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Chapter 4: Problem 3

(a) In what direction does a planet move relative to the horizon over the course of one night? (b) The answer to (a) is the same whether the planet is in direct motion or retrograde motion. What does this tell you about the speed at which planets move on the celestial sphere?

Short Answer

Expert verified
Over the course of a night, planets move relative to the horizon from the east to west, which is due to the rotation of the Earth. The same direction applies whether the planet is in direct motion or retrograde motion, indicating that the speed at which planets move on the celestial sphere is slower compared to the rotational speed of Earth.

Step by step solution

01

Determine the Direction of a Planet’s Motion

As the Earth rotates on its axis, stars and other celestial bodies, including planets, seem to move from east to west. This is because observers on Earth are turning from west to east due to the Earth's rotation. Therefore, over a single night, a planet seems to rise in the east and set in the west, moving relative to the horizon from east to west.
02

Unveiling the Mystery of Retrograde Motion

Sometimes, planets appear to move from west to east relative to the stars in the sky. This is called 'direct motion'. But at other times, they appear to reverse direction, moving from east to west. This 'retrograde motion' is an optical illusion due to the motion of Earth and these planets around the Sun. It does not mean that the planet has suddenly started moving backwards.
03

Understand Relative Speed of Planets

Regardless of whether the planet is in direct motion or retrograde motion, they still rise in the east and set in the west over the course of one night because this is dictated by the Earth's rotation. However, the speed at which planets traverse is significantly slower than the stars due to its own motion around the Sun, hence sometimes, depending on the relative positions of Earth and the planet around the Sun, the planet may appear to move in the opposite direction (retrograde).

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Most popular questions from this chapter

Use the Starry Night Enthusiast \({ }^{\mathrm{TM}}\) program to observe the moons of Jupiter. Display the entire celestial sphere (select Guides \(>\) Atlas in the Favourites menu) and center on the planet Jupiter (double-click the entry for Jupiter in the Find pane). Then use the zoom controls at the right- hand end of the toolbar (at the top of the main window) to adjust your view to about \(20^{\prime} \times\) \(14^{\prime}\) so that you can see the planet and its four Galilean satellites. (Click on the \(+\) button or press the \(+\) key on the keyboard to zoom in and click on the - button or press the key on the keyboard to zoom out.) This is equivalent to increasing the magnification of a telescope while looking at Jupiter. (a) Click on the Time Flow Rate control (immedi-ately to the right of the date and time display) and set the discrete time step to 2 hours. Using the Step Forward button (just to the right of the Forward button), observe and draw the positions of the moons relative to Jupiter at 2-hour intervals. (b) From your drawings, can you tell which moon orbits closest to Jupiter and which orbits farthest away? Explain your reasoning. (c) Are there times when only three of the satellites are visible? What happens to the fourth moon at those times?

What is an epicycle? How is it important in Ptolemy's explanation of the retrograde motions of the planets?

What are the foci of an ellipse? If the Sun is at one focus of a planet's orbit, what is at the other focus?

Use the Starry Night Enthusiast \({ }^{\mathrm{TM}}\) program to observe the changing appearance of Mercury. Display the entire celestial sphere (select Guides > Atlas in the Favourites menu) and center on Mercury (double-click the entry for Mercury in the Find pane); then use the zoom controls at the right-hand end of the toolbar (at the top of the main window) to adjust your view so that you can clearly see details on the planet's surface. (Click on the + button to zoom in and on the - button to zoom out.) (a) Click on the Time Flow Rate control (immediately to the right of the date and time display) and set the discrete time step to 1 day. Using the Step Forward button, observe and record the changes in Mercury's phase and apparent size from one day to the next. Run time forward for some time to see these changes more graphically. (b) Explain why the phase and apparent size change in the way that you observe.

In what direction does a planet move relative to the stars when it is in direct motion? When it is in retrograde motion? How do these compare with the direction in which we see the Sun move relative to the stars?
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