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Transcript
NATS 1311 - From the Cosmos to Earth Seasons occur because even though the Earth's axis remains pointed toward Polaris throughout the year, the orientation of the axis relative to the Sun changes as the Earth orbits the Sun. Around the time of the summer solstice, the Northern Hemisphere has summer because it is tipped toward the Sun, and the Southern Hemisphere has winter because it is tipped away from the Sun. The situation is reversed around the time of the winter solstice when the Northern Hemisphere has winter and the Southern Hemisphere has summer. At the equinoxes, both hemispheres receive equal amounts of light. NATS 1311 - From the Cosmos to Earth Why Does Flux Sunlight Vary Animation NATS 1311 - From the Cosmos to Earth Antarctica June 21 December 21 NATS 1311 - From the Cosmos to Earth In the summer hemisphere, the sun follows a longer and higher path. The sunlight is more intense - more direct and more concentrated. In the winter hemisphere, the sun follows a shorter and lower path. The sunlight is less direct and less intense. Why are the warmest days one to two months after summer solstice? NATS 1311 - From the Cosmos to Earth The Relationship of the Celestial Equator and the Ecliptic Plane The solstices are when the celestial equator and the ecliptic have the greatest separation - when the Sun is highest in the sky. The equinoxes are when ecliptic and the celestial equator cross - when the Sun crosses from north/south of the celestial equator to south/north of the equator Northern spring equinox is when the Sun passes from south of the celestial equator to north. NATS 1311 - From the Cosmos to Earth Why are the seasons more extreme in the Northern hemisphere? NATS 1311 - From the Cosmos to Earth 1. Most of Earth’s land mass in in the Northern Hemisphere. Water takes longer to heat or cool than soil or rock. The water temperature remains relatively constant, thereby moderating the climate. 2. Earth is slightly farther from the sun during northern summer solsitce moves slower in its orbit so summer/winter is 2 - 3 days longer/shorter. This effect is more important then the slightly more intense sunlight due to Earth being closer/farther away. NATS 1311 - From the Cosmos to Earth Five Major Circles of Latitude 1. The Arctic Circle (66.5 degrees N) 2. Tropic of Cancer (23.5 degrees N) 3. The Equator 4. The Tropic Capricorn (23.5 degrees S) 5. The Antarctic Circle (66.5 degrees S) What is special about these latitude circles? NATS 1311 - From the Cosmos to Earth Five Major Circles of Latitude The Arctic and Antarctic Circles - One day a year the sun shines all day and one day a year it doesn’t shine at all. Tropic of Cancer (Capricorn) - The sun is never directly overhead at higher latitudes. NATS 1311 - From the Cosmos to Earth Daily Paths of the Sun at Equinoxes and Solstices At the north pole At 40º latitude At the equator At the tropic of Cancer NATS 1311 - From the Cosmos to Earth Celestial Coordinate System Equatorial System –Declination - - Angle of a star North or South of the Celestial Equator Measured in Degrees –Right Ascension - Measure of Angular Distance of a Star East of the Spring Equinox Measured in Hours, Minutes, Seconds In this coordinate system the position of a star is specified by the right ascension and the declination. This system is used more than any other by astronomers. NATS 1311 - From the Cosmos to Earth Coordinate Systems Geographic Latitude - lines of latitude parallel to Earth’s equator - labeled north or south relative to equator - from 90º N to 90º S Celestial Declination - lines of declination parallel to celestial equator - labeled positive or negative relative to celestial equator - from 90º to +90º Longitude - lines of longitude extend from North Pole to South Pole - by international Right ascension - lines of right ascension run from north celestial pole to south treaty, longitude 0 (the prime meridian) celestial pole - by convention 0 runs through runs through Greenwich, England spring equinox - measured in hours, minutes and seconds east of spring equinox - one hour is 15º NATS 1311 - From the Cosmos to Earth NATS 1311 - From the Cosmos to Earth Annual Motion of the Sun The R.A. of the Sun… increases about 2 hours per month The Declination of the Sun… varies between –23º and +23º NATS 1311 - From the Cosmos to Earth Local Skies Lines of constant declination cross the sky at different altitudes, depending on your