Week 3--Lecture 4
Week 3--Lecture 4 Astronomy 104
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This 2 page Class Notes was uploaded by Caitlin Acierno on Saturday February 6, 2016. The Class Notes belongs to Astronomy 104 at University of Wisconsin - Madison taught by Jim Lattis in Spring 2016. Since its upload, it has received 23 views.
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Date Created: 02/06/16
Plane of ecliptic Plane or Moon’s orbit—5 degrees tipped Intersection: Lines of Nodes Ascending node, descending node Crossing points where the two lines intersect and the Moon’s orbit passes through. Most full moons and new moons don’t result in eclipses because of this? Phase measures only the angle away from Earth-Sun line At angles of 0 degrees and 180 degrees, you have “syzygy” Term sometimes also applied to alignments of Sun, Earth, and a planet North-South motion of Moon, i.e. the inclination of the plane of the lunar orbit wrt Earth’s orbit (ecliptic) is needed to understand… Orbital inclination adds North-South motion. Tilt ~5 degrees New moon near node Possible solar eclipse Full moon near node Possible lunar eclipse Lunar orbit “Precesses,” i.e. nodes move westward: “regression of the nodes” So “eclipse seasons” do not happen every 6 months but come a few days earlier each cycle Nodes complete rotation in ~18 2/3 years 360 degrees/18.7 years = ~19 degrees/year Or ~20 days/year = 10 days earlier each “season” 3x18 2/3 years = 56 years, so lunar eclipse patterns repeat pretty close to every 56 years. Stonehenge (Aubrey Holes) 56 “holes” might have marked events in lunar eclipse cycles Galileo, 1609 “Ashen Light” o Also called “earthshine,” “old moon in new moon’s arms” o Sunlight, twice reflected o Nearly new Moon means nearly full Earth More Coordinates: Equatorial Rotation of Earth: West to East o California follows New York Counter-clockwise as viewed from above North Pole o “right hand” rule Consider Mintaka, in Orion o Rises due East, sets due West o Traces celestial equator o Question: Where on Earth would you see it at the zenith? Equatorial Coordinates 90 degrees north and south of celestial equator o North celestial pole (NCP) o South Celestial Pole (SCP) Celestial objects ascend daily in the east and descend in the west, paths centered on celestial poles If path crosses horizon, object will rise and set If close enough to the pose, it’s circumpolar. Review Motions of the Sun Annual solar motion o Earth orbits ccw viewed from North o Sun appears to shift eastward wrt stars o Sun’s eastward motion makes sidereal day 4 minutes shorter than solar day o Sun’s apparent annual motion (eastward) is opposite to apparent diurnal motion (westward) of stars. Lunar Eclipses Inclination of lunar orbit to ecliptic Don’t get one every full moon because it is not commonly at one of the nodes. Must occur at full moon Visible across the entire hemisphere, so more common in our experience than solar eclipses Umbra; penumbra Moon can be reddish in umbra Fun, nearly effortless to watch No longer scientifically important. o Used to be one of the few ways for astronomers to measure longitude on the earth.
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