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Origin of Modern Astronomy
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Early history of astronomy
Ancient Greeks • Used philosophical
arguments to explain natural phenomena
• Most ancient Greeks held a geocentric (Earth-centered) view of the universe• Earth was a
motionless sphere at the center of the universe
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Ancient Greeks (cont’d)
• Stars were on the celestial sphere • Transparent, hollow sphere• Celestial sphere turns daily around Earth
• Seven heavenly bodies (planetai) • Changed position in sky • The seven wanderers included the
• Sun• Moon• Mercury through Saturn (excluding
Earth)
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Aristarchus
• (312-230 B.C.) was the first Greek to profess a Sun-centered, or heliocentric, universe
• Planets exhibit an apparent westward drift –Called retrograde motion
»Occurs as Earth, with its faster orbital speed, overtakes another planet
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• Explain the reason for the observed motion of Mars relative to the background of stars.
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Ptolemaic system • A.D. 141• Geocentric model• To explain retrograde motion, Ptolemy used two
motions for the planets• Large orbital circles, called deferents, and• Small circles, called epicycles
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The universe according to Ptolemy, second century A.D.
Figure 21.4 A
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Retrograde motion as explained by Ptolemy
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Birth of modern astronomy
• 1500s and 1600s1. Nicolaus Copernicus (1473-1543)
• Concluded Earth was a planet• Constructed a model of the solar system that
put the Sun at the center, but he used circular orbits for the planets
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Early history of astronomy
2. Tycho Brahe (1546-1601) • Tried to find stellar parallax – the apparent
shift in a star's position due to the revolution of Earth
• Did not believe in the Copernican system because he was unable to observe stellar parallax
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Early history of astronomy
3. Johannes Kepler (1571-1630) • Planets revolve around the Sun• Three laws of planetary motion
• Orbits of the planets are elliptical• Planets revolve around the Sun at varying
speed • There is a proportional relation between a
planet's orbital period and its distance to the Sun (measured in astronomical units (AU’s) – one AU averages 93 million miles)
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Kepler’s law of equal areas
Figure 21.10
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Early history of astronomy 4. Galileo Galilei (1564-1642)
• Supported Copernican theory• Constructed an astronomical telescope in
1609• Four large moons of Jupiter• Planets appeared as disks• Phases of Venus• Features on the Moon• Sunspots
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Early history of astronomy 5. Sir Isaac Newton (1643-1727)
• Law of universal gravitation• Proved that the force of gravity, combined
with the tendency of a planet to remain in straight-line motion, results in the elliptical orbits discovered by Kepler
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Constellations
Configuration of stars named in honor of mythological characters or great heroes
Today 88 constellations are recognizedConstellations divide the sky into units,
like state boundaries in the United StatesThe brightest stars in a constellation are
identified in order of their brightness by the letters of the Greek alphabet – alpha, beta, and so on
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Positions in the sky Stars appear to be fixed on a spherical
shell (the celestial sphere) that surrounds Earth
Equatorial system of location • A coordinate system that divides the celestial
sphere• Similar to the latitude-longitude system that
is used on Earth's surface
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Positions in the sky • Two locational components
• Declination – the angular distance north or south of the celestial equator
• Right ascension – the angular distance measured eastward along the celestial equator from the position of the vernal equinox
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Astronomical coordinate system on the celestial sphere
Figure 21.16
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Navigation StarsAstronauts use “nav stars” (navigation stars) for space navigation. They are trained toidentify various starfield patterns and the navigation stars that are found within thepatterns. The measuring of the position of the navigation stars can help to determineone’s current position in space or help to chart a course to a different location. Chartinga course to a body in space is no easy matter, especially since, in space, things arealways moving!Take the example of the moon. If one were to leave from the Earth on a trip to themoon, and charted a course directly towards the moon on the day of departure, theship would not reach the moon! By the time the ship reached the correct distance tothe moon, the moon would have moved in its orbit around the Earth and would nolonger be in the same place! In order to reach the moon, the navigator must be ableto predict the future position of the moon for the time that it will take to reach theproper distance, and then chart a course to that predicted position by determining thenav stars in that area.
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Given that the moon moves eastward through the zodiac at the approximate rate of 54 minutes of right ascension per day, determine the following information for each of the problem:
Your ship departs Earth’s orbit on September 4. The moonis located at 2 hours 25 minutes. Your expected arrival date is September 9.
a) The name of the starfield (constellation) for the position of the moon for the first date given.
b) The approximate position of the moon as measured in right ascension (using the units of hours and minutes) for the day of arrival.
c) The appropriate nav star to use for the trip (the one closest to the predicted position of the moon for the arrival date). Note: it should be in the zodiac. List the nav star’s number.
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Earth motions Two primary motions
• Rotation • Turning, or spinning, of a body on its axis• Two measurements for rotation
• Mean solar day – the time interval from one noon to the next, about 24 hours
• Sidereal day – the time it takes for Earth to make one complete rotation (360º) with respect to a star other than the Sun – 23 hours, 56 minutes, 4 seconds
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The difference between a solar day and a sidereal day
Figure 21.19
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Earth motions Two primary motions
• Revolution• The motion of a body, such as a planet or moon,
along a path around some point in space• Earth's orbit is elliptical
• Earth is closest to the Sun (perihelion) in January
• Earth is farthest from the Sun (aphelion) in July
• The plane of the ecliptic is an imaginary plane that connects Earth's orbit with the celestial sphere
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Earth motions Other Earth motions
• Precession• Very slow Earth movement• Direction in which Earth's axis points continually
changes• Movement with the solar system in the
direction of the star Vega• Revolution with the Sun around the galaxy• Movement with the galaxy within the
universe
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Precession of Earth
Figure 21.22
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Motions of the Earth-Moon system
Phases of the Moon• The relative positions of the Sun, Earth, and
Moon constantly change• Lunar phases are a consequence of the
motion of the Moon and the sunlight that is reflected from its surface
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Phases of the Moon
Figure 21.23
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Motions of the Earth-Moon system
Lunar motions • Earth-Moon
• Synodic month • Cycle of the phases• Takes 29 1/2 days
• Sidereal month • True period of the Moon's revolution around
Earth• Takes 27 1/3 days
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Motions of the Earth-Moon system
Lunar motions • Earth-Moon
• The difference of two days between the synodic and sidereal cycles is due to the Earth-Moon system also moving in an orbit around the Sun
• Moon's period of rotation about its axis and its revolution around Earth are the same, 27 1/3 days• Causes the same lunar hemisphere to always face
Earth
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The difference between the sidereal month and the synodic month
Figure 21.24
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Motions of the Earth-Moon system
Eclipses • Simply shadow effects that were first
understood by the early Greeks • Two types of eclipses
• Solar eclipse • Moon moves in a line directly between Earth
and the Sun• Can only occur during the new-Moon phase
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Solar eclipse
Figure 21.25
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Motions of the Earth-Moon system
Eclipses • Two types of eclipses
• Lunar eclipse • Moon moves within the shadow of Earth• Only occurs during the full-Moon phase• For any eclipse to take place, the Moon must
be in the plane of the ecliptic at the time of new- or full-Moon
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Lunar eclipse
Figure 21.26
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Motions of the Earth-Moon system
Because the Moon's orbit is inclined about 5 degrees to the plane of the ecliptic, during most of the times of new- and full-Moon the Moon is above or below the plane, and no eclipse can occur
• The usual number of eclipses is four per year
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