History of Astronomy

Early Ideas of the Heavens The Shape of the Earth Pythagoras taught as early as 500 B.C. that the Earth was round, based on the belief that the sphere is the perfect shape used by the gods By 300 B.C., Aristotle presented naked-eye observations for the Earth’s spherical shape: Shape of Earth’s shadow on the Moon during an eclipse A traveler moving south will see stars previously hidden by the southern horizon Figure 1.18 History of Astronomy

Earth is Round clip History of Astronomy

Early Ideas of the Heavens Eratosthenes Eratosthenes (276-195 B.C.) made the first measurement of the Earth’s size He obtained a value of 25,000 miles for the circumference, a value very close to today’s value His method entailed measuring the shadow length of a stick set vertically in the ground in the town of Alexandria on the summer solstice at noon, converting the shadow length to an angle of solar light incidence, and using the distance to Syene, a town where no shadow is cast at noon on the summer solstice Figure 1.18 History of Astronomy

Astronomy in the Renaissance Nicolaus Copernicus (1473-1543) Sun is at the center of the universe, motionless; stars are motionless around the edge Planets all revolve around the sun (6 total including Earth) Moon revolves around Earth Earth rotates on axis causing apparent daily motion of the heavens Earth revolves around sun causing sun's annual movements Retrograde motion of planets is due to relative planetary motions Planetary orbits are perfect circles Copernicus was the first to accurately determine the relative distances of the planets from the sun. Figure 1.25, Figure 1.26 Animation: The Retrograde Motion of Mars According to the Heliocentric Model History of Astronomy

Planet Copernican Distance Real Distance Mercury 0.38 AU 0.39 AU Venus 0.75 AU 0.72 AU Earth 1.00 AU 1.00 AU Mars 1.52 AU 1.52 AU Jupiter 5.22 AU 5.20 AU Saturn 9.17 AU 9.54 AU

Astronomy in the Renaissance Johannes Kepler (1571-1630) Kepler’s Three Laws: Planets move in elliptical orbits with the Sun at one focus of the ellipse The orbital speed of a planet varies so that a line joining the Sun and the planet will sweep out equal areas in equal time intervals The amount of time a planet takes to orbit the Sun is related to its orbit’s size, such that the period, P, squared is proportional to the semimajor axis, a, cubed: P2 = a3 where P is measured in years and a is measured in AU Figure 1.27, Figure 1.28 Animation: Kepler’s Law History of Astronomy

Astronomy in the Renaissance Johannes Kepler (continued) Consequences of Kepler’s laws: Second law implies that the closer a planet is to the Sun, the faster it moves Third law implies that a planet with a larger average distance from the Sun, which is the semimajor axis distance, will take longer to circle the Sun Third law hints at the nature of the force holding the planets in orbit Third law can be used to determine the semimajor axis, a, if the period, P, is known, a measurement that is not difficult to make History of Astronomy

Kepler's three laws. (A) A planet moves in an elliptical orbit with the Sun at one focus. (B) A planet moves so that a line from it to the Sun sweeps out equal areas in equal times. Thus the planet moves fastest when nearest the Sun. (C) The square of a planet's orbital period (in years) equals the cube of the semimajor axis of its orbit (in AU), the planet's distance from the Sun if the orbit is a circle. History of Astronomy Back

Astronomy in the Renaissance Galileo (1564-1642) Contemporary of Kepler First person to use the telescope to study the heavens and offer interpretations The Moon’s surface has features similar to that of the Earth Þ The Moon is a ball of rock The Sun has spots Þ The Sun is not perfect, changes its appearance, and rotates Jupiter has four objects orbiting it Þ The objects are moons and they are not circling Earth Milky Way is populated by uncountable number of stars Þ Earth-centered universe is too simple Venus undergoes full phase cycle Þ Venus must circle Sun Figure 1.29 History of Astronomy

