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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. NATS 1311 - From the Cosmos to Earth Mean Solar Day Length of solar day varies over course of year - averages about 24 hours mean solar day. Two reasons for variance.: 1. Earth's orbit is not a perfect circle - it’s an ellipse - Earth moves faster when it is nearest the Sun and slower when it is farthest from the Sun. 2. Earth's axial tilt - the Sun appears to move at an angle to equator during the year - apparently moves fast or slow depending on whether it is apparently far from or close to the equator. Apparent solar days are shorter in March and September than they are in June or December. Solar day may differ from a mean solar day by as much as nearly 22 s shorter to nearly 29 s longer. Because many of these long or short days occur in succession, the difference builds up to as much as nearly 17 minutes early or a little over 14 minutes late. NATS 1311 - From the Cosmos to Earth Synodic Month vs Sidereal Month Synodic month - the time it takes for the moon to complete its cycle of phases - or to come back to the same position with respect to the Earth-Sun line - about 29 1/2 days Sidereal month - the time it takes for for the moon to complete one orbit relative to the position of the stars - about 27 1/3 days NATS 1311 - From the Cosmos to Earth Synodic vs Sidereal Month Animation NATS 1311 - From the Cosmos to Earth Tropical vs Sidereal Year Sidereal year - time it takes for Earth to complete one orbit relative to the stars or the time for the Sun to return to the same position in respect to the stars - the orbital period of the Earth Tropical year - calendar year - time from spring equinox to spring equinox about 20 minutes shorter than a sidereal year - because of precession. Changes not only the orientation of Earth’s axis but the location in Earth’s orbit of the seasons. 26,000 year precession period means that location of solstices and equinoxes among the stars shifts by 1/26,0000 around the orbit. 1/26,000th of a year is about 20 minutes. NATS 1311 - From the Cosmos to Earth A tropical year is about 365.242191 days long - requires leap year every 4 years to keep solstices and equinoxes on same calendar date. After 100 years, in error by more than 3/4 day: 100 tropical years - (75 years + 25 leap years) = 36524.2191-36525 = 0.7809 days Skip leap year every 100 years After 400 years error of nearly a day: 400 tropical years - (304 regular years + 96 leap years)=146,096.876 146,096 = 0.876 So add a leap year every 400 years. Earth’s rotation slowing - tidal drag - add about 29 leap seconds every 100 years. NATS 1311 - From the Cosmos to Earth Leap Seconds Ephemeris Time - the average mean solar time between 1750 and 1890 (centered on 1820) - the period during which the observations on which Simon Newcomb's Tables of the Sun, which formed the basis of all astronomical ephemerides from 1900 through 1983, were performed. Universal Time (UT) - timescale based on the rotation of the Earth. - a modern continuation of the Greenwich Mean Time (GMT), i.e., the mean solar time on the meridian of Greenwich, England. Ephemeris Time and Universal Time are different because the Earth’s rotation is slowing down due primarily due to tidal friction ΔT - the time difference obtained by subtracting Universal Time from Ephemeris Time. NATS 1311 - From the Cosmos to Earth Initially, second defined as 1/86,400th of a mean solar day in the year 1820. Now use stable atomic clocks one second =time for a cesium atom to make 9,192,631,770 vibrations Rotation of Earth slowing down - tidal friction, atmospheric circulation, internal effects, transfer of angular momentum to the Moon orbital motion, etc… To a first approximation - tidal forces slow Earth's rate of rotation by 2.3 ms/day/cy. - melting of continental ice sheets at the end of the last ice age removed their tremendous weight - allowed land under them to begin to isostatically rebound upward in the polar regions - continues to this day - causes Earth's rate of rotation to speed up by 0.6 ms/day/cy. The net tidal acceleration or the change in the length of the mean solar day is +1.7 ms/day/cy. NATS 1311 - From the Cosmos to Earth dT (1.7ms/day /cy)t dt t in centuries dT (1.7ms/day /cy)tdt dT 1.7ms/day /cy tdt 1 2 T (1.7ms/day /cy) t 2 1.7ms/day /cy 62s /cy 2 T 31s /cy 2 t 2 year 1820 Let t 100 2 year 1820 T 31s /cy 2 100 NATS 1311 - From the Cosmos to Earth Actual T varies significantly because of aforementioned effects. - all values of ΔT before 1955 depend on observations of the Moon, either via eclipses or occultations - now, orientation of the Earth relative to an inertial reference frame formed by extra-galactic radio sources is used NATS 1311 - From the Cosmos to Earth Modern Time Scales Temps Atomique International (TAI) or International Atomic Time weighted average of the time kept by about 200 cesium atomic clocks in over 50 national laboratories worldwide - available since 1955 UT1 - timescale defined by the Earth's rotation - computed by the International Earth Rotation and Reference Systems Service (IERS). - TAI was defined such that TAI = UT1 on January 1, 1958. Coordinated Universal Time (UTC) - basis for legal time worldwide, - TAI became the international standard on which UTC is based on January 1, 1972. - UTC always differs from TAI by an integral number of seconds. - in mid 2005, behind TAI by 32 seconds - difference due to leap seconds periodically inserted into UTC to keep it from drifting more than 0.9 seconds from UT1 23 leap seconds since first leap second added in 1972 - last was added on December 31, 2005 - first in 7 years