Introduction

Chapter 14. Discovering Planets

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Author: Scott Miller, Pennsylvania State College

Editor: Grace L. Deming, University of Maryland

Uranus and Neptune
Uranus and Neptune: Two planets not observable with the unaided eye.

The goals of this module: After completing this exercise, you should be able to:

  1. Explain how astronomers distinguish between observations of planets and stars.
  2. Explain how astronomers used the basic laws of physics to calculate the existence of Neptune.

In this module you will explore:

  1. How a planet appears to move through the sky relative to the stars.
  2. How astronomers used motion to detect the existence of undiscovered planets in our solar system.
  3. How astronomers use a series of observations, and not just one, to determine the existence and orbital parameters of a planet.

Why you are doing it: Only five planets can be observed without a telescope (the first six planets, minus Earth). Objects further out in our solar system were discovered with the use of a telescope. Given the size of the entire sky and the small fraction of it observed through a telescope, it is amazing that astronomers have been able to detect as many objects as they do. The detection of the seventh and eighth planets in our solar system not only changed our view of the solar system, but it also tripled the scale of it.

Even before the invention of telescopes, ancient astronomers looked up into the sky and noted what they saw. They observed thousands of stars in the sky, and plotted their positions. Some of the names of the patterns of stars they recognized, like Orion, Cygnus, and Sagittarius, we still use today. They knew where each star belonged. Before telescopes, astronomers also knew that there existed a number of planets as well (5 to be exact). Looking up in the sky without a telescope, though, it's difficult to tell the difference between a star and a planet, so how did they know?

The above animation shows a view of the night time sky, with what appears to be a field of stars. Rapid click on the arrows at the bottom of the animation to advance the observation forwards or backwards in time. Each time you click about two weeks of time have passed. Do all of the objects appear stationary?

Question 1.

Advance the animation to the last frame. Which of the labeled objects appeared to move relative to background stars from one observation to the next?

A.
B.
C.
D.
E.

3
Try again. You may not notice the object move from one frame to the next, but flip through a number of frames and you'll see one of the objects moving relative to the others.
Correct. You may not notice it from one night to the next, but over time it becomes apparent that object B appears to move relative to the background stars. Astronomers noticed that some objects in the sky seem to "wander" relative to background stars, and called these objects planets (which is from the Greek word for wanderer). Before telescopes were invented, astronomers knew of all the planets out to Saturn. It wasn't until much later that more planets were discovered.
Incorrect. You may not notice it from one night to the next, but over time it becomes apparent that object B appears to move relative to the background stars. Astronomers noticed that some objects in the sky seem to "wander" relative to background stars, and called these objects planets (which is from the Greek word for wanderer). Before telescopes were invented, astronomers knew of all the planets out to Saturn. It wasn't until much later that more planets were discovered.
William Herschel
William Herschel

Before the invention of the telescope, ancient astronomers knew of the existence of five planets in the sky: Mercury, Venus, Mars, Jupiter and Saturn. These planets are visible to the unaided eye, and astronomers noted that they moved relative to the background stars. Up until the 1700's, these were the only planets that astronomers were aware of. In 1781, though, that all changed. On March 13, William Herschel, a musician turned mathematician and astronomer, was systematically observing the sky with a telescope when he happened to observe what appeared to be a fuzzy blue-green object. At first Herschel believed that this object was a comet, since comets appear fuzzy when viewed through a telescope. Over the course of the year, though, as Herschel continued to observe this object, he determined that its orbit was not one that would be expected of a comet. Comets move on highly elliptical orbits, which bring them close to the Sun. This object's orbit was nearly circular, and it resided beyond Saturn. In fact, Herschel had discovered the seventh planet of our solar system. Herschel wanted to name the new planet Georgium Sidus (George's Star) after England's King George III, but many in the astronomical community (especially those from other countries) refused to call the planet by this name, and eventually it came to be known as Uranus (pronounced YOOR-uh-nus).

Question 2.