Astronomy in the Renaissance Galileo (continued) Convicted of heresy, Galileo was placed under house arrest for the remainder of his life, a gentle punishment for any individual convicted during the Inquisition. On 31 October 1992, 350 years after Galileo's death, Pope John Paul II gave an address on behalf of the Catholic Church in which he admitted that errors had been made by the theological advisors in the case of Galileo. The Church however never admitted that they were wrong in declaring Galileo a heretic History of Astronomy

Isaac Newton & Birth of Astrophysics Isaac Newton (1642-1727) was born the year Galileo died He made major advances in mathematics, physics, and astronomy He pioneered the modern studies of motion, optics, and gravity and discovered the mathematical methods of calculus It was not until the 20th century that Newton’s laws of motion and gravity were modified by the theories of relativity History of Astronomy

Big Bang Theory Hubble’s Law (1920’s) – All other galaxies objects of the universe are moving away from our galaxy Redshift vs. Blueshift? https://www.youtube.com/watch?v=th_9ZR2I0_w&index=95&list=PL908547EAA7E4AE74

Nebular Hypothesis Kant and Laplace (1800’s) Theory that our Solar System originated from the collapse of a Nebula http://csep10.phys.utk.edu/astr161/lect/solarsys/nebular.html

The Sun hides from our view stars that lie beyond it The Sun hides from our view stars that lie beyond it. As we move around the Sun, those stars become visible, and the ones previously seen are hidden. Thus the constellations change with the seasons. History of Astronomy Back

The Sun's path across the background stars is called the ecliptic The Sun's path across the background stars is called the ecliptic. The Sun appears to lie in Taurus in June, in Cancer during August, in Virgo during October, and so forth. Note that the ecliptic is also where the Earth's orbital plane cuts the celestial sphere. History of Astronomy Back

The Earth's rotation axis is tilted by 23.5° with respect to its orbit. The direction of the tilt remains the same as the Earth moves around the Sun. Thus for part of the year the Sun lies north of the celestial equator, whereas for another part it lies south of the celestial equator. History of Astronomy Back

These five diagrams show the Sun's position as the sky changes with the seasons. Although the Earth moves around the Sun, it looks to us on the Earth as if the Sun moves around us. Notice that because the Earth's spin axis is tilted, the Sun is north of the celestial equator half of the year (late March to late September) and south of the celestial equator for the other half of the year (late September to late March). History of Astronomy Back

Stonehenge, a stone monument built by the ancient Britons on Salisbury Plain, England. Its orientation marks the seasonal rising and setting points of the Sun. (Courtesy Tony Stone/Rob Talbot.) History of Astronomy Back

A planet's eastward drift against the background stars plotted on the celestial sphere. Note: Star maps usually have east on the left and west on the right, so that they depict the sky when looking south. History of Astronomy Back

The position of Mars marked out on the background stars and showing its retrograde motion. In what constellation is Mars in October 1994? (Use the star charts on the inside covers of the book to identify the constellations.) History of Astronomy Back

Why we see retrograde motion Why we see retrograde motion. (Object sizes and distances are exaggerated for clarity.) History of Astronomy Back

(A) The cycle of the phases of the Moon from new to full and back again. (B) The Moon's phases are caused by our seeing different amounts of its illuminated surface. The pictures in the dark squares show how the Moon looks to us on Earth. History of Astronomy Back

A solar eclipse occurs when the Moon passes between the Sun and the Earth so that the Moon's shadow strikes the Earth. The photo inset shows what the eclipse looks like from Earth. (Photo courtesy of Dennis di Cicco.) History of Astronomy Back

A lunar eclipse occurs when the Earth passes between the Sun and Moon, causing the Earth's shadow to fall on the Moon. Some sunlight leaks through the Earth's atmosphere casting a deep reddish light on the Moon. The photo inset shows what the eclipse looks like from Earth. (Photo courtesy of Dennis di Cicco.) History of Astronomy Back

(A) During a lunar eclipse, we see that the Earth's shadow on the Moon is curved. Thus the Earth must be round. (B) As a traveler moves from north to south on the Earth, the stars that are visible change. Some disappear below the northern horizon, whereas others, previously hidden, become visible above the southern horizon. This variation would not occur on a flat Earth. History of Astronomy Back