Astronomy Chapter 2 notes
Astronomy Chapter 2 notes PHYS 1350
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This 7 page Class Notes was uploaded by Lisa Render on Wednesday October 5, 2016. The Class Notes belongs to PHYS 1350 at University of Nebraska at Omaha taught by Charles St Lucas in Fall 2016. Since its upload, it has received 3 views. For similar materials see Principles of Astronomy in Physics at University of Nebraska at Omaha.
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Date Created: 10/05/16
2.1 Patterns in the Night Sky Monday, August 29, 2016 12:53 PM Discoveringthe Universe for Yourself • We used to believe in an system where everything revolvedaround the earth ○ Galileo was exoneratedfor writing otherwise • The sun rises and sets from east to west and so do the stars in a way What does the universe look like from Earth? • With the naked eye, we can see more than 2000stars as well as the Milky Way ○ Have to get pretty far away from the light pollution of the city to really see most of what is possible • 88 constellationsfill the entire sky ○ (Same number of keys on a piano) ○ A constellation is a region of the sky • Star distances ○ The big dipper (ursa major) Apparent shape of the big dipper traced on a plane 50 light years from earth Closest being 63 light years away and furthest being 210 light years away ○ The brightest stars in a constellationmay actually be quite far away from each other • The celestial sphere ○ Stars at different distances all appear to lie of the celestial sphere ○ The ecliptic is the sun's apparent path through the celestial sphere ○ • The "ecliptic": Sun's apparent path among the stars ○ We see different stars at different time of the year Zodiac signs and stars • The Milky Way ○ Band of light that makes a circle around the celestial sphere ○ What is it? Our view into the plane of our galaxy ○ When we look out of the galactic plane, we have a clear view to the distant universe ○ When we look in any direction into the galactic plane, we see the stars and interstellar louds that make up the Milky Way in the night sky • The Local Sky ○ An objects altitude (above horizon) and direction (along horizon) specify its location in your local sky Zenith □ 90 degrees □ The point directly overhead Horizon □ 0 degrees □ All points 90 degrees away from zenith degrees azimuth Azimuth Meridian □ Line passing through zenith and connecting N and S points on the horizon Celestial Noon ○ Angular size The angle that appears to separate two objects in the sky Arm outstretched □ Pinky can equal 1 degree □ Fist can equal 10 degrees ○ Angular measurements Full circle = 360 degrees 1 degree = 60 arcminutes 1 arcminute = 60 arcseconds Basis of time measurement 1 degree is 3600arcseconds Why do star rise and set? • Earth rotates west to east, so stars appear to circle from east to west • Our view from earth ○ Stars never near the north celestial pole are circumpolar and never set ○ We cannot see stars near the south celestial pole ○ All other stars (and Sun, Moon, planets) rise in east and set in west The north celestial pole never sets ○ The north celestial pole never sets Why do the consternations we see depend on latitude and time of year? • Because earth orbit changes the apparent direction of the Sun relative to the stars • The sky varies as Earth orbits the Sun ○ As earth orbits the Sun, the Sun appears to moveeastward along the ecliptic ○ At midnight, the stars on our meridian are opposite the Sun in the sky • Coordinates on the earth ○ Latitude: position north or south of equator ○ Longitude: position east or west of prime meridian (run through Greenwich England) ○ The sky varies with latitude You can find your latitude due to Polaris (the star) Degrees from the ground to the north star = degrees North latitude • Sidereal ○ Sidereal time (star time) ○ Solar day = 24 hours ○ Sidereal day (earth's rotation period) = 23 hours, 56 minutes 4 minutes slower x 365 days = a whole other year 2.2 The Reason for Seasons Wednesday, August 31, 2016 1:04 PM What causes the seasons? • FALSE: earth is closer to the sun in the summer and farther from the sun in the winter ○ Seasons are opposite in North and South hemispheres (so distance cannot be the reason) ○ The real reason for seasons involves Earth's axis tilt • Indirect and direct sunlight on the earth depending on the axis of the earth and angle that sunlight hits the earth ○ Direct (90 degrees to 73 degrees) = summer ○ Indirect/angled (26 degrees) = winter • The suns altitude also changes with seasons ○ Sun's position at noon in summer Higher altitude means more direct sunlight ○ Sun's position at noon in winter Lower altitude means less direct sunlight • 4 landmarks of our rotation around the sun (seasons) 1. Summer