On March 13, 1781, William Herschel concluded that he had discovered a new planet when he observed a fuzzy blue-green object in the night sky.

A.
B.

3
Try again.
Correct. While Herschel first observed the object on March 13, it wasn't until after many observations that Herschel was able to determine aspects of Uranus' orbit which led him to conclude that what he was observing was not a comet, but rather a planet.
Incorrect. While Herschel first observed the object on March 13, it wasn't until after many observations that Herschel was able to determine aspects of Uranus' orbit which led him to conclude that what he was observing was not a comet, but rather a planet.
Uranus Orbit

After Herschel's discovery of Uranus, astronomers realized that he was not the first to observe the planet. Since 1690, Uranus had been observed at least 20 times, but without noticing its motion relative to background stars. It appeared simply as yet another dim star in a field of stars. Based on its distance away from the Sun, Uranus appears to move through the sky only 4° per year.

Question Sequence

Question 3.

Assuming that the ecliptic is equally divided among the twelve zodiacal constellations, how many years does it take Uranus to travel through one constellation?

3
Try again.
Correct. A full circle contains 360°. If there are twelve signs of the zodiac, then each one would cover 360°/12 = 30°. If Uranus appears to move 4° in one year, then it would take 30°/(4°/year) = 7.5 years to move through one constellation.
Incorrect. A full circle contains 360°. If there are twelve signs of the zodiac, then each one would cover 360°/12 = 30°. If Uranus appears to move 4° in one year, then it would take 30°/(4°/year) = 7.5 years to move through one constellation.
The Solar System

After Herschel's discovery of Uranus in 1781, astronomers continued to track its motion. Over time, it became apparent that Uranus was not following the path it was supposed to take, according to Kepler's laws. By 1830, Uranus was 2 arcminutes away from its predicted position.

Some astronomers began to worry that Newton's law of gravitation was not accurate at large distances. Others, though, provided another alternative. Two scientists, Le Verrier in France and Adams in England (working independently), were investigating the possibility that there existed an object with enough mass to cause slight changes (perturbations) in Uranus' orbit, pulling it away from its predicted orbit. Calculations suggested that Uranus had sped up in its orbit as it approached this massive object, and then slowed down after passing by. Using their calculations and the latest star maps, another astronomer, Galle in Germany, searched for and found the eighth planet in our solar system, Neptune.

Question 5.

The Sun is roughly 20,000 times more massive than Neptune, and roughly twice as far away from Uranus than Neptune. How much stronger is the gravitational force of the Sun on Uranus compared to the gravitational force of Neptune on Uranus?

3
Try again.
Correct. Newton's law of gravitation states that the force of gravity is directly proportional to the masses of the object and inversely to the square of their distance of separation. Since the Sun is 20,000 times more massive, it would pull with 20,000 times as much force, if located the same distance away as Neptune. The Sun is twice as far away, through, so it's pull needs to be decreased by a factor of 4 (2, squared). The net result is that Neptune's pull on Uranus is 1/5000th as strong as the Sun's. While this is small, it is not negligible.
Incorrect. Newton's law of gravitation states that the force of gravity is directly proportional to the masses of the object and inversely to the square of their distance of separation. Since the Sun is 20,000 times more massive, it would pull with 20,000 times as much force, if located the same distance away as Neptune. The Sun is twice as far away, through, so it's pull needs to be decreased by a factor of 4 (2, squared). The net result is that Neptune's pull on Uranus is 1/5000th as strong as the Sun's. While this is small, it is not negligible.

Indepth Activity: Discovering Planets

Question 6.

William Herschel is credited with discovering the planet Uranus in 1781:

A.
B.
C.
D.
E.

Correct. While Herschel discovered Uranus in 1781, Uranus had been observed since 1690. Astronomers just didn't realize that what they were looking at was a planet.
Incorrect. While Herschel discovered Uranus in 1781, Uranus had been observed since 1690. Astronomers just didn't realize that what they were looking at was a planet.

Question 7.