location on Earth. declination line = your latitude - goes through your zenith the altitude of the N or S celestial pole = your latitude NATS 1311 - From the Cosmos to Earth Local Skies NATS 1311 - From the Cosmos to Earth Sun’s Altitude vs Latitude and Season Animation NATS 1311 - From the Cosmos to Earth Celestial Navigation Determining latitude Find celestial pole - latitude equal to angular altitude - in northern hemisphere Polaris is within 1º of celestial pole For more precision - use star with known declination determine angular altitude as it crosses your meridian imaginary half circle drawn from your horizon due south, through zenith (point directly overhead) to horizon due north - or when star is at its highest altitude in the sky. Ancients used crossstaff or Jacob’s ladder to determine angular altitude. Modern device called a sextant. Sextant NATS 1311 - From the Cosmos to Earth Vega crosses your meridian in the southern sky at 78º 44’. You know it crosses you meridian at 38º 44’ north of the celestial equator. So the celestial equator must cross you meridian at an altitude of 40º so your latitude is 50º. The formula for latitude is Elevation declination north 90 south north/south of zenith. Sun can also be used if you know the date and the Sun’s declination on that date. NATS 1311 - From the Cosmos to Earth Celestial Navigation Determining longitude Need to compare current positions of objects in your sky with positions at known longitude - Greenwich (0º Longitude). For instance - use sundial to determine local solar time is 3:00 PM. If time at Greenwich is 1:00 PM, you are two hours east of Greenwich and your longitude is 15º X 2 = 30º East Longitude. Accurate determination of longitude required invention of clock that could remain accurate on a rocking ship. By early 1700s, considered so important, British government offered large monetary prize for the solution claimed by John Harrison in 1761. Clock lost only 5 seconds during a 9week voyage. NATS 1311 - From the Cosmos to Earth Precession (a) A spinning top slowly wobbles, or precesses, more slowly than it spins. (b) The Earth's axis also precesses. Each precession cycle takes about 26,000 years. Note that the axis tilt remains about the same throughout the cycle, but changing orientation of the axis means that Polaris is only a temporary North Star. NATS 1311 - From the Cosmos to Earth Precession Movie NATS 1311 - From the Cosmos to Earth Gravitational Attraction The Sun’s gravity (and the Moon’s to a lesser degree) tugs on the Earth trying to straighten out its rotational axis. However, like any rotating object, the Earth tends to keep spinning around the same axis. The result is that gravity succeeds only in making the axis precess. NATS 1311 - From the Cosmos to Earth Sun’s Path Through the Zodiac Animation When astrology began - few thousand years ago - astrological sign supposed to represent the constellation Sun appeared in on your birth date. However, because of precession - no longer case for most people signs are about a month off. Sign actually corresponds to constellation the Sun would have appeared in on your birthday 2000 years ago NATS 1311 - From the Cosmos to Earth 26,000 yrs Climate Changes 41,000 yrs 100,000 yrs Changes in Earth’s orbit and orientation cause cyclic changes in climate - ice ages. Mildest period about 5,000 years ago - headed for another ice age. NATS 1311 - From the Cosmos to Earth Milankovitch Theory Variations in Earth's orbit, the resulting changes in solar energy flux at high latitude, and the observed glacial cycles. Milankovitch Theory - precession of equinoxes, variations in tilt of Earth's axis (obliquity) and changes in eccentricity of the Earth's orbit responsible for observed 100 kyr cycle in ice ages by varying amount of sunlight received by the Earth particularly noticeable in high northern latitude summer. NATS 1311 - From the Cosmos to Earth Lunar Phases Moon’s appearance and rise and set times change with the cycle of the lunar phases The sun illuminates one side (half) of the moon - appearance varies with side of moon facing the Earth - the angle between the sun-Earth line and the Earth-moon line. NATS 1311 - From the Cosmos to Earth NATS 1311 - From the Cosmos to Earth Lunar Phases NATS 1311 - From the Cosmos to Earth Lunar Phases NATS 1311 - From the Cosmos to Earth Lunar Phase Terminology Phases of the Moon’s 29.5 day cycle new crescent first quarter gibbous waxing full gibbous last quarter crescent waning