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1.4 ORIGIN OF MODERN ASTRONOMY

1.4 ORIGIN OF MODERN ASTRONOMY. The Roots of Astronomy. The study of the astronomy of ancient peoples has been termed archeoastronomy . Back in the stone and bronze ages, humans were interested in the cyclic motions in the sky.

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1.4 ORIGIN OF MODERN ASTRONOMY

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  1. 1.4ORIGIN OF MODERN ASTRONOMY

  2. The Roots of Astronomy • The study of the astronomy of ancient peoples has been termed archeoastronomy. • Back in the stone and bronze ages, humans were interested in the cyclic motions in the sky. • Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance. • Used for calendars or to predict eclipses. Newgrange Monument in Ireland built around 3200 B.C. Sunlight shines down a central passageway on the day of the winter solstice.

  3. The Roots of Astronomy • Perhaps the best known example isStonehenge, located on Salisbury Plain in southern England. • Built in stages from 3000 B.C. to 1800 B.C. • Alignments with locations of sunset, sunrise, moonset, and moonrise at summer and winter solstices. Summer Solstice Heelstone

  4. Other Examples • Chaco Canyon, in New Mexico, features slits in the rock formation producing what is known as a “Sun Dagger”, indicating the day of the summer solstice.

  5. Astronomy of Greece • The Golden Age of Astronomy originated and was centered in Greece. • First preserved written documents about ancient astronomy were from ancient Greek philosophy. • Greeks tried to understand the motions of the sky and describe them in terms of mathematical models, rather than physical models.

  6. Greek Astronomers • Thales of Miletus (624-547 B.C.) • Stated the universe is “rational”. • Mysteries because they were unknown, not unknowable. • Pythagoras (570-500 B.C.) • Geometric and mathematical relations: Pythagorean Theorem • Plato (428-347 B.C.) • Philosopher, not astronomer • Influenced the notion of rotating spheres around Earth.

  7. Greek Astronomers • Eudoxus(409-356 B.C.) • Student of Plato; applied his principle of spheres and devised a model of 27 to show the motions of the universe around Earth. • Aristotle (384-322 B.C.) • Like most of his predecessors, Aristotle believed in a geocentric universe, one in which the Earth is at the center. • Expanded the sphere model of Eudoxus to 55 total spheres, with fixed stars in the background. • Knew Earth was spherical in shape due to the shadow it cast on the moon during lunar eclipses.

  8. The Ancient Universe • Ancient astronomers believed Earth did not move because they failed to study parallax, the apparent motion of an object due to the motion of the observer. • Parallax is difficultto see with the unaided eye, thus the use of modern telescopes have helped us better determine distances to stars.

  9. Greek Astronomers • Aristarchus (310-230 B.C.) • First to propose the Earth rotated on its axis and revolved around the Sun. • Eratosthenes(276-194 B.C.) • Discovered a way to measure Earth’s radius. • Used the well at Syene (Aswan) in southern Egypt.

  10. The Well At Syene • Eratosthenes learned sunlight shone vertically into the well on the day of the summer solstice. • Concluded the Sun was at the zenith at Syene. • On that same day in Alexandria, he heard the Sun was 7.2° south of the zenith. • 360° (full circle) / 7.2° = 50 • Discovered the distance from Alexandria to Syene was 1/50 of Earth’s circumference.

  11. The Well At Syene • Travelers told him it took 50 days to travel from Alexandria to Syene; he knew a camel could travel about 100 stadia (singular of stadium) per day. • 50 x 100 = 5000 stadia • If 5000 stadia = 1/50 of Earth’s circumference, then the total circumference = 250,000 stadia • Circumference formula of a sphere = 2πr • Earth’s radius = 40,000 stadia

  12. How Close Was Eratosthenes? • The stadium had different lengths in ancient times, but if you assume 6 stadia = 1 km., he was only off by 4%. • If he used the Olympic stadium, his result was 14% too big. • In any case, this was a much better measurement of Earth’s radius than Aristotle’s, which was much too small (40% of the true radius). Earth’s actual radius = 6378 km. Eratosthenes’ estimation = 6666 km.