solstice June 21st Highest path 2. Autumnal equinox September 22nd 3. Winter solstice December21st Lowestpath 4. Vernal equinox March 20th • We can recognize solstices and equinoxes by the Sun's path across the sky 2.3 The Moon, Our Constant Companion Wednesday, August 31, 2016 1:25 PM Why do we see phases of the Moon? • Phases 1. New moon (0) 2. Waxing crescent (1/4) 3. First quarter (1/2) 4. Waxing gibbous (3/4) 5. Full moon (1/1) 6. Waning gibbous (3/4) 7. Third quarter (1/2) 8. Waning crescent (1/4) 9. New moon (0) • Lunar phases are a consequencesof the Moon's 27.3 day orbit around Earth • The moon makes a sinusoidal path through space ○ 13 sine waves, 12 months • We only see 1 side of the moon. ○ Synchronous rotation The moon rotates exactly once with each orbit This is why only one side is visible from earth There isn't a "dark side" but there is a far side ○ It rotateson its axis once in the SAME time it takes to orbit us once So the moon is always facing us from the same way What causes eclipses • The earth and moon cast shadows ○ When either passes through the other's shadow, we have an eclipse • Lunar eclipses are red the same reason that sunsets are red ○ Blue light is scattered out at that angle • During a total lunar eclipse, the Moon movesinto the darkest part of Earth's shadow ○ The moon appears orange during a total eclipse • During a total solar eclipse, the Moon blocks out sunlight from reaching part of Earth's surface ○ Solar eclipses are also relativelyrare ○ 3 hours south to see a solar eclipse on August 21st 2017(grand island)? When can eclipses occur? • Lunar exclipses can occur only at full moon ○ Penumbral Moon passes through the penumbra (blue) ○ Partial Part of the moon passes through the umbra ○ Total Moon passes entirely through umbra (red/orange) • Solar eclipses can occur only at new moon ○ Partial Occurs in the lighter area surround the area of totality ○ Total Occurs in the small central region of the moon's shadow ○ Annular If the moon's umbral shadow does not reach Earth, an annular eclipse occurs in the small central region in the small central region Ring of light around the sun Why don’t we have an eclipse at every new and full moon? • The moon's orbit is tilted 5 degrees to the ecliptic plane • So we have about two eclipse season each year with a lunar eclipse at new moon and solar eclipse at full moon • Predicting eclipses ○ Eclipses recur with the 18-year 11 1/3 day saros cycle,but type and location may vary ○ Eclipse tracks shown here mark places where a total solar eclipse is seen 2.4 The Ancient Mystery of the Planets Friday, September 2, 2016 1:20 PM Planets vs. stars • We can tell them apart because stars are much farther away • Planets don’t twinkle like stars do when we look at them Why was planetary motion so hard to explain? • Planets know in Ancient Times ○ Mercury Difficult to see Always close to Sun in sky ○ Venus Very bright when visible Morning or evening "star" ○ Mars Noticeably red ○ Jupiter Very bright ○ Saturn Moderately bright • Planets follow paths close to the ecliptic, the Sun's path among the stars • Planets usually move slightly eastward relative to the stars from night to night • But sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion ○ We see apparent retrograde motion when we pass by a planet in its orbit ○ Appears to go backwards in orbit because one planet is on a quicker/closer orbit • Apparent retrograde motion ○ Occurs when we "lap" another planet ○ Hard to explain if you think our world is the center of the universe. Why did the ancient Greeks reject the real explanation for planetary motion? • Their inability to observe stellar parallax was a major factor • Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines ○ The term is derived from the Greek word parallaxis, meaning "alteration" • The Greeks know that the lack of observable parallax could mean one of two things: 1. Stars are so far away that stellar parallax is too small to notice with the naked eye 2. Earth does not orbit the sun, the sun is the center of the universe ○ With rare exceptions, such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away So begins the long, historical showdown between Earth-centered and Sun-centered systems Religion fucked this up real bad too ○ Most Greeks concluded that Earth must be stationary because they thought the stars could not be so far away as to make parallax undetectable Suns closest neighbors • Alpha centari 4 light years away (distance 1839) ○ Proxima centauri red dwarf next to alpha centari (distance 1917) • Barnard's star 6 light years away (distance 1916) • WISE 1049-5319 over 6 light years (distance 2013) • WISE 0855-0714 over 6 light years (distance 2014)
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