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1 Intro To Astronomy


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1 Intro To Astronomy

  1. 1. Intro to Astronomy
  2. 2. Space is Big <ul><li>For distances within our solar system, we use a unit of distance known as the Astronomical Unit (AU). </li></ul><ul><li>1 AU is defined as the average distance from the Earth to the Sun, roughly 1.5 x 10 8 kilometers (93 million miles) </li></ul>
  3. 3. Really Big <ul><li>When dealing with objects outside of our solar system, the AU is too small to be effective, so we use the light-year. </li></ul><ul><li>A light-year (ly) is defined as the distance a beam of light travels in one year. </li></ul><ul><li>1 ly = 10 trillion km (6 trillion mi) </li></ul>
  4. 4. For Comparison... <ul><li>It takes a beam of light roughly 8 minutes to travel from the Sun to the Earth </li></ul><ul><li>Proxima Centauri (the nearest star to us, after our Sun) is over 4 light years away </li></ul>
  5. 5. The Celestial Sphere <ul><li>Imagine the Earth at the center of a clear, hollow globe with the stars glued to the inside. </li></ul><ul><li>Everything we use to navigate on Earth can be “copied” onto the Celestial Sphere (latitude, longitude, the equator, and the poles) </li></ul>
  6. 7. Angular Measurement <ul><li>A full circle contains 360 degrees </li></ul><ul><li>1 o can be broken further into arc minutes (60’ in 1 o ) </li></ul><ul><li>Arc minutes can be broken again into arc seconds (60” in 1’) </li></ul>
  7. 8. Angular Measures <ul><li>The Sun and Moon both cover an area of about 0.5 o – half the size of a finger held at arm’s length </li></ul><ul><li>At arm’s length, a hand spans about 15 o (also the amount of sky covered by the Sun’s motion in one hour) </li></ul>
  8. 9. Celestial Coordinates <ul><li>Declination (dec) is the equivalent of latitude on the Celestial Sphere </li></ul><ul><li>dec is measured in degrees north or south of the Celestial Equator </li></ul>
  9. 10. Celestial Coordinates (cont.) <ul><li>Right Ascension (RA) is the longitude equivalent on the C.S. </li></ul><ul><li>RA is measured in hours, minutes, and seconds </li></ul><ul><li>The Prime Meridian of RA is wherever the Sun is on the C.S. at the vernal equinox (first day of spring) </li></ul>
  10. 11. Orbital Motion <ul><li>Solar Day: the time it takes the Sun to return to a specific spot in the sky (24 hours) </li></ul><ul><li>Sidereal Day: the time it takes Earth to complete one full rotation in its orbit (23 hours, 56 minutes) </li></ul><ul><li>The 4 minute per day difference gives us leap years </li></ul>
  11. 13. Seasonal Changes <ul><li>Earth’s orbit around the Sun causes us to see different constellations in the sky </li></ul>
  12. 14. The Zodiac <ul><li>The ecliptic is the Sun’s path along the Celestial Sphere. </li></ul><ul><li>The Zodiac is made up of the 12 constellations that the Sun travels through along the ecliptic. </li></ul><ul><li>Due to position, the constellation of your sign can only be seen 6 months before/after your birth month. </li></ul>
  13. 16. Seasonal Changes (cont.) <ul><li>Earth rotates on its axis, which is tilted 23 ½ degrees to its orbit. </li></ul><ul><li>On the Celestial Sphere, the ecliptic is tilted the same 23 ½ degrees. </li></ul><ul><li>This tilt is what gives us the four seasons. </li></ul>
  14. 17. Four Seasons
  15. 18. Four Seasons (cont.) <ul><li>Vernal Equinox – March 21st </li></ul><ul><li>Autumnal Equinox – September 21 st </li></ul><ul><ul><li>12 hours of night and day - everywhere </li></ul></ul><ul><li>Summer Solstice – June 21 st </li></ul><ul><ul><li>Most sunlight of the year </li></ul></ul><ul><li>Winter Solstice – December 21 st </li></ul><ul><ul><li>Least sunlight of the year </li></ul></ul>
  16. 19. Distance & Size <ul><li>We can triangulate the distance to an object we can’t directly measure </li></ul>
  17. 20. Distance & Size (cont.) <ul><li>With really large distances, triangulation less reliable. </li></ul><ul><li>Rather than used a measure baseline, we use the missing angle of the triangle, or parallax </li></ul>
  18. 21. Distance & Size (cont.) <ul><li>Try this: </li></ul><ul><li>Hold a pencil in front of your face and let your eyes focus on the wall. First close your left eye, and then open it and close your right eye. </li></ul><ul><li>The apparently difference in position of the pencil is parallax </li></ul>
  19. 23. Review <ul><li>By this point, you should be able to: </li></ul><ul><ul><li>Describe the Celestial Sphere </li></ul></ul><ul><ul><li>Use angular measurements to find objects in space </li></ul></ul><ul><ul><li>Explain the apparent motion of the Sun and stars with the actual motion of the Earth </li></ul></ul><ul><ul><li>Explain how to gauge size and distance of faraway object </li></ul></ul>
