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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鈥檚 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

The mass of Saturn is approximately 100 times that of Earth, and the semimajor axis of Saturn's orbit is approximately \(10 \mathrm{AU}\). To this approximation, how does the gravitational force that the Sun exerts on Saturn compare to the gravitational force that the Sun exerts on the Earth? How do the accelerations of Saturn and the Earth compare?

It is quite probable that within a few weeks of your reading this chapter one of the planets will be near opposition or greatest eastern elongation, making it readily visible in the evening sky. Select a planet that is at or near such a configuration by searching the World Wide Web or by consulting a reference book, such as the current issue of the Astronomical Almanac or the pamphlet entitled Astronomical Phenomena (both published by the U.S. government). At that configuration, would you expect the planet to be moving rapidly or slowly from night to night against the background stars? Verify your expectations by observing the planet once a week for a month, recording your observations on a star chart.

Which planets can never be seen at opposition? Which planets can never be seen at inferior conjunction? Explain your answers.

How did Copernicus determine that the orbits of Mercury and Venus must be smaller than the Earth's orbit? How did he determine that the orbits of Mars, Jupiter, and Saturn must be larger than the Earth's orbit?

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.
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