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

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

Celestial Sphere • The view of an observer at the north pole AST 2010: Chapter 1

Celestial Sphere • The view of an observer at the equator AST 2010: Chapter 1

Apparent Motion of the Sun 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 3000 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 Bristish Isles, one finds spectacular monuments that, one now believes, were used to track the motion of the Sun and the Moon • Pythagoras already knew (2500 years ago) the Earth is round • Aristotle (384 –322 BC) summarized the knowledge of his day… AST 2010: Chapter 1

Summary of Aristotle’s Astronomy • Progression of the Moon phases • Sun being farther away from the Earth than the Moon • The roundness of the Earth • Earth’s shadow on the Moon during Moon eclipses is always round • The apparent height of the Northern star (now Polaris) decreases as one travels south – inconsistent with a flat Earth, but explainable with a round Earth AST 2010: Chapter 1

Ancient Astronomy (2) • Aristarchus (310-230 BC) of Samos, Greece, suggested long before Copernicus that the Earth moves around the Sun • His ideas were, however, dismissed by Aristotle on the basis that there is no observable parallax of the stars • Aristotle made quite reasonable conclusions based on his observations which in the end turned out to be wrong!!! AST 2010: Chapter 1

Parallax • This is the apparent shift in the position of an object as a result of the motion of the observer q d L L ~ d/ q (radians) AST 2010: Chapter 1

Erastosthenes: A Brilliant Astronomer • Erastosthenes, a Greek living in Alexandria around 200 BC, 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 BC • 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 • Ptolemy, about the year 140 AD, wrote Almagest, which is a huge compilation of astronomical data • His most important contribution was a geometrical representation of the observed motion 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… Epicycle Earth Equant point Center of eccentric Deferent AST 2010: Chapter 1

Planet’s orbit Earth’s orbit E D E E D B C C C Sun D A B A B A 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 • Copernicus’ model still retained the Aristotelian idea of uniform circular motions for the planets and used small epicycles to get the details of the retrograde loops correct AST 2010: Chapter 1

Galileo Galilei (1564--1642 C.E.) was the first person we know of that used the 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. He found many new things when he looked through his telescope: • The superior light-gathering power of his telescope over the naked eye enabled him to see many, many new fainter stars that were never seen before. This made Bruno's argument more plausible. • The superior resolution and magnification over the naked eye enabled him to see pits and craters on the Moon and spots on the Sun. This meant that the Earth is not only a place of change and decay! 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

Phases of Venus (1) • Galileo also made the important discovery that Venus goes through a complete set of phases • The gibbous and full phases of Venus were impossible in the Ptolemaic model, but possible in Copernican model • In the Ptolemaic model, Venus was always approximately between the Earth and the Sun and was never found further away from the Earth than the Sun AST 2010: Chapter 1

Phases of Venus (2) • Problem: • According to the Ptolemaic model, Venus should always be in a crescent, new, or quarter phase • Solution: • Venus has a gibbous or full phase because it orbits the Sun • With respect to our viewpoint, Venus could get on the other side of, or behind, the Sun further away from us than the Sun • This is a prime example of the scientific method and progress… AST 2010: Chapter 1

Galileo’s Battle with the Church • Galileo’s astronomical observations confirmed the Copernican heliocentric model of the universe • This eventually put him in conflict with the authorities of the 17th century Church, who still upheld the geocentric ideas of Aristotle and Ptolemy • For Galileo himself, there was no contradiction between the authority of the Church (in matters of religion and morality) and the authority of nature (as revealed by experiments, in matters of science) • Although his observations were consistent with the heliocentric model, they could also be explained with a geocentric model like Tycho Brahe's • But for Galileo, the observations were enough because he had already been convinced of the heliocentric system before he used his telescope, and his observations only confirmed his belief 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 AST 2010: Chapter 1

A Need for Better Technology! • 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 views on how things move were wrong AST 2010: Chapter 1

Aristotle’s Views on Motion • 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's observations of moving objects contradicted the long-unchallenged physics of Aristotle • 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 AST 2010: Chapter 1

Galileo’s Studies of Motion (3) • Galileo's studies on how forces operate also provided the foundation to prove that the Earth spins on its axis • Although the stars and Sun appear to rise and set every night or day, they are actually stationary — itis the Earth that moves AST 2010: Chapter 1

Beginning of a Revolution • 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