1 Intro To Astronomy
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1 Intro To Astronomy






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1 Intro To Astronomy 1 Intro To Astronomy Presentation Transcript

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