Observing the Sky

Observing the Sky AST 2010: Chapter 1

Some Basic Terms & Definitions • Angles • Zenith & nadir • Celestial sphere • Horizon • Ecliptic AST 2010: Chapter 1

45o 360o What’s an Angle? • In astronomy as in geometry, one measures angles in degrees or in radians • A full circle makes 360o or 2p radians AST 2010: Chapter 1

More about Angles • Examples Note: p = 3.1415927 ... AST 2010: Chapter 1

Earthly Questions • Do you believe the Earth is round? • If you do (or don’t), what are your reasons? AST 2010: Chapter 1

Dome of the Sky • The sky on a clear night appears to the naked eye as a great hollow dome that is part of a great sphere which is turning around the observer • This great sphere is called the celestial sphere • The top of the dome is called the zenith • The horizon is where the dome meets the Earth • The point on the celestial sphere that is directly opposite the zenith is called the nadir AST 2010: Chapter 1

Celestial Sphere • Today we know that the apparent turning of the celestial sphere is caused by the Earth’s spinning on its axis, which passes through the Earth’s North and South Poles • If this axis is extended outward, the points where it meets the celestial sphere are defined as the north celestial pole and the south celestial pole • The celestial equator lies halfway between the celestial poles AST 2010: Chapter 1

The turning of the sky looks different depending on your latitude on Earth To an observer at the Earth’s North Pole, the stars circle the zenith They do not rise and set AST 2010: Chapter 1

To an observer at the Earth’s equator, the celestial poles are on the horizon The stars rise straight up and set straight down To an observer at an intermediate latitude, the north celestial pole is at some position between overhead and the horizon The stars rise and set at an angle to the horizon

Apparent Motion of the Sun • Because of the Earth’s revolution around the Sun, its position on our sky changes day by day • The path that the Sun appears to take around the celestial sphere is called the ecliptic AST 2010: Chapter 1

1 day Sun Earth ~1o 1o = 24 hours/360 ~ 4 minutes Rising and Setting of the Sun • The Sun gradually changes position on the celestial sphere, moving each day by one degree • It also rises 4 minutes later each day • Why? AST 2010: Chapter 1

Constellations • On a clear night, about 3,000 stars are visible to the naked eye in sky around us • Ancient Chinese, Egyptians, and Greeks have grouped them into constellations • Today we use “constellation” to mean one of the 88 sectors of the sky • Some of the old names are still in use • Constellations are listed in Appendix 14 AST 2010: Chapter 1

Ancient Astronomy (1) • Babylonian, Assyrian, and Egyptian astronomers knew the approximate length of the year • The Mayans of Central America developed a calendar based on the planet Venus • In the British Isles, one finds spectacular monuments (such as Stonehenge) that, one now believes, were used to track the motion of the Sun and the Moon • Pythagoras already suggested (2500 years ago) that the Earth should be a sphere • Aristotle (384–322 B.C.) summarized the knowledge of his day… AST 2010: Chapter 1

Summary of Aristotle’s Astronomy • The progression of the Moon’s phases results from our seeing different portions of the Moon’s sunlit hemisphere during the month • The Sun being farther away from the Earth than the Moon occasionally causes the Moon to hide the Sun temporarily from view • We call this a solar eclipse • The Earth must be round for 2 reasons: • The Earth’s shadow on the Moon during Moon eclipses is always round • The apparent height of the Northern Star (now called Polaris) decreases as one travels south, which is inconsistent with a flat Earth, but explainable with a round Earth AST 2010: Chapter 1

Ancient Astronomy (2) • Aristarchus (310-230 B.C.) of Samos, Greece, suggested that the Earth was moving around the Sun • His ideas were, however, dismissed by Aristotle and most of the ancient Greek scholars on the basis that they did not find any observable parallax of the stars • Parallax is the apparent shift in the position of an object as a result of the motion of the observer AST 2010: Chapter 1

Stellar Parallax • Movie demonstrating the (greatly exaggerated) parallax motion of a nearby star relative to distant stars in the background, as seen from the Earth • Note that in the movie none of the stars is moving, and that the apparent motion of the nearby star is only a reflection of the Earth’s orbital motion • Note also that the parallax is less if the nearby star is farther away from the Earth • Astronomers in Aristarchus’ time were not able to observe stellar parallax because they did not have the tools that were sufficiently precise • Their inability to detect stellar parallax led them to accept the geocentric view of the universe

Erastosthenes: a Brilliant Astronomer • Erastosthenes, a Greek living in Alexandria around 200 B.C., made a measurement of the size of the Earth based on a very simple, but powerful observation • He noticed that on the first day of summer at Syenne, Egypt, sunlight struck the bottom of a vertical well at noon AST 2010: Chapter 1

1/50 of a circle Erastosthenes’ Measurement of the Earth (1) • At the same date and time, in Alexandria, Egypt, he observed that the Sun was not directly overhead (or striking the bottom of a vertical well) • He observed further that the light rays from the sun were instead making an angle of 1/50 of a full circle with the vertical (approximately 7 degrees) AST 2010: Chapter 1

