Lecture 4 The Moon Motion of the Moon: sidereal vs synodic month extra events: phases, eclipses Jan 25, 2006 Astro 100 Lecture 4 1 Dealing with multiple motions We now know that many of the confusing apparent motions in the sky are due to multiple independent real motions. In the solar system, it is not as hard as it could be, since most motions are counterclockwise (viewed by observer above Earth's north pole). Body aid - point thumb of right hand North; the direction your fingers curl is the typical solar system motion But none of the motions are lined up exactly, nor are they commensurate (eg one year is not even number of days or months). Keep track with "ephemeris". Jan 25, 2006 Astro 100 Lecture 4 2
Reprise: Diurnal + Annual Motion Earth CCW rotation (exactly W to E) causes apparent daily ("diurnal") steady motion of stars E to W in one sidereal day (rotation of celestial sphere) Earth-sun slow orbital motion in CCW direction causes additional apparent motion of Sun approximately W to E among stars (on the celestial sphere). Added to diurnal apparent motion, total apparent motion of the sun is E to W slightly more slowly than stars ==> solar day slightly longer than sidereal Apparent motion W to E among stars (counter to diurnal motion) is called "direct" motion. Jan 25, 2006 Astro 100 Lecture 4 3 Motion of the Moon Moves (fairly uniformly) approximately W to E (direct) among stars at 15 deg / day. Is due to actual motion: orbit of Moon about Earth (CCW again!) in one sidereal "month" 27 d 8 h Moon's path in sky is near but not quite same as sun's path (ecliptic): orbit tilted 5 deg. Because of fairly fast direct motion, Moon rises about one hour later each day. (If we count "lunar days" by meridian crossing of Moon, would be about 25 hours). Because sun also has direct motion (albeit slower), moon takes longer than sidereal month to pass sun in sky. This is "synodic month" 29 d 13 h Jan 25, 2006 Astro 100 Lecture 4 4
Phases Moon exhibits "Phases" which correlate with synodic month: Full when opposite sun, "Quarter" when at 90 deg, "New" when near sun. ==> Moon seen by reflected light from sun Verification: Moon gets very faint ("lunar eclipse") when Earth is in the way: Jan 25, 2006 Astro 100 Lecture 4 5 Eclipses Lunar eclipse: Sun - Earth - Moon lineup. Moon in Earth's shadow. Happens at FULL MOON. Solar eclipse: Sun - Moon - Earth lineup> (A very small part of) Earth in Moon's shadow. Happens at NEW MOON. ==> Moon closer than Sun Don't have two eclipses every month since Moon's orbit tilted 5 deg to earth's (Moon's path not exactly on ecliptic). Opportunity every ½ "eclipse year" ½ x 343 day. Jan 25, 2006 Astro 100 Lecture 4 6
Predicting Eclipses Some kind of lunar eclipse visible to half the earth every 6 months. Totality duration hours. Since Moon and Sun have about same angular size, although there is some kind of solar eclipse visible somewhere on Earth every six months, the shadow is very small (< 150 miles). Totality duration < 7 minutes. Since synodic month (new moon to new moon) not an exact multiple of eclipse year, shadow hits different part of Earth each time. Do get rough repetition every 18 years ("saros cycle") Jan 25, 2006 Astro 100 Lecture 4 7 Moon's Orbit and Phases Sun New Full Jan 25, 2006 Astro 100 Lecture 4 8
Lunar Eclipse N E W S Jan 25, 2006 Astro 100 Lecture 4 9 Solar Eclipse N E W S Jan 25, 2006 Astro 100 Lecture 4 10
Shadow of Moon Jan 25, 2006 Astro 100 Lecture 4 11