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Observing the SkyObserving the Sky

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Some Basic Terms & DefinitionsSome Basic Terms & DefinitionsAnglesZenith & nadirCelestial sphere

Horizon

Ecliptic

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360o

What’s an Angle?What’s an Angle?In astronomy as in geometry, one measures angles in degrees or in radiansA full circle makes 360o or 2 radians

45o

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More about AnglesMore about AnglesExamples

360o 2

180o

90o /2

45o /4

Note:

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Earthly QuestionsEarthly Questions

Do you believe the Earth is round? Do you believe the Earth is round?

If you do (or don’t), what are your If you do (or don’t), what are your reasons?reasons?

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Dome of the SkyDome of the SkyThe 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 sphereThe top of the dome is called the zenithThe horizon is where the dome meets the EarthThe point on the celestial sphere that is directly opposite the zenith is called the nadir

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Celestial SphereCelestial SphereToday 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 PolesIf this axis is extended outward, the points where it meets the celestial sphere are defined as the north celestial pole and the south celestial poleThe celestial equator lies halfway between the celestial poles

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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

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

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Apparent Motion of the SunApparent Motion of the SunBecause of the Earth’s revolution around the Sun, its position on our sky changes day by dayThe path that the Sun appears to take around the celestial sphere is called the ecliptic

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Sun Earth

1 day

~1o

1o = 24 hours/360 ~ 4 minutes

Rising and Setting of the SunRising and Setting of the Sun

The Sun gradually changes position on the celestial sphere, moving each day by one degreeIt also rises 4 minutes later each dayWhy?

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ConstellationsConstellationsOn a clear night, about 3,000 stars are visible to the naked eye in sky around usAncient Chinese, Egyptians, and Greeks have grouped them into constellationsToday we use “constellation” to mean one of the 88 sectors of the sky

Some of the old names are still in useConstellations are listed in Appendix 14

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Ancient Astronomy (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 VenusIn 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 MoonPythagoras already suggested (2500 years ago) that the Earth should be a sphereAristotle (384–322 B.C.) summarized the knowledge of his day…

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Summary of Aristotle’s AstronomySummary of Aristotle’s AstronomyThe progression of the Moon’s phases results from our seeing different portions of the Moon’s sunlit hemisphere during the monthThe 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 roundThe 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

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Ancient Astronomy (2)Ancient Astronomy (2)Aristarchus (310-230 B.C.) of Samos, Greece, suggested that the Earth was moving around the SunHis 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

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Stellar ParallaxStellar ParallaxMovie 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 preciseTheir inability to detect stellar parallax led them to accept the geocentric view of the universe

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Erastosthenes: a Brilliant AstronomerErastosthenes: 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 observationHe noticed that on the first day of summer at Syenne, Egypt, sunlight struck the bottom of a vertical well at noon

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Erastosthenes’ Measurement of the Earth (1)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)

1/50 of a circle

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Distance ~ 5,000 stadia

Angle ~ 1/50 of a full circle

Circumference ~ 50 x 5,000 = 250,000 stadia ~ 40,000 km

Erastosthenes’ Measurement of the Earth (2)Erastosthenes’ Measurement of the Earth (2)

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Erastosthenes’ measurement of the Earth (3)Erastosthenes’ measurement of the Earth (3)

He knew the distance between Syenne and Alexandria to be 5,000 stadiaHe 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 is believed that he came within 20% of the actual value of 40,000 km

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HipparchusHipparchusHipparchus, 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 objectsHe discovered that the position in the sky of the north celestial pole had altered over the previous century and a halfThis phenomenon is called precession

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PtolemyPtolemyClaudius Ptolemaeus (Ptolemy), in Alexandria about the year 140 A.D., wrote Almagest, which is a huge compilation of astronomical dataHis 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

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Ptolemy’s Cosmological SystemPtolemy’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

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Retrograde Planet Motion: the Modern ViewRetrograde Planet Motion: the Modern View

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Nicolaus CopernicusNicolaus CopernicusCopernicus (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 scienceHe found many deficiencies in the Ptolemaic modelHe developed a heliocentric, or Sun-centered, model of the solar system

He believed that any model of planetary motions must account for observationsThe 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

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Galileo (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

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Galileo (2)Galileo (2)His telescope enabled him to see many, many new fainter stars that had never been seen beforeThe 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

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Galilean SatellitesGalilean 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 systemThe moons are not moving around the Earth, but are centered on JupiterHe concluded that perhaps other objects, including the planets, did not move around the Earth

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Definitive Evidence of Earth’s MotionDefinitive Evidence of Earth’s MotionIn 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 1830sThe 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 minuteThe parallax angle of even the nearest star is less than one arc second

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Galileo’s Studies of Motion (1)Galileo’s Studies of Motion (1)Galileo also made advances in understanding how ordinary objects move here on the EarthHe set up experiments to see how things move under different circumstancesHe found that Aristotle's long-unchallenged views on how things move were wrongAristotle’s views:

In order for something to keep moving, at even a constant speed, a force must be continually appliedA falling object falls at a constant speedHeavier things always fall more quickly than lighter things

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Galileo’s Studies of Motion (2)Galileo’s Studies of Motion (2)Galileo discovered that

An object's motion is changed only by having a force act on itObjects falling to the ground accelerate as they fallAll 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

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Beginning of a RevolutionBeginning of a RevolutionGalileo’s astronomical observations confirmed the Copernican heliocentric model of the solar systemThe 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 unimportantThere may be other places like Earth in the universeAs the Earth was demoted from its position at the center of the universe, so too was humanity