Brahe's model of the universe differ from that of Ptolemy or Copernicus?Explain the points of dispute.

    Tycho Brahes model of the universe differed from Ptolemy and Copernicus in that in

    many ways it was a hybrid of the two conflicting models of the universe. Brahe rejected both of

    the previous models. He rejected the Ptolemaic model because of its inaccuracy (p. 50). He

    also rejected the Copernican model because he was not able to detect parallax of the stars. This

    lack of relative movement was evidence to Brahe that the Earth had a fixed position, and that

    other objects moved. His geocentric model was different from Ptolemy in that planets, other

    than the Earth, revolved around the Sun.

    4. Newton and Kepler

    Discuss how Newton's law of universal gravitation explained or clarified the orbital circularmotion of planets. Consider Kepler's second and third laws to help you in your explanation

    It is important to notice that Keplers three laws are empirical (p. 54). They accurately

    describe the phenomenon of the motions of the planets but do not explain them. Eight decades

    after Kepler published the three laws, Newton proved that Keplers laws are a result of Newtons

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    law of universal gravitation. For Example, Keplers third law of planetary motion can be derived

    from Newtons laws of motion (p. 65).

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    References

    Seeds, M. A., & Backman, D. E. (2010).Horizons: Exploring the Universe(11th ed.). Belmont,

    CA: Thomson Brooks/Cole.