Ancient Astronomy: Anaxagoras to Hipparchus

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Questions and Answers

Which observation made by Anaxagoras contributed to the understanding that Earth is spherical?

  • Ships disappearing hull first over the horizon.
  • The phases of the moon correlate with its position relative to the sun.
  • The circular shape of Earth's shadow on the Moon during a lunar eclipse. (correct)
  • The changing position of the North Star at different latitudes.

What reasoning did Aristotle use to support the idea of a spherical Earth?

  • The consistent rising and setting points of the sun throughout the year.
  • The cyclical patterns of solar eclipses.
  • The correlation between the distance to the North Star and latitude. (correct)
  • The observation that constellations appear the same from different locations.

What was the primary method used by Eratosthenes to determine the circumference of Earth?

  • Using shadows cast by a stick at noon during the summer solstice in two different locations. (correct)
  • Calculating the distance to the moon based on its apparent size.
  • Observing the curvature of the Earth from high mountain peaks.
  • Measuring the time it took to travel a known distance by ship.

How did Eratosthenes calculate the circumference of the Earth using his observations?

<p>By measuring the angle of a shadow and the distance between two cities. (D)</p> Signup and view all the answers

What astronomical phenomenon did Anaxagoras explain?

<p>The causes of the phases of the moon. (B)</p> Signup and view all the answers

Which model of the universe did Aristarchus propose?

<p>A heliocentric model with the Sun at the center. (B)</p> Signup and view all the answers

What significant contribution did Hipparchus make to astronomy?

<p>He created a catalog of stars and developed a method for predicting lunar eclipses. (D)</p> Signup and view all the answers

What is the main concept behind Claudius Ptolemy's geocentric model?

<p>The Sun, Moon, and planets orbit the Earth in circular paths. (A)</p> Signup and view all the answers

Which of the following tools were used by Babylonian and Egyptian astronomers to study the Sun?

<p>Gnomons (D)</p> Signup and view all the answers

What is the cause of the the phases of the moon?

<p>The varying amounts of the sunlit surface of the Moon visible from Earth. (A)</p> Signup and view all the answers

Which statement accurately describes the difference in frequency between lunar and solar eclipses?

<p>Lunar eclipses occur more frequently. (B)</p> Signup and view all the answers

What distinguishes diurnal motion from annual motion of stars?

<p>Diurnal motion is the daily movement, while annual motion is yearly. (A)</p> Signup and view all the answers

What is precession of the equinoxes?

<p>The wobble of Earth’s axis, causing a gradual shift in the orientation of the Earth’s rotational axis over a period of 25,920 years. (B)</p> Signup and view all the answers

Which planets were known to astronomers before the invention of telescopes?

<p>Mercury, Venus, Mars, Jupiter, and Saturn. (D)</p> Signup and view all the answers

What kind of work is Tycho Brahe known for?

<p>Making accurate observations of the movement of celestial bodies. (D)</p> Signup and view all the answers

What was the main difference between Tycho Brahe’s model of the universe and the Copernican model?

<p>Brahe's model had the Earth at the center, with the Sun, Moon, and stars orbiting the Earth, and other planets orbiting the Sun, while the Copernican model had the Sun at the center . (C)</p> Signup and view all the answers

How did Brahe’s meticulously collected data contribute to Kepler's laws of planetary motion?

<p>Brahe’s data provided the empirical evidence that Kepler used to formulate his laws. (A)</p> Signup and view all the answers

What is Kepler’s First Law of Planetary Motion?

<p>Planets follow an oval or ellipse orbit around the Sun. (A)</p> Signup and view all the answers

What does Kepler’s Second Law of Planetary Motion describe?

<p>The rate at which a planet covers area in its orbit. (D)</p> Signup and view all the answers

According to Kepler’s Third Law, what is the relationship between a planet’s orbital period and its distance from the Sun?

<p>The square of a planet’s orbital period is proportional to the cube of a planet's average distance from the sun. (B)</p> Signup and view all the answers

Which of the following observations solidified the understanding that the Earth is not a perfect sphere?

<p>Detailed measurements showing that the Earth bulges at the equator. (A)</p> Signup and view all the answers

How does the concept of the 'oblate spheroid' describe the true shape of the Earth?

