Lab_2-_Competing_worldviews_2023.pdf

Names: Astrophysics Lab 2- Competing Worldviews 1 / 11 Astrophysics Lab 2 Competing Worldviews A- Motion with respect to the background stars We've already seen that when viewed from the surface of the Earth, the entire celestial sphere appears to rotate once a day around the celestial poles. By observing the sky night after night, we quickly recognize fixed and unchanging patterns of stars called constellations. However, careful observations also allow us to see a limited number of objects whose position with respect to the fixed background pattern of stars changes over time. For the first part of this lab, you will be observing these objects' motion with respect to the background stars in an accelerated fashion through the use of Stellarium . Your teacher will project a reproduction of the sky while you take write down your observations. i) Motion of the Sun Briefly describe the Sun's motion with respect to the background stars. Specifically, • Which direction is it moving in? (For example, E-W, N-S, NE-SSW, etc...) Is this motion uniform? • How long does it take for the Sun to come back to the same point with respect to the background stars?

Astrophysics Lab 2- Competing Worldviews 2 / 11 ii) Motion of the Moon Briefly describe the Moon's motion with respect to the background stars. Specifically, • Which direction is it moving in? (For example, E-W, N-S, NE-SW, etc...) Is this motion uniform? • How long does it take for the Moon to come back to the same point with respect to the background stars? iii) Motion of the inferior planets (Mercury and Venus) Briefly describe Mercury and Venus's motions with respect to the background stars. Make sure to note any particularly striking and/or intriguing characteristics. Note also what these two planets' motions have in common and what distinguishes them from each other.

Astrophysics Lab 2- Competing Worldviews 3 / 11 iv) Motion of the superior planets (Mars, Jupiter and Saturn) Briefly describe Mars, Jupiter and Saturn's motions with respect to the background stars. Make sure to note any particularly striking and/or intriguing characteristics. Note also what these three planets' motions have in common and what distinguishes them from each other.

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Astrophysics Lab 2- Competing Worldviews 4 / 11 B- The Geocentric Model You will now use the Nebraska Astronomy Applet Project (NAAP) to examine the observed motion of the planets in two different models of the solar system. Either open the NAAP app on your computer or go to https://astro.unl.edu/naap/ . Open the lab on Solar System Models and then read the material in the Basic Observations, Elongation and Early Modeling sections. i) The inferior planets Open the Ptolemaic Model Simulator (click on the name in the brackets beside the Early Modeling)and choose the Venus preset (make sure to click OK for the settings to take effect). The Geocentric Model assumed that the Earth was fixed and at the center of the Universe. The stars, Sun, Moon and planets were all assumed to revolve around the Earth. The simplest hypothesis would have all these objects move in circular orbits around the Earth. However, notice how Venus' observed motion requires its behavior to be much more complex. This seemingly complex path can however be explained with a limited number of concepts. The first of these is to say that Venus (and the other planets) are moving on a circle called an epicycle , whose center is itself moving on a circle called the deferent , initially hypothesized to be centered on the Earth. The inferior planets, Venus and Mercury, were both thought in this model to be closer to the Earth than the Sun, as shown in the animation.

Astrophysics Lab 2- Competing Worldviews 5 / 11 Play around with the animation, and observe Venus' behavior for a while before answering the following questions. • In this model, how is it that the inferior planets never stray too far from the Sun as seen from the Earth? • In the space below, show the arrangement of the Sun, Venus and Earth at inferior conjunction, greatest eastern and greatest western elongations. (Make sure to label all objects on the diagram, and include orbits, epicycles, deferents, etc... when applicable.) • Also, explain why it is impossible in this model for the inferior planets to be at superior conjunction.

Astrophysics Lab 2- Competing Worldviews 6 / 11 ii) The superior planets Now choose the Mars preset, once again remembering to press OK for the settings to take effect. In this animation, you must keep in mind that even though the Sun is shown beyond the orbit of Mars, it is actually placed inside the orbit of the superior planets in the Geocentric Model. The animation therefore is showing you the apparent position of the Sun with respect to the background stars and not its actual position according to the model (I guess this was likely done so as not to crowd the central part of the simulation with too many objects). Notice that in the case of Mars, the simplicity of the Geocentric Model starts to fade. In order to fit observations, the Earth is no longer quite at the center of the deferent. In addition, the center of the deferent is not even the point around which the epicycle's rotation is uniform. Instead, the epicycle's rotation is uniform around a new, different point called the equant . Play around with the animation, and observe Mars' behavior for a while before answering the following questions. • In this model, why is it that the superior planets do stray very far from the Sun as seen from the Earth?

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Astrophysics Lab 2- Competing Worldviews 7 / 11 • In the space below, show the arrangement of the Sun, Mars and Earth at superior conjunction, opposition, eastern and western quadratures. (Make sure to label all objects on the diagram, and include orbits, epicycles, deferents, etc... when applicable. Make sure to place the Sun where it really is placed according to the Geocentric Model, not where the animation places it.) • Also explain why the superior planets are never in inferior conjunction in this model.

Astrophysics Lab 2- Competing Worldviews 8 / 11 C- The Heliocentric Model Read the Heliocentrism and Elongations and Configurations sections. (You may omit any reference to the advanced page and the Copernican Derivations section). Answer the following question: • Looking at the animation of the Copernican (Heliocentric) Solar System on the Heliocentrism page, what relationship do you notice between the period of a planet's orbit and its distance from the Sun? i) The inferior planets Open the Planetary Configurations Simulator and choose Earth as the observer's planet and Venus as the target planet. The Heliocentric Model assumes that the Sun is fixed and at the center of the Solar System. The apparent motion of the Sun with respect to the background stars now comes from the changing perspective from the Earth as it orbits the Sun. Let's now see how the Heliocentric Model explains all the observations we made previously. Play around with the animation, and observe Venus' behavior for a while before answering the following questions. • In this model, how is it that the inferior planets never stray too far from the Sun as seen from the Earth?

Astrophysics Lab 2- Competing Worldviews 9 / 11 • In the space below, show the arrangement of the Sun, Venus and Earth at inferior and superior conjunction, greatest eastern and greatest western elongations. (Make sure to label all objects on the diagram.) • Point out a major difference between the Geocentric model and the Heliocentric model when it comes to the possible configurations for the inferior planets.

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Astrophysics Lab 2- Competing Worldviews 10 / 11 ii) The superior planets Now choose Mars as the target planet. Play around with the animation, and observe Mars' behavior for a while before answering the following questions. • In this model, why is it that the superior planets do stray very far from the Sun as seen from the Earth? • In the space below, show the arrangement of the Sun, Mars and Earth at conjunction, opposition, eastern and western quadratures. (Make sure to label all objects on the diagram.) • Also explain why the superior planets are never in inferior conjunction in this model.

Astrophysics Lab 2- Competing Worldviews 11 / 11 • Explain how the Heliocentric Model accounts for the observed retrograde motion of planets. Compare this to the manner in which the Geocentric Model accounts for this behavior. Which of the two models seems simpler?