the earth and the universe origin of modern astronomy image credit: nasa/esa/hubble heritage team...
TRANSCRIPT
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The Earth and the Universe
Origin of Modern Astronomy
Image Credit: NASA/ESA/Hubble Heritage Team (AURA/STScI)
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Early history of astronomy
Ancient Greeks
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Ancient Greeks
• Used philosophical arguments to explain natural phenomena
• Also used some observational data
• Most ancient Greeks held a geocentric (Earth-centered) view of the universe
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Ancient Greeks
• Anaxagoras (~500 B.C.)– Reasoned that the Moon shines by reflected
sunlight– Only one half of the moons is illuminated at a
time
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Ancient Greeks
• Aristotle (384-322 B.C.) – Earth is spherical.
• Aristarchus (312-230 B.C.) – Proposed a heliocentric universe
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Ancient Greeks
• Eratosthenes (276-194 B.C.). – First successful attempt to
establish the size of Earth
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Ancient Greeks
• Hipparchus (~200 B.C.) – Calculated the length of the year to within 6.5
minutes – Precession of the equinoxes – Star catalog
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Ancient Greeks
• Planets exhibit an apparent westward drift– Called retrograde motion– Occurs as Earth, with its faster orbital speed,
overtakes another planet
(This animated gif was converted from a QuickTime movie obtained from Dr. Ted Snow's web site at The University of Colorado, Boulder.)
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Ancient Greeks
• Ptolemaic system– Earth is stationary– Planets, Sun & stars orbit about the Earth– How to explain retrograde motion?
• planets orbited in small
circles (epicycles),
• revolving along large
circles (deferents).
(This animated gif was obtained from Dr. Stephen J. Daunt's Astronomy 161 web site at The University of Tennesee, Knoxville.)
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The universe according to Ptolemy, 2nd century A.D.
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Retrograde motion as explained by Ptolemy
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Early history of astronomy
Birth of modern astronomy
(1500s and 1600s)
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Birth of modern astronomy
• Nicolaus Copernicus (1473-1543)– Concluded Earth was a planet– Heliocentric Universe– Ushered out old astronomy
(This animated gif was obtained from Dr. Stephen J. Daunt's Astronomy 161 web site at The University of Tennesee, Knoxville.)
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Birth of modern astronomy
• Tycho Brahe (1546-1601)– Precise observer– Tried to find stellar
parallax– Did not believe in the
Copernican system because he was unable to observe stellar parallax
Image from: http://csep10.phys.utk.edu/astr161/lect/retrograde/copernican.html
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Image from: http://csep10.phys.utk.edu/astr161/lect/retrograde/copernican.html
Parallax is larger for closer objects
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Birth of modern astronomy
• Johannes Kepler (1571-1630)– Ushered in new astronomy– Planets revolve around the Sun
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Birth of modern astronomy
• Johannes Kepler (1571-1630)– 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
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Kepler’s law of equal areas
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Birth of modern astronomy
• Galileo Galilei (1564-1642)– Supported Copernican theory– Used experimental data– Constructed an astronomical
telescope in 1609
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Birth of modern astronomy
• Galileo Galilei (1564-1642)– Galileo's discoveries using the telescope
• Four large moons of Jupiter
• Planets appeared as disks
• Phases of Venus
• Features on the Moon
• Sunspots
– Tried and convicted by the Inquisition
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Birth of modern astronomy
• Sir Isaac Newton (1643-1727)– Law of universal gravitation– Gravity causes elliptical orbits– No gravity = straight line motion
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Modern Astronomy
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Constellations
• Configuration of stars named in honor of mythological characters or great heroes
• Today 88 constellations are recognized
http://www.astronomy.net/constellations/cancer.html
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Constellations
• Constellations divide the sky into units, like state boundaries in the United States
• The 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
• 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
Image from: http://csep10.phys.utk.edu/astr161/lect/celestial/celestial.html
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Positions in the sky
• Equatorial system of location– Two locational components
• Declination – the angular distance north or south of the celestial equator
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Positions in the sky
• Equatorial system of location– Two locational components
• Right ascension – the angular distance measured eastward along the celestial equator from the position of the vernal equinox
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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 (360 ) 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 & a sidereal day
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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 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
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Earth motions
• Other Earth motions– 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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Motions of the Earth-Moon system
• Phases of the Moon– When viewed from above the North Pole, the
Moon orbits Earth in a counter-clockwise (eastward) direction
– 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
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Motions of the Earth-Moon system
• Lunar motions– Earth-Moon
• Synodic month– Cycle of the phases
– Takes 29 days
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Motions of the Earth-Moon system
• Lunar motions– Earth-Moon
• Sidereal month– True period of the Moon's revolution around
Earth
» Takes 27 days
– 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
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The difference between the sidereal month and
the synodic month
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Motions of the Earth-Moon system
– Moon's period of rotation about its axis and its revolution around Earth are the same, 27 days
• Causes the same lunar hemisphere to always face Earth
• Causes high surface temperature on the day side of the Moon
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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
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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
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Motions of the Earth-Moon system
• Eclipses – Two types of eclipses
• Lunar eclipse – 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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