Questions and Answers
From where on Earth could you observe all of the stars during the course of a year?
the equator
What fraction of the sky can be seen from the North Pole?
half
Give four ways to demonstrate that Earth is spherical.
- The shadow cast on the Moon is always round. 2. Ships appear to sink as they sail away due to Earth's curvature. 3. Satellites show Earth looks round from every direction. 4. The Sun's altitude changes with longitude.
Explain why we see retrograde motion of the planets in geocentric cosmology.
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Explain why we see retrograde motion of the planets in heliocentric cosmology.
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In what ways did the work of Copernicus and Galileo differ from the views of the ancient Greeks?
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What were four of Galileo's discoveries that were important to astronomy?
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Explain the origin of the magnitude designation for determining the brightness of stars.
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Stars in Ursa Minor are above the horizon during the day.
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How many degrees does the Sun move per day relative to the fixed stars?
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How many days does it take for the Sun to return to its original location relative to the fixed stars?
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How many degrees does the Moon move per day relative to the fixed stars?
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How many days does it take for the Moon to return to its original location relative to the fixed stars?
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Explain how the zodiacal constellations are different from the other constellations.
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Study Notes
Observational Points
- All stars can be observed from the equator over a year, although high-declination stars may be near the horizon.
- From the North Pole, only half of the sky is visible, and this view remains constant throughout the year.
Evidence of Earth's Sphericity
- Shadows cast during lunar eclipses are always round.
- Ships seem to sink as they distance themselves from the observer due to the Earth's curvature.
- Satellite images consistently show Earth's round shape from all perspectives.
- The varying altitude of the Sun at different longitudes indicates a spherical Earth; if flat, the Sun would be at the same position for everyone.
Retrograde Motion of Planets
- In the geocentric model, Mars moves on an epicycle and a deferent, creating the illusion of retrograde motion.
- In the heliocentric model, Mars appears to move backward relative to stars due to Earth’s faster orbit, confirmed by Mars' brightness when closest to Earth.
Copernicus and Galileo vs. Ancient Views
- Copernicus and Galileo posited that Earth rotates and revolves around the Sun, opposing the static Earth-centered model.
- Galileo emphasized experimental observation instead of purely theoretical reasoning and utilized the telescope, a tool absent in ancient Greek astronomy.
Key Discoveries by Galileo
- Observations of Venus' phases supported the heliocentric theory.
- Discovery of Jupiter's moons showed that not all celestial bodies orbit Earth.
- Detailed observations of the Moon revealed surface features resembling Earth.
- The Milky Way was identified as a collection of unresolved stars, expanding our understanding of the galaxy.
Magnitude System for Star Brightness
- Hipparchus initiated the magnitude classification, ranking the brightest stars as "first magnitude" and progressively fainter stars as "second magnitude," and so forth.
- The scale is inverted; smaller numbers represent brighter stars due to historical context.
Circumpolar Stars and Visibility
- Ursa Minor's stars, including Polaris, are circumpolar and remain above the horizon at all times.
- Daylight renders these stars invisible despite their constant presence in the sky.
Solar and Lunar Movement
- The Sun moves approximately 1° daily relative to fixed stars, completing a cycle in about 365.25 days.
- The Moon shifts around 12° daily and returns to its original star position in roughly 27.3 days.
Zodiacal Constellations
- Zodiacal constellations intersect with the ecliptic, representing the Sun's apparent annual path.
- The Sun appears to move through these constellations, unlike the surrounding constellations which do not intersect the ecliptic.
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Description
Explore the fascinating concepts of Earth's sphericity and the retrograde motion of planets in this quiz. Delve into the observational evidence provided by ancient astronomers like Copernicus and Galileo. Test your knowledge on how these theories revolutionized our understanding of the cosmos.
