2 OBSERVING THE SKY: THE BIRTH OF ASTRONOMY

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1 2 OBSERVING THE SKY: THE BIRTH OF ASTRONOMY The Sky Above Did you ever lie flat on your back in an open field and look up? If so, what did the sky look like? Most people think it appears to look like an upside bowl. Astronomers call that "bowl" the celestial sphere. E W The Sky Above The sky behaves as if the universe is geocentric Stars all lie on celestial sphere Stars appear to move across the sky uniformly E W 3 3 1

2 2.1 The Sky Above Celestial sphere The point directly overhead is called the zenith Horizon: where the celestial sphere meets the earth Four compass points: north, south, east, west E W Looks can be deceiving Sky only appears to move due to Earth's rotation on its axis 24 hours North pole ~ north celestial pole South pole ~ south celestial pole Halfway between the poles on Earth's surface is the equator Celestial equator: projection of equator onto celestial sphere The sky above the North Pole is very different from our sky At the north pole, North Celestial Pole is directly overhead Celestial equator is parallel to the horizon 6 6 2

3 2.1 The view is also very different at the equator At the equator, Celestial equator passes overhead North Celestial Pole on the northern horizon South Celestial Pole on the southern horizon The sky above Long Island is in between these two extremes On Long Island, North Celestial Pole is 40 degrees off the northern horizon = latitude Celestial equator is 50 degrees off the southern horizon The sky actually consists of several "parts" 9 9 3

4 2.1 As the Earth turns, the whole sky appears to pivot around the celestial pole Most stars rise in the east and set in the west Some never rise or set, but always visible Circumpolar Right now, Polaris is at the NCP. Not always, as we will see later Circumpolar constellations Ursa Major, Cassiopeia, Ursa Minor circumpolar constellations from our latitude All stars north of the celestial equator are circumpolar from the North Pole No stars are circumpolar from the equator Rising and Setting of the Sun Motion of the Sun The Sun does more than just rise and set, it also appears to move eastward among the stars Stars are there, even though we can't see them because of sunlight scattered in our atmosphere Sun completely circles the celestial sphere in 1 year

5 2.1 The Sky Above Motion of the Sun Sun completely circles the celestial sphere in 1 year Therefore, different stars are visible during different seasons The Sun always follows the same path Motion of the Sun Sun always follows the ecliptic. The Sun is not moving -- we are Ecliptic is actually the plane of Earth's orbit around the Sun The Ecliptic does not lie along the celestial equator The Ecliptic does not lie along the celestial equator, but is tilted at 23.5 degrees

6 2.1 The Moon and planets always lie near the ecliptic Moon and planets ("wanderers") can vary a little above or below in a zone called the zodiac ("zoo"), since all but one are living creatures. 12 "signs" or, more correctly, constellations Trivia: which is not alive? There are pictures in the stars! Constellations: star drawings Formed by our ancestors Egyptians Greeks Sumerians Chinese Today, sky is divided into 88 official constellations Designated by International Astronomical Union Think streets in a town Streets connect houses, but there is much more land in a town than just that Seasonal constellations Circumpolar constellations Big Dipper, Cassiopeia, Little Dipper: circumpolar constellations from our latitude All stars north of the celestial equator are circumpolar from the North Pole No stars are circumpolar from the equator

7 2.1 Asterisms are unofficial patterns that are easily recognizable Examples: Big Dipper Little Dipper Winter Football of Stars Summer Triangle Great Square of Pegasus Ancient Astronomy Astronomy: oldest science? Babylonians, Sumerians, Assyrians, Egyptians knew the length of the year Egyptians had a 365 day calendar Kept track of the rising time of the bright star Sirius in the predawn sky, which coincided with the flooding of the Nile and the hottest days of the year (dog days of summer) Astronomy: oldest science? Mayans in Central America had a sophisticated calendar based on the appearances of the planet Venus 500 BC: eastern Mediterranean cultures knew Earth was round Evidence: shadow of Earth on Moon during a lunar eclipse is always round Also, as travelers went north or south, different stars are visible at night

8 2.2 How Big is Earth? Eratosthenes Greek astronomermathematician living in Alexandria, Egypt: 200 BC Measured earth's diameter Answer was very close to modern value Earth s Place in the Universe Ptolemy Egyptian astronomer: 140 AD Compiled a monumental work called the Almagest ("the Greatest") Main source of info on Greek astronomy Set up the geocentric universe Earth Geocentric versus Heliocentric Universe

9 2.2 Hipparchus was one of the greatest ancient astronomers -25 Sun (-26) Cataloged 850 stars seen with the naked eye Assigned them a set of coordinates and divided them into categories based on their apparent brightness -- magnitudes Brightest were labeled 1st mag, then 2nd, etc. to 6th mag Naked-eye limits Inner city (0) Outer city (+2) Suburbs (+4) Rural (+6) Full Moon (-13) Venus (-4) Jupiter (-2) Sirius (-1.4) Vega (0) Polaris (+2) M31 (+4) 50-mm binoculars (+9) M57 (+9) inch telescope (+13) Pluto (+13) 10-inch telescope (+14) 30-inch telescope (+16) inch Hale telescope (+22) 394-inch Keck telescopes (+29) Hubble Space Telescope (+30) Hipparchus also discovered that the Earth wobbles like a top Precession Celestial poles trace a circle among the stars over 26,000 years Caused by the gravity of the Sun and Moon pulling at our equatorial bulge Thuban - north star in 3,000 BC Vega - north star in 14,000 AD Astrology and astronomy were once one and the same Ancient origins -- Babylonians about 2,000 BC Greeks absorbed Babylonian culture, bringing along astrology Natal astrology: belief that planets affect personalities and fortune Zodiac: 12 sectors or signs System set up over 2,000 years ago Since then, precession! All signs off by one constellation Therefore, during Aries (Mar 21- Apr 20), the Sun is actually in Pisces

10 2.3 There are no known "invisible" forces that could affect a person Astrologers argue that there must be unknown forces at work, but where is the evidence of their existence? Based on moment of birth. What about moment of conception? Would you be a different person if you were born a few hours earlier or later? See "Testing Astrology" in textbook Copernicus marked the birth of modern astronomy Polish astronomer, 16 th century Devised the heliocentric universe Better explained motions of the planets -- much simpler than geocentric BUT one objection was if the Earth were moving, why don't we feel the motion? Answer: everything is moving at the same speed, so the relative motion is canceled out Galileo was the father of experimental science Before Galileo, everything was based on reason, not necessarily testing of theories First to use a telescope to observe the night sky Discovered: Craters on the Moon Moons going around Jupiter Phases of Venus There were more stars in the sky than could be seen with the eye alone

11 2.4 Birth of Modern Astronomy Galileo Phases of Venus Ptolemy versus Copernicus Birth of Modern Astronomy Galileo wrote a book that proclaimed his observations as law Church argued that Galileo was a heretic and threw him in jail for his proclaiming a heliocentric solar system Pope John Paul II finally said that the Church was wrong about Galileo in 1992! Celestial Sphere Celestial Equator Celestial Pole Circumpolar Horizon Geocentric universe Heliocentric universe Chapter 2 Review Words to know Zenith Rotation Revolution Zodiac Constellation Asterism Magnitude 33 Famous dead astronomers Ptolemy Hipparchus Eratosthenes Copernicus Galileo 33 11