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Chapter 1

Chapter 1. Introduction to Astronomy. The Scientific Method. Observations lead to theories and laws Laws are concise statements that summaries a large number of observations. Theories are explanations for observed phenomena.

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Chapter 1

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  1. Chapter 1 Introduction to Astronomy

  2. The Scientific Method • Observations lead to theories and laws • Laws are concise statements that summaries a large number of observations. • Theories are explanations for observed phenomena. • Theories are more powerful than laws because of there predictive power.

  3. Examples of Laws & Theories • Newton’s Law of Gravity: Mathematically describes the gravitational force between two objects. • Keplers 3rd Law: Mathematically describe the relationship between the orbital period of a planet and its distance from the sun. • The Big Bang Theory: Explains how the universe as come to look the way it does today. • Evolutionary Theory: Explains the why life varies in form and distribution.

  4. Where are we? • Earth is average and has no special place in the universe • The earth revolves around the sun with 8 planets and many smaller bodies. • The Sun revolve around the center of the Milky Way Galaxy. • The Milky Way is one of billions of galaxies in the Universe. • The Universe is the totality of all space, time, matter, and energy.

  5. The Milky Way galaxy is about 100,000 light years across.

  6. The Night Sky • There are about 3000 stars visible to the naked eye on any clear night. Twice that many if you stay up all night.

  7. What are Constellations • Constellations are groups of stars that have meaning for the observer. • Stars in a constellation are not actually close together.

  8. Constellations • There are 88 named constellations in the sky. • 12 constellations form the Zodiac. • Constellations that are always visible form a specific location are called circumpolar. • Circumpolar constellations circle a point.

  9. The Celestial Sphere • Stars appear to be located on a rotating sphere surrounding the earth. • The location of constellations are fixed on the sphere.

  10. Celestial Sphere Details • The projection of the earth’s equator onto the sphere is called the celestial equator. • A circular plane marked by the celestial equator is called the equatorial plane. • The projection of the north & south poles are called the north & south celestial poles. • The star that marks the point in the sky closest to the celestial pole is called the pole star. Currently the northern pole star is Polaris.

  11. The Ecliptic • The suns path along the celestial sphere is the ecliptic. • the vernal and autumnal equinoxes are the points where the ecliptic intersects the celestial equator.

  12. The Zodiac • The 12 constellations that lie along the ecliptic are called the zodiac.

  13. Angular Measure • The location of stars on the celestial sphere is measured in angles. • 360degrees of arc in a circle • 60 minutes of arc per degree • 60 seconds of arc per minute of arc. • One fist held at arms length is approximately 10 degrees. • 1 finger at arms length is about 1 degrees.

  14. Powers of 10 The scale of the universe

  15. Motions of the Earth Keeping the seasons and calendar aligned

  16. Solar and Sidereal Days • A solar day is the time from solar noon on one day to solar noon the next day. • The length of a solar day is the result of the earth’s rotation on its axis and its revolving around the sun. The mean solar day is 24 hours. • A day measured by the stars is a sidereal day. • A sidereal day (23h 56m) is about 3.9 minutes shorter than a mean solar day. • The length of a sidereal day depends only on the rotational motion of the earth.

  17. The Sidereal Day

  18. Time Zones • The meridian is a line on the celestial sphere that passes through the north and south celestial poles and though your zenith. • Solar noon occurs when the sun is on your local meridian. • Each one hour the sun moves 15o to the west. • Time zones are placed every15o to create 24, one-hour time zones.

  19. World Time Zones

  20. Astronomical Adjustments • Year doesn’t quite have a whole number of solar days in it—leap years take care of this. • Add extra day every 4 years • Omit years that are multiples of 100 but not of 400 • Omit years that are multiples of 1000 but not of 4000 • This will work for 20,000 years.

  21. Seasons • 1 year is the time it takes the earth to complete one revolution around the sun. • The earth is tilted 23.5o relative to the plane of the orbit. • It’s the tilt of the earth not how close the earth is to the sun that causes the seasons.

