Astronomy Chapter 3 HW Notes

Suppose a comet orbits the Sun on a highly eccentric orbit with an average (semimajor axis) distance of 1 AU. How long does it take to complete each orbit, and how do we know?

1 year, which we know from Kepler's third law. Kepler's third law tells us that any object with the same average distance as Earth will orbit in the same time of 1 year.

1. Earth is located at one (blank) of the Moon's orbit.target 2. According to Kepler's second law, Jupiter will be traveling most slowly around the Sun when at (blank). 3. Earth orbits in the shape of a/an (blank) around the Sun. 4. The mathematical form of Kepler's third law measures the period in years and the (blank) in astronomical units (AU). 5. According to Kepler's second law, Pluto will be traveling fastest around the Sun when at (blank) 6. The extent to which Mars' orbit differs from a perfect circle is called its eccentricity.

1. focus 2. aphelion 3. ellipse 4. semimajor axis 5. perihelion 6. eccentricity

Which of these four ellipses has the greatest eccentricity?

4 Eccentricity describes the amount by which an ellipse is stretched out compared to a perfect circle. Because Ellipse 4 is the most stretched out of the four ellipses shown, it has the greatest eccentricity.

This diagram shows a planet at four points in its orbit around the Sun. At which of the points shown is it traveling slowest in its orbit? (The planet is not real, as all the planets of our solar system have orbits much more circular than the one shown.)

4 Kepler's second law tells us that a planet sweeps out equal areas in equal times as it moves along its orbit, which means it moves faster when it is closer to the Sun and slower when it is farther from the Sun. Point 4 is farthest from the Sun, which is why it is the point at which the planet moves slowest.

What do we mean by a geocentric model of the universe?

A model designed to explain what we see in the sky while having the Earth located in the center of the universe. The geocentric model developed in ancient Greece and was generally accepted until the time of the Copernican revolution.

According to Kepler's third law (p2 = a3), how does a planet's mass affect its orbit around the Sun?

A planet's mass has no effect on its orbit around the Sun. Kepler's third law makes no allowance for planetary mass, and in fact the planet's mass has virtually no effect on its orbit of the Sun. (The Sun's mass has a major effect, however.)

Which of the following is part of a good scientific theory?

A scientific theory must make testable predictions that, if found to be incorrect, could lead to its own modification or demise. A scientific theory should be based on natural processes and should not invoke the supernatural or divine .A scientific theory must explain a wide variety of phenomena observed in the natural world. A scientific theory does not have to be proven true beyond all doubt to be accepted

How many times should a theory be tested before it is accepted?

A theory needs a wide body of evidence and lots of verfication and testing Einstein's theory of relativity has been tested and verified thousands of times.

When would a new Venus be highest in the sky?

At noon A new Venus occurs when Venus is directly between the Sun and Earth, which means a new Venus will be highest in the sky at the same time that the Sun is highest in the sky, which is around noon (local time).

Imagine that Venus is in its full phase today. If we could see it, at what time would the full Venus be highest in the sky?

At noon Because Venus is full when it is on the opposite side of the Sun from Earth, the Sun and Venus both appear to move through the sky together at that time. Venus therefore rises with the Sun, reaches its highest point at noon, and sets with the Sun.

What is true about scientific theories?

Even the strongest theories can never be proved true beyond all doubt. A theory must explain a wide range of observations or experiments. A theory is not essentially an educated guess

In Ptolemy's Earth-centered model for the solar system, Venus's phase is never full as viewed from Earth because it always lies between Earth and the Sun. In reality, as Galileo first recognized, Venus is __________.

Full whenever it is on the opposite side of the Sun from Earth A full Venus always occurs when it is on the opposite side of the Sun as viewed from Earth. Galileo used this fact as evidence for the Sun-centered view of the solar system: The fact that Venus goes through all the phases must mean it goes all the way around the Sun. In contrast, in the Ptolemaic model, Venus only varies between new and crescent phases.

What practical value did astronomy offer to ancient civilizations?

It helped them keep track of time and seasons, and it was used by some cultures for navigation.

What are orbital periods affected by?

Kepler's third law tells us that the orbital period of the planet depends on its average distance from its star, but not on the planet's mass. As Newton later showed with his version of Kepler's third law, this is actually an approximation that works well whenever the planet's mass is small compared to the mass of the star.

When would you expect to see Venus high in the sky at midnight?

Never For Venus to be high in the sky at midnight, it would have to be on the opposite side of our sky from the Sun. But that never occurs because Venus is closer than Earth to the Sun.

Can God existing be tested by scientific means?

No

What is major and semimajor axis?

Note that the line that passes through the star and connects the nearest and farthest points of the planet's orbit is called the major axis, and half this line is the semimajor axis — which we consider the planet's average distance from the star.

How do orbital distances affect orbital periods?

Recall that the time it takes a planet to complete an orbit is called its orbital period. The pattern found in this Part illutrates one of the ideas that are part of Kepler's third law: Planets with larger average orbital distances have longer orbital periods.

Part complete In the Greek geocentric model, the retrograde motion of a planet occurs when:

The planet actually goes backward in its orbit around Earth.

How did the Ptolemaic model explain the apparent retrograde motion of the planets?

The planets moved along small circles that moved on larger circles around the Earth. This created a "loop-the-loop" motion that made the planets in the model appear to sometimes go backward as viewed from Earth.

How do orbital distances affect speed?

This pattern illustrates another of the ideas that are part of Kepler's third law: Planets with larger average orbital distances have slower average speeds.

In Ptolemy's Earth-centered model for the solar system, Venus always stays close to the Sun in the sky and, because it always stays between Earth and the Sun, its phases range only between new and crescent. The following statements are all true and were all observed by Galileo. Which one provides evidence that Venus orbits the Sun and not Earth?

We sometimes see gibbous (nearly but not quite full) Venus In the Ptolemaic system, we should never see more than a crescent for Venus. Because we do in fact see more, the Ptolemaic model must be wrong. The full range of phases that we see for Venus is consistent only with the idea that Venus orbits the Sun. Galileo was the first to observe the phases of Venus — and hence to find this evidence in support of the Sun-centered system — because he was the first to observe Venus through a telescope. Without a telescope, we cannot tell that Venus goes through phases.

Scientific thinking is _________.

based on everyday ideas of observation and trial-and-error experiments

This diagram shows a graphical interpretation of Kepler's third law. Suppose there were a planet in our solar system that orbited the Sun with an average speed of 11 km/s. In terms of its average distance, this planet would be located:

between Jupiter and Saturn Notice that Jupiter orbits at about 13 km/s and Saturn at about 10 km/s, so a planet orbiting at 11 km/s would have to be located between Jupiter and Saturn.

Tycho Brahe's contribution to astronomy included:

collecting data that enabled Kepler to discover the laws of planetary motion.

Ptolemy was important in the history of astronomy because he _________.

developed a model of the solar system that made sufficiently accurate predictions of planetary positions to remain in use for many centuries His model was the culmination of the work of many other Greek astronomers in developing the Greek geocentric (Earth-centered) model of the universe.