<p>It describes the phenomenon of Earth being flattened at the poles and bulging at the equator. (D)</p> Signup and view all the answers

What advantage did Brahe's Tychonic model provide over the earlier geocentric models?

<p>More accurate predictions of planetary positions due to extensive data collection. (B)</p> Signup and view all the answers

How did observations of ships disappearing hull first over the horizon contribute to the understanding of Earth's shape?

<p>Indicated a curved surface, with the hull disappearing before the mast. (B)</p> Signup and view all the answers

What challenge did Kepler overcome using Brahe's data when developing his laws of planetary motion?

<p>Defining planetary orbits as ellipses rather than perfect circles. (A)</p> Signup and view all the answers

Why was the use of a gnomon important in early astronomy?

<p>It could systematically observe the motion of the sun. (C)</p> Signup and view all the answers

How did ancient astronomers explain retrograde motion before the invention of telescopes?

<p>By proposing that planets moved in epicycles - small circles whose centers moved along larger circles. (B)</p> Signup and view all the answers

Which of the following concepts was NOT a contribution of ancient Greek astronomers to the field?

<p>Precise measurements of the distances to other galaxies. (C)</p> Signup and view all the answers

If Eratosthenes had underestimated the distance between Alexandria and Syene, how would this have affected his calculation of Earth's circumference?

<p>His calculation of Earth's circumference would have been an underestimate. (D)</p> Signup and view all the answers

How did the belief in uniform circular motion hinder the development of accurate astronomical models?

<p>It complicated models with unnecessary epicycles to match observations. (A)</p> Signup and view all the answers

In what way did Tycho Brahe's design of astronomical instruments improve the accuracy of observations?

<p>They were larger and more stable, allowing for finer angular measurements. (C)</p> Signup and view all the answers

Why was Kepler's first law (law of ellipses) important, even though the elliptical orbits are close to circular?

<p>Eliminated the need for complex epicycles used in previous models. (D)</p> Signup and view all the answers

What would be the consequence if a planet did not follow Kepler's 2nd law, i.e. Law of Equal Areas?

<p>Its speed would be constant throughout the orbit without any changes. (D)</p> Signup and view all the answers

According to Kepler’s Third Law, how would increasing the distance to a planet with respect to the sun affect the planet's orbital period?

<p>The planet's orbital period would increase with an increase in its distance from the sun. (C)</p> Signup and view all the answers

How has modern astronomy expanded upon the work of early astronomers like Brahe and Kepler?

<p>By using telescopes and space-based instruments to discover planets and stars. (A)</p> Signup and view all the answers

Which of the following arguments for a spherical Earth relies on observations of celestial objects?

<p>Observing that different stars are visible from different latitudes. (A)</p> Signup and view all the answers

What critical flaw in Ptolemy's geocentric model was directly addressed through Kepler's laws?

<p>The assumption that all orbits are perfectly circular. (B)</p> Signup and view all the answers

How did the collaborative relationship between Brahe and Kepler, despite its challenges, advance astronomical understanding?

<p>Brahe’s accurate data combined with Kepler’s mathematical skill and insight led to new planetary motion laws. (B)</p> Signup and view all the answers

Flashcards

What is an oblate spheroid?

The Earth's true shape; bulging at the equator and squeezed at the poles.

Who was Anaxagoras?

Anaxagoras supported the spherical Earth and noticed the circular shadow cast during lunar eclipses.

What observations did Aristotle make?

Aristotle noted the changing position of the North Star and ships disappearing hull first over the horizon.

Who was Eratosthenes?

Eratosthenes measured Earth's circumference using a stick, geometry, and distance between Alexandria and Syene.

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What is a gnomon?

Babylonians and Egyptians used it to systematically observe the motion of the Sun.

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Who explained moon phases?

Anaxagoras explained what causes the phases of the moon.

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Who proposed heliocentrism?

Aristarchus proposed a Sun-centered (heliocentric) model of the Universe.

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Who was Hipparchus?

Hipparchus compiled of 850 stars and a method for predicting lunar eclipses.

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Who proposed the geocentric model?

