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Transcript
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 NATS 1311 - From the Cosmos to Earth New Moon First Quarter Full Moon NATS 1311 - From the Cosmos to Earth Full Moon Third Quarter New Moon NATS 1311 - From the Cosmos to Earth Earthshine The dark portion of the lunar face is not totally dark - you can see the outline of the full face of the Moon even when the Moon is not full - in particular the crescent phase. Because the crescent phase is nearly a new moon as seen from Earth, the Earth is nearly full as viewed from the moon. The light of Earth illuminates the night moonscape - just as the full moon illuminates the Earth landscape. Because Earth is much larger than the Moon, the full earth is much bigger and brighter in the lunar sky than the full moon is in Earth's sky. This faint light illuminating the “dark” portion of the Moon's face is often called the ashen light or earthshine. NATS 1311 - From the Cosmos to Earth The “Dark” Side of the Moon Near Side Far Side The Moon is tidal locked with the Earth - one side faces the Earth at all times - term dark side would better be called the far side - the hemisphere that never can be seen from Earth. Was not seen until first spacecraft orbited the moon and sent back pictures of the far side. NATS 1311 - From the Cosmos to Earth Names of the Full Moons January February March April May June July August September October November December Wolf Moon Snow Moon Worm Moon Pink Moon Flower Moon Strawberry Moon Buck Moon Sturgeon Moon Harvest Moon Hunter's Moon Beaver Moon Cold Moon NATS 1311 - From the Cosmos to Earth Blue Moon Modern folklore - a Blue Moon is the second full Moon in a calendar month can occur in any month but February, which is always shorter than the time between successive full Moons (29 1/2 days). Ecclesiastical version - occurs when a season has four full Moons, rather than the usual three - the third is the Blue Moon - found only in February, May, August, and November, one month before the next equinox or solstice. The result of following rules laid down as part of the Gregorian calendar reform in 1582. The ecclesiastical vernal (spring) equinox always falls on March 21st, regardless of the position of the Sun. Lent begins on Ash Wednesday, 46 days before Easter, and must contain the Lenten Moon, considered to be the last full Moon of winter. The first full Moon of spring is called the Egg Moon (or Easter Moon, or Paschal Moon) and must fall within the week before Easter. Only by naming the third moon the Blue Moon will the names of the other full Moons, such as the Moon Before Yule and the Moon After Yule, fall at the proper times relative to the solstices and equinoxes. NATS 1311 - From the Cosmos to Earth Astronomical Time Periods NATS 1311 - From the Cosmos to Earth Definitions of a Day • Sidereal Day – Time from one transit of a star across the meridian to the next. – Related to the Stars • Apparent Solar Day – Time from one transit of sun across the meridian to the next. – From one high noon to the next – Related to the sun • Mean Solar Day – Time between successive transits of mean sun. – Average of apparent solar days over one year. – Defined to be 24 Hours NATS 1311 - From the Cosmos to Earth Sidereal Day Sidereal - “related to the stars” - the time it takes for any star to make a circuit of the sky - about 23 hours 56 minutes. Measure of the Earth’s rotation - varies about 1 second in 45,000 years. Today defined relative to an ensemble of extra-galactic radio sources. NATS 1311 - From the Cosmos to Earth Solar Day The time it takes for the Sun to make one circuit around the local sky length varies over course of year (up to 25 seconds longer or shorter) but averages 24 hours. NATS 1311 - From the Cosmos to Earth Why is a Sidereal Day Shorter than a Solar Day? One full rotation represents a sidereal day - but while orbiting the Sun, Earth travels in its orbit (about 1 degree per day). So the Earth must rotate slightly farther to point back at the Sun - solar day.