Ch. 22 Section 2 Sun-Earth-Moon

Move Like the Earth Rotation – Turning or spinning of a body on its axis Revolution – Motion of a body along a path around some point in space Precession – Slow movement/wobble of object on its axis

2X Rotation Solar Day – Time interval from one noon to the next (~24 hrs) Sidereal Day – Time it takes for Earth to make one complete rotation (360°) with respect to a star other than our sun. – 23 hours, 56 minutes & 4 seconds( in solar time) 4 minutes shorter than mean solar day – Modeling Moons Motion Lab (sidereal vs. lunar month) It takes Earth 23 hours and 56 minutes to make one rotation with respect to the stars (sidereal day). However, after Earth has completed one sidereal day, point Y has not yet returned to the “noon position” with respect to the sun. Earth has to rotate another 4 minutes to complete the solar day. Why do we use solar day vs. sidereal day?

Be part of the revolution! Earth revolves around the sun in an elliptical orbit – ~107,000 km/hr Average distance from sun ~ 150 million km – Due to elliptical orbit distance varies Perihelion: Earth closest to sun (~147 million km) occurs around January 3/year Aphelion: Earth farthest from sun (~152 million km) occurs around July 4/year

Ecliptic Plane Ecliptic plane: path planets Ecliptic plane travel around Sun Orbit  ellipse with the Sun at one focus. If you made this ellipse a solid surface and extended it infinitively, then you would have the ecliptic plane. – Our entire Solar System can be thought of as flat because all of the planets’ orbits are near or on this plane. Apparent path of sun & celestial equator an angle of 23.5 °

Eccentricity of Orbits Eccentricity – Ratio of the distance between the foci and the length of the major axis. – The eccentricity is: 0 for a circle 1 for a straight line – Pluto has a large eccentricity. Why?

Tilt Causes Seasons Spring & Fall Equinox: – Equal radiation in both hemispheres – Axis perp. to rays – Sun directly over equator – On ecliptic plane/celest. sphere intersection Winter Solstice: – N hemi tilted away Sun – less direct radiation Summer Solstice: – N hemi tilted to Sun – more direct radiation

Precession Earth’s axis maintains approx. the same angle of tilt – Direction in which the axis points continually changes – Axis traces a circle on the sky Similar to the wobble of a toy top The North Celestial Pole traces out a precessional circle around the pole of the ecliptic – the equinoxes precess backwards around the ecliptic, at the rate of arc-seconds per year (around 26,000 years for a complete cycle).

Earth & Moon Moon completes one orbit of the Earth in 1 month Moon’s orbit is ellipse shaped Distance to Earth varies by about 6 %, averaging 384,401 kilometers – Apogee: Moon’s location furthest from Earth – Perigee: Moon’s location closest to Earth

Phases of the Moon Change in the amount of the moon that appears lit. Waxing: light increasing Waning(whining  bad): light decreasing Crescent: less than half lit (crescent roll) Gibbous: more than half lit Quarter: half lit

Months, Weeks, & Days?? The year is also divided into 12 months. Why 12? – There are 12.4 lunar synodic periods (cycles of phases) during a year. The word for Month derives from the word for the Moon. Months are divided into Weeks: – The week is traditionally divided into 7 days; Seven for the 7 planets/moving bodies visible to the naked eye (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn)

Sidereal vs. Lunar Month Synchronous rotation: rate of orbit and rotation equal…see same side of moon Sidereal month: 360 ° orbit of M around E, 27.3d Lunar month: new moon to new moon, 29.5 d Moonrise 50 min later each day b/c E has to turn an additional 13 ° Moon’s orbit creates tides…GRAVITATIONAL PULL OF MOON ON EARTH Modeling Moon’s Motions Activity

Solar Eclipse [ S un M oon E arth ]  order Moon blocks Sun from our view – Total: entirely blocked – Partial: portion blocked – Penumbra: shadowy region cast by moon – Umbra: darkest shadow cast by moon, cone-shaped – Next eclipse: partial on 10/23 – Solar eclipse in India 2010 Solar eclipse in India 2010

Lunar Eclipse [S un E arth M oon ] Moon passes through Earth’s shadow – Umbra: darkest shadow behind Earth…ultra dark – Penumbra: shadowy area cast behind Earth…partly – Total lunar eclipse: entire M in E’s shadow – Partial lunar eclipse: partial shadow of E on M – Next eclipse: total lunar 4/14 Why don’t eclipses occur every time there’s a new/full moon? – Moon’s orbit is inclined ~5° to the plane that contains the Earth & sun Total lunar eclipse in Hawaii 2007 Total lunar eclipse in Hawaii 2007

You’re Invited to a Meteor Shower! Geminids Comet of Origin: 3200 Phaethon Radiant: constellation Gemini Active: Dec. 4-17, 2013 Peak Activity: Dec , 2013 Peak Activity Meteor Count: Approximately 120 meteors per hour Meteor Velocity: 22 miles (35 kilometers) per second Notes: The Geminids are typically one of the best and most reliable of the annual meteor showers. A nearly full moon will block out some of the best meteors this year, but the Geminids are so bright it should still be a decent show. This shower is considered one of the best opportunities for younger viewers because it gets going around 9 or 10 p.m.

Poor Comet Ison 2-4 mile nucleus; relatively small (according to Hubble images) Thought of as comet of century early on due to brightness near Jupiter Originated from the Oort Cloud Comet broke up a couple of days before its passage by the sun. – Intense heat and gravitational pull from the sun pulled the nucleus apart and vaporized its entire ice-water reservoir in a matter of hours. Net result of this will be at least one or two, if not a handful, of extra shooting stars in the sky over a couple of nights