  22. Seasons

  23. Precession • Precession is change in the direction of the earth’s rotational axis. • Precession is a 26000 year cycle. • As the axis moves the pole star

  24. Precession

  25. Future & Past Pole Stars • The pole star is not always Polaris. It varies according the Earth’s precession.

  26. Consequences of Precession • Because of precession the constellations change relative to the seasons. In 13000 years Orion will be a summer constellation

  27. Lunar Cycles

  28. Lunar Phases • Moon takes about 29.5 days to go through whole cycle of phases—synodic month • Phases are due to different amounts of sunlit portion being visible from Earth • Time to make full 360° around Earth, sidereal month, is about 2 days shorter

  29. Phases of the Moon • Waxing Crescent • 1st Quarter • Waxing Gibbous • Full • Waning Gibbous • 3rd Quarter • Waning Crescent • New

  30. Eclipses • A lunar eclipse occurs when the moon passes into the shadow of the earth. • Lunar eclipses only occur during a full moon. • A solar eclipse occurs when the moon passes between the earth and sun. • Solar eclipses only occur during a new moon.

  31. Lunar eclipse • Partial when only part of Moon is in shadow • Total when it’s all in shadow

  32. Eclipses don’t occur every month because the Earth’s and Moon’s orbits are not in the same plane.

  33. Solar Eclipse • Partial when only part of Sun is blocked (penumbra shadow) • Total when it all is blocked (umbra shadow) • Annular when Moon is too far from Earth for total

  34. Earth Coordinate System

  35. Terrestrial Coordinates (North & South) • The earth rotates on an axis that runs from the north pole to the south pole. • Midway between the poles, an imaginary line called the equator divides the earth into equal halves. • Latitude is an angle measured north or south from the equator. • Lines of latitude are also called parallels.

  36. Latitude Latitude measure your angular distance from the equator.

  37. Important Latitudes • The Tropic of Cancer is the circle of latitude on the earth that marks the northernmost path of the sun (23.5oN). • The Tropic of Capricorn is the circle of latitude on the earth that marks the southernmost path of the sun — the apparent "equator" (23.5oS). • The Arctic Circle marks the point where north of the circle experiences a continuous 24 hours of daylight at least once each year (66.5oN). • The Antarctic Circle marks the point where south of the circle experiences a continuous 24 hours of daylight at least once each year (66.5oS).

  38. Terrestrial Coordinates (East & West) • Longitude is the angle measured east or west around the equator. • The zero point for longitude is a point just south of Greenwich, England. • The line of longitude passing through Greenwich is called the prime meridian or the Greenwich line.

  39. The Prime meridian passes through the Royal Observatory in Greenwich, England.

  40. A brass strip set in the ground marks a portion of the prime meridian.

  41. Celestial Navigation Two Systems: Alt-Az and Equitorial

  42. The Celestial Sphere • The celestial sphere is an imaginary sphere surrounding the earth to which the stars are attached. • The celestial sphere is divided into to two halves by the celestial equation which is the projection of the earth’s equator onto the sphere. • The top and bottom of the celestial sphere is marked by the celestial poles which are the projections of the earth’s north and south poles.

  43. Alt-Az System • Also know as the local system • Zenith is the point directly overhead. • The celestial horizon is the great circle 90o from the zenith. • Altitude is the angular distance above the celestial horizon. • Azimuth is the angular distance measured east from north. • The point opposite the zenith is called the Nardir. • The altitude of the north celestial pole is the same as the observer's latitude. • Example: 45o altitude and 270o azimuth would be 45o above the horizon and due west.

  44. The horizon System (alt/az) • Azimuth is the angle measured from north to east along the horizon.

  45. The Alt/Az System gives different coordinates for each location on earth. EARTH

  46. The Equatorial System • Declination measures the angle above and below the celestial equator. • Right ascension measures east from the vernal equinox. • Right ascension is usually measure in hours, minutes & seconds.

  47. Comparing Coordinate Systems • The horizon system (alt/az) is easier to use and understand. • The equatorial system gives the same coordinates regardless of your location. • Modern astronomers use software to transform the equatorial coordinate to local coordinates.

  48. End Chapter 1

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