Claudius Ptolemy proposed that the Sun, moon, and planets move in circular orbits around the Earth (geocentric model).

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What is Diurnal Motion?

The daily movement of celestial bodies across the sky.

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What is Annual Motion?

Apparent yearly movement of stars due to Earth's orbit around the sun.

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Who was Tycho Brahe?

Tycho Brahe was a Danish astronomer who made accurate observations of celestial bodies.

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What is the Tychonic Model?

The Earth is the center with the moon, sun, and stars revolving around it, while other planets revolve around the sun.

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Who was Johannes Kepler?

Johannes Kepler to be Brahe's assistant.

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What enabled Kepler's laws?

Brahe's extensive data to discover his laws of planetary motion.

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What is Kepler's 1st Law?

Planets follow elliptical orbits with the Sun at one focus.

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What is Kepler's 2nd Law?

A line from the Sun to a planet sweeps out equal areas in equal times.

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What is Kepler's 3rd Law?

The square of a planet's orbital period is proportional to the cube of its average distance from the Sun.

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Study Notes

Ancient Astronomy

  • Anaxagoras (500-430 B.C.) supported the idea of a spherical Earth
  • He observed that the Earth's shadow on the moon during a lunar eclipse was circular
  • Aristotle (around 340 B.C.) argued for a spherical Earth based on:
    • The position of the North Star
    • The shape of the Moon and the Sun
    • Ships disappearing over the horizon
  • Eratosthenes calculated the Earth's circumference
    • He used a stick, knowledge of the distance from Alexandria to Syene, and geometry
    • He measured the angle of a shadow cast by a stick at noon on the summer solstice in Alexandria, which was about 7.2 degrees (1/50 of a complete circle)
    • Alexandria to Syene is 500 miles
    • From these measurements, Eratosthenes computed the circumference of the Earth to be approximately 40,000 kilometers
  • Anaxagoras explained the causes of the phases of the Moon
  • Aristarchus proposed a heliocentric model, stating the Sun was the center of the Universe
  • Hipparchus compiled a catalog of 850 stars and developed a method for predicting lunar eclipses
  • Claudius Ptolemy proposed the geocentric model, stating that the Sun, Moon, and other planets move in circular orbits around the Earth

Astronomical Phenomena Before Telescopes

  • Babylonians and Egyptians used a gnomon to systematically observe the motion of the sun
  • Ancient civilizations observed and tracked the phases of the moon
  • Lunar eclipses occur when the Earth passes between the sun and the moon
  • Solar eclipses occur when the moon passes between the sun and the Earth
  • Diurnal motion refers to the daily motion of celestial objects across the sky
  • Annual motion is the apparent yearly movement of stars due to the Earth's revolution around the Sun
  • Precession of the Equinoxes refers to the rotation of the heavens over a period of 25,920 years
  • The planets Mercury, Venus, Mars, Jupiter, and Saturn were discovered before the invention of telescopes

Modern Astronomy

  • Tycho Brahe was a Danish astronomer who made accurate observations of the movements of celestial bodies
  • Brahe's model had the Earth at the center of the universe with the moon, sun, and stars revolving around it, while the other planets revolved around the sun
  • Johannes Kepler, a German astronomer, mathematician, and astrologer, was Brahe's assistant
  • Brahe and Kepler had an unsteady working relationship
  • Kepler used Brahe's data to formulate his laws of planetary motion
  • Kepler's first law (Law of Ellipses): planets follow an oval or elliptical orbit
  • Kepler's second law (Law of Equal Areas): an imaginary line drawn from the center of the Sun to the center of the planet sweeps out an equal area of space in equal time intervals
  • Kepler's third law (Law of Harmonies): the square of a planet's orbital period is proportional to the cube of the planet's average distance from the Sun

Other Key Points

  • Earth's true shape is an oblate spheroid, bulging at the equator and squeezed at the poles
  • Ancient civilizations were able to explain some astronomical phenomena, and some of these explanations have proven to be accurate
  • Ancient civilizations tried to explain astronomical phenomena through observation
  • Modern understanding of the motion of heavenly bodies has been made possible by the discoveries of astronomers like Brahe and Kepler

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