  20. 24. Motions of the Planets <ul><li>‘ Planets’ comes from the Greek word: ‘planetes’ which means “wanderer” </li></ul><ul><li>As viewed from Earth, the planets of our solar system all exhibit retrograde motion </li></ul><ul><li>Like the Moon, planets are visible because of reflected sunlight </li></ul>
  21. 25. The Geocentric Universe <ul><li>Until the 16 th century, astronomers believed that the Earth was the center of the universe </li></ul><ul><li>As a result, everything (the Sun, Moon, planets and stars) revolved around us </li></ul><ul><li>Astronomers tried everything to fit observations into this theory </li></ul>
  22. 26. The Heliocentric Model <ul><li>Nicholas Copernicus proposed the idea of a Sun-centered universe in the 16 th century </li></ul><ul><li>In fear of persecution, Copernicus kept his ideas secret until he died in 1543 </li></ul>
  23. 27. Galileo & Kepler <ul><li>Galileo Galilei was the first astronomer to use a telescope for observing the night sky </li></ul><ul><li>Using his telescope, he discovered: </li></ul><ul><ul><li>Sunspots </li></ul></ul><ul><ul><li>Lunar terrain </li></ul></ul><ul><ul><li>Moons orbiting Jupiter </li></ul></ul><ul><ul><li>The phases of Venus </li></ul></ul>
  24. 28. Eppur si muove <ul><li>For supporting Copernicus’ ideas, Galileo was arrested and sentenced to death </li></ul><ul><li>He was spared the ultimate punishment and instead sentenced to house arrest for retracing his claims </li></ul><ul><li>Supposedly, he muttered “And yet, it moves” under his breath after he recanted </li></ul>
  25. 29. Kepler’s Laws <ul><li>The planets revolve around the Sun in elliptical (not circular) paths </li></ul><ul><ul><li>Perihelion: when a planet is closest to the Sun </li></ul></ul><ul><ul><li>Aphelion: when a planet is farthest from the Sun </li></ul></ul>
  26. 31. Kepler’s Laws (cont.) <ul><li>Planetary orbits sweep out equal areas of the ellipse in equal amounts of time </li></ul>
  27. 33. Kepler’s Laws (cont.) <ul><li>The square of an orbital period is proportional to the cube of its semi-major axis </li></ul><ul><li>P 2 (in Earth years) = a 3 (in AU) </li></ul>
  28. 34. Kepler’s Laws (cont.)
  29. 35. Kepler’s Laws (cont.) <ul><li>During Kepler’s life, there were only 6 known planets – those that can be seen without a telescope </li></ul><ul><li>Kepler’s 3 rd Law works for Uranus, Neptune, and Pluto even though these were discovered after the 3 rd law was written! </li></ul>
  30. 36. Solar System Dimensions <ul><li>Recall: the astronomical unit is defined as the mean (average) distance from the Earth to the Sun </li></ul><ul><li>This was done because until recently, we lacked the technology to directly measure distances outside of Earth </li></ul>
  31. 37. Dimensions (cont.) <ul><li>Today, we use radar imaging to directly measure the distance between planets </li></ul><ul><li>We send radio waves toward a nearby object (Venus, for example) and wait for the echo to come back </li></ul><ul><li>Multiply the round trip travel time by the speed of light and we calculate double the distance to the object </li></ul>
  32. 39. Example: Venus <ul><li>At its closest, Venus is 0.3 AU from Earth </li></ul><ul><li>A RADAR signal takes 300 s to reach Venus and return to Earth </li></ul><ul><li>300,000 km/s * (300 s / 2) = 45,000,000 km = 0.3 AU </li></ul><ul><li>Therefore, 1 AU = 150,000,000 km </li></ul>
  33. 40. Gravity <ul><li>The force due to gravity is continuous and always attractive </li></ul><ul><li>Unlike magnets, there is no ‘repulsive’ gravity </li></ul>
  34. 41. Gravity (cont.) <ul><li>All objects constantly exert a gravitational force on each other – even you and me. </li></ul><ul><li>The force is only dependent on the mass of the objects and the distance between them </li></ul>
  35. 42. Newton’s Law of Gravitation <ul><li>F = Gravitational Force </li></ul><ul><li>G = Gravitational Constant = 6.67 x 10 -11 N m 2 / kg 2 </li></ul><ul><li>M 1 = Mass of object #1 </li></ul><ul><li>m 2 = Mass of object #2 </li></ul><ul><li>r = Distance between objects </li></ul>
  36. 43. Important Notes <ul><li>The force decreases exponentially with distance </li></ul><ul><ul><li>If you’re twice as far away, the force is 2 2 times weaker (1/4 as strong) </li></ul></ul><ul><li>No matter how big r gets, the force never reaches zero (gravity exerts an effect everywhere) </li></ul>
  37. 44. Example: The Lunar Diet <ul><li>How much would you weigh on the Moon? </li></ul><ul><li>F = Force or Weight </li></ul><ul><li>G = Gravitational Constant = 6.67 x 10 -11 N m 2 / kg 2 </li></ul><ul><li>M 1 = Mass of the Moon (7.3477×10 22 kg) </li></ul><ul><li>m 2 = Mass of you (~70kg) </li></ul><ul><li>r = 1,737,000 m (Moon radius) </li></ul>
  38. 45. Example (cont.) <ul><li>This compares to a weight on Earth of: </li></ul><ul><li>W = (70 kg) * (9.8 m/s 2 ) = 686 N </li></ul><ul><li>Or, roughly, you’d weigh 1/6 as much on the Moon </li></ul>