Neptune was the first planet discovered:

A.
B.
C.
D.
E.

Correct. Five planets are observable in the night sky, and Uranus was discovered by accident. Neptune was discovered based on calculations of an object required to cause perturbations in Uranus' orbit as it deviated from what Kepler's laws predicted.
Incorrect. Five planets are observable in the night sky, and Uranus was discovered by accident. Neptune was discovered based on calculations of an object required to cause perturbations in Uranus' orbit as it deviated from what Kepler's laws predicted.

Question 8.

After Uranus’ discovery in 1781:

A.
B.
C.
D.
E.

Correct. Astronomers had to observe Uranus for many years before they had enough data to determine that Uranus' orbit deviated from its predicted path.
Incorrect. Astronomers had to observe Uranus for many years before they had enough data to determine that Uranus' orbit deviated from its predicted path.

Question 9.

How do astronomers distinguish between planets and stars?

A.
B.
C.
D.
E.

Correct. When Herschel first observed Uranus, it appeared as a fuzzy blue-green object, which he thought was a comet. It was only after continued observations that he determined that it didn't move like a comet, but rather like a planet.
Incorrect. When Herschel first observed Uranus, it appeared as a fuzzy blue-green object, which he thought was a comet. It was only after continued observations that he determined that it didn't move like a comet, but rather like a planet.

Question 10.

Which planets have been observed in the night sky since ancient times?

A.
B.
C.
D.
E.

Correct. Before the invention of the telescope, ancient astronomers knew of the existence of five planets in the sky: Mercury, Venus, Mars, Jupiter and Saturn. Earth was not considered a planet until after the adoption in modern times of the heliocentric model.
Incorrect. Before the invention of the telescope, ancient astronomers knew of the existence of five planets in the sky: Mercury, Venus, Mars, Jupiter and Saturn. Earth was not considered a planet until after the adoption in modern times of the heliocentric model.

Question 11.

Uranus moves very slowly:

A.
B.
C.
D.
E.

Correct. Before the invention of the telescope, ancient astronomers knew of the existence of five planets in the sky: Mercury, Venus, Mars, Jupiter and Saturn. Earth was not considered a planet until after the adoption in modern times of the heliocentric model.
Incorrect. Before the invention of the telescope, ancient astronomers knew of the existence of five planets in the sky: Mercury, Venus, Mars, Jupiter and Saturn. Earth was not considered a planet until after the adoption in modern times of the heliocentric model.

Question 12.

Newton’s law of gravity describes the gravitational force between two objects. Which object exerts the greatest gravitational force on Uranus?

A.
B.
C.
D.
E.

Correct. Before the invention of the telescope, ancient astronomers knew of the existence of five planets in the sky: Mercury, Venus, Mars, Jupiter and Saturn. Earth was not considered a planet until after the adoption in modern times of the heliocentric model.
Incorrect. Before the invention of the telescope, ancient astronomers knew of the existence of five planets in the sky: Mercury, Venus, Mars, Jupiter and Saturn. Earth was not considered a planet until after the adoption in modern times of the heliocentric model.

Question 13.

When Herschel first observed Uranus, he thought it was a comet. What did he observe about Uranus that convinced him it wasn’t a comet?

A.
B.
C.
D.
E.

Correct. When Herschel first observed Uranus, it appeared as a fuzzy blue-green object, which he thought was a comet. It was only after continued observations that he determined that it didn't move like a comet, but rather like a planet.
Incorrect. When Herschel first observed Uranus, it appeared as a fuzzy blue-green object, which he thought was a comet. It was only after continued observations that he determined that it didn't move like a comet, but rather like a planet.

Question 14.

If Neptune takes roughly 164.9 years to orbit the Sun, how far away from the Sun is it?

A.
B.
C.
D.

Correct. Kepler's third law states that P2 = a3. 164.92 = 27192.01. If a3 = 27192.01, then a = 30.1 AU.
Incorrect. Kepler's third law states that P2 = a3. 164.92 = 27192.01. If a3 = 27192.01, then a = 30.1 AU.

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