  13. Later Refinements • Hipparchus (190-120 B.C.) • Credited with inventing trigonometry and creating the first star catalog (850 stars). • Ptolemy(A.D. 90-168) • Created a mathematical model of the universe in which planets followed a small circle called the epicycle. This slid around a larger circle called the deferent.

  14. Retrograde Motion • A big problem for ancient astronomers was planetary motion. • Planets did not move at a constant rate; occasionally they would stop and move westward before resuming their eastward motion. • This backward motion is called retrograde motion.

  15. The Copernican Revolution • Nicholas Copernicus(1473-1543) • Proposed a heliocentric universe, one in which the Sun was at the center. • Explained daily and annual cycles of the sky by saying the Earth rotates on its axis and revolves around the Sun.

  16. The Copernican Revolution • Copernicus could explain retrograde motion without Ptolemy’s epicycles. • Occurs when Earth passes another planet. • However, Copernicus still was a firm believer of uniform circular motion. http://faculty.fullerton.edu/cmcconnell/Planets.html

  17. Brahe’s Legacy • The Copernican hypothesis solved the problem of the place of Earth, but didn’t explain planetary motion. • If the planets don’t move in uniform circular motion, how do they move? • Tycho Brahe(1546-1601) • Known for precise observations of the stars and planets. • Built an observatory on the island of Hveen, just off the Danish coast. • Became international center of astronomical study.

  18. Brahe’s Legacy • Still believed in a partial geocentric universe. • Rejected the Ptolemaic universe; devised a model in which the Earth was the immobile center around which the Sun and moon moved. • Other planets circled the Sun.

  19. Kepler’s 3 Laws of Planetary Motion • Brahe hired an assistant to prove the validity of his hypothesis. • Johannes Kepler(1571-1630) • Became imperial mathematician when Brahe died in November of 1601, at his request. • In college, he became a believer of the Copernican hypothesis (heliocentric). • Abandoned both uniform motion and circular motion. • Planets move around the Sun in elliptical paths with non-uniform velocities.

  20. Kepler’s 3 Laws of Planetary Motion 1. The orbits of the planets are ellipses with the Sun at one focus. The semimajor axis (a), is half of the longest diameter; the average distance between a planet and the Sun. Eccentricity = c/a c The eccentricity (e) of an ellipse is half the distance between the foci (c) divided by the semimajor axis (a).

  21. Eccentricities of Ellipses 3) 1) 2) e = 0.02 e = 0.1 e = 0.2 5) 4) e = 0.4 e = 0.6

  22. Eccentricities of Planetary Orbits • The orbits of planets are virtually indistinguishable from circles. Earth: e = 0.0167 Pluto: e = 0.248

  23. Kepler’s 3 Laws of Planetary Motion 2. A line from a planet to the Sun sweeps out equal areas in equal intervals of time.

  24. Kepler’s 3 Laws of Planetary Motion 3. A planet’s orbital period (P) squared is proportional to its semi-major axis (a), or average distance from the Sun, cubed. P2 = a3 Py= period in years aAU = distance in AU

  25. Telescopic Observations • Galileo Galilei (1564-1642) • Built his own telescope in 1609. • Observed astronomical features and reported major discoveries.

  26. Discoveries of Galileo (1) • The moon was imperfect. • Mountains and valleys dominant. • Noticed other surface features on the moon as well.

  27. Discoveries of Galileo (2) • 4 major moons of Jupiter: Galilean moons • Io, Europa, Ganymede, Callisto Rings of Saturn – what he saw

  28. Galilean Moons of Jupiter

  29. Discoveries of Galileo (3) • Found the Sun was also imperfect. • Recognized dark regions on the Sun, now known as sunspots (cooler regions).

  30. Discoveries of Galileo (4) • Recognized Venus goes through phases. • Similar to the moon  signified Venus revolved around the Sun rather than the Earth.

  31. Modern Astronomy • Began during the 99 years between the deaths of Copernicus and Galileo (1543-1642). • Marked the change from the Ptolemaic model of the universe to the Copernican model. • This period marks the beginning of the modern scientific method. • Scientists beginning with Copernicus and including Brahe, Kepler, and Galileo depended more on evidence, observation, and measurement rather than on first principles.

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