Erastosthenes’ Measurement of the Earth (2) Angle ~ 1/50 of a full circle Distance ~ 5,000 stadia Circumference ~ 50 x 5,000 = 250,000 stadia ~ 40,000 km AST 2010: Chapter 1

Erastosthenes’ measurement of the Earth (3) • He knew the distance between Syenne and Alexandria to be 5,000 stadia • He was thus able to deduce that the Earth’s circumference is 50 x 5,000 = 250,000 stadia • Despite the uncertainties with his definition of stadia, it isbelieved that he came within 20% of the actual value of 40,000 km AST 2010: Chapter 1

Hipparchus • Hipparchus, born in Nicaea (now Turkey), erected a large observatory on the island of Rhodes around 150 B.C. • He established a pioneering compilation of a very large number of stars and stellar objects • He discovered that the position in the sky of the north celestial pole had altered over the previous century and a half • This phenomenon is called precession AST 2010: Chapter 1

Ptolemy • Claudius Ptolemaeus (Ptolemy), in Alexandria about the year 140 A.D., wrote Almagest, which is a huge compilation of astronomical data • His most important contribution was a geometrical representation of the observed motions of the planets in the solar system based on a geocentric system • The Earth was at the center of things • His complicated geocentric model would endure more than a thousand years AST 2010: Chapter 1

Ptolemy’s Cosmological System • A main circular orbit: the deferent • Small circles within: epicycles • Explanation of the retrograde motion of the planets • In use until the Renaissance … • Movie showing 5-year geocentric motions of the Sun, Mars, Mercury, and Venus AST 2010: Chapter 1

Retrograde Planet Motion: the Modern View AST 2010: Chapter 1

Nicolaus Copernicus • Copernicus (1473-1543), a Polish cleric trained in law and medicine, but interested in astronomy and mathematics, initiated an intellectual revolution that would lead to the emergence of modern science • He found many deficiencies in the Ptolemaic model • He developed a heliocentric, or Sun-centered, model of the solar system • He believed that any model of planetary motions must account for observations • The Ptolemaic model not only failed to do that, but also was clumsy and not elegant • Movie comparing the Ptolemaic (geocentric) and Copernican (heliocentric) solar systems AST 2010: Chapter 1

Galileo (1) • Galileo Galilei (1564-1642 AD) was the first person known to use a telescope for astronomical observations (starting in 1609) • The telescope was originally used as a naval tool to assess the strength of the opponent's fleet from a great distance • Galileo found many new things when he looked through his telescope AST 2010: Chapter 1

Galileo (2) • His telescope enabled him to see many, many new fainter stars that had never been seen before • The superior resolution and magnification of his telescope enabled him to • see pits and craters on the Moon • see spots on the Sun : • these objects are not static • they decay, they are not god-like. • discover four moons orbiting Jupiter AST 2010: Chapter 1

Galilean Satellites • The 4 moons of Jupiter are Io, Europa, Ganymede, and Callisto • They are called Galilean satellites in Galileo’s honor • Jupiter and its moons form a mini-model of the heliocentric system • The moons are not moving around the Earth, but are centered on Jupiter • He concluded that perhaps other objects, including the planets, did not move around the Earth AST 2010: Chapter 1

Definitive Evidence of Earth’s Motion • In 1729, James Bradley (1693-1762) discovered that a telescope has to be slightly tilted because of the Earth's motion • The direction of the telescope must be tilted constantly as the Earth orbits the Sun • Over a century later, Friedrich W. Bessel (1784-1846) provided further evidence for the Earth's motion by measuring the parallax of a nearby star in the late 1830s • The measurements of Bradley and Bessel required technology and precision beyond that of Galileo's time • The telescope tilt angle is less than half an arc minute • The parallax angle of even the nearest star is less than one arc second AST 2010: Chapter 1

Galileo’s Studies of Motion (1) • Galileo also made advances in understanding how ordinary objects move here on the Earth • He set up experiments to see how things move under different circumstances • He found that Aristotle's long-unchallenged views on how things move were wrong • Aristotle’s views: • In order for something to keep moving, at even a constant speed, a force must be continually applied • A falling object falls at a constant speed • Heavier things always fall more quickly than lighter things AST 2010: Chapter 1

Galileo’s Studies of Motion (2) • Galileo discovered that • An object's motion is changed only by having a force act on it • Objects falling to the ground accelerate as they fall • All objects, regardless of their size, fall with the same acceleration in the absence of air drag • Galileo's studies on how forces operate also provided the foundation to prove that the Earth spins on its axis

Beginning of a Revolution • Galileo’s astronomical observations confirmed the Copernican heliocentric model of the solar system • The new ideas from Copernicus and Galileo began a revolution in our conception of the cosmos (universe) • It became clear that the universe is a vast place and that the Earth’s role in it is relatively unimportant • There may be other places like Earth in the universe • As the Earth was demoted from its position at the center of the universe, so too was humanity AST 2010: Chapter 1

Observing the Sky