Student Astronomical SocietyStudent Astronomical SocietyFirst Meeting!!!!!First Meeting!!!!!

Wednesday, September 147:00 PM

5th Floor of the Physics BuildingThere will be refreshments!!!

Student Astronomical SocietyStudent Astronomical SocietyFirst Meeting!!!!!First Meeting!!!!!

Wednesday, September 14Wednesday, September 14

Wednesday, September 147:00 PM

5th Floor of the Physics BuildingThere will be refreshments!!!

7:00 PM5th Floor of the Physics Building

There will be refreshments!!!7:00 PM

5th Floor of the Physics BuildingThere will be refreshments!!!

Astronomy 1010Astronomy 1010Introduction to AstronomyIntroduction to Astronomy

Lecture 10

Why does the moons appearance Why does the moons appearance change?change?

Why do we see different phases of the Moon?Why do we see different phases of the Moon?a) The motion of the Moon in its orbit around the Earth

causes us to see different amounts of the Earth's shadow falling on the Moon.

b) The motion of the Moon in its orbit around the Earth causes us to see different amounts of the sunlit side of the Moon.

c) The distance of the Moon from the Earth changes because of the elliptical orbit of the Moon, causing the sunlit side of the Moon to move relative to the Earth.

d) The rotation of the Moon around its own axis causes us to see different amounts of the sunlit side of the Moon.

The Phases of the MoonThe Phases of the MoonFrom Earth, we see different portions of the Moon’s surface lit by the sun, causing the phases of the moon.

U.S. Naval ObservatoryAstronomical Applications Department

Sun or Moon Rise/Set Table for One Year

Sky Gazer Program – Explore, Phases of the Planets, Phases of the Moon

Moon Phaseshttp://www.valdosta.edu/~cbarnbau/astro_demos/frameset_moon.html

The Phases of the MoonThe Phases of the MoonEvening Sky

The Phases of the MoonThe Phases of the MoonMorning Sky

Suppose that on a given evening you notice Suppose that on a given evening you notice that the Moon has a crescent shape. This that the Moon has a crescent shape. This

simple observation tells yousimple observation tells youa) that at that particular time the Moon is closer to

the Sun than you are.

b) that at that particular time the Moon is farther from the Sun than you are.

c) that the line from you to the Moon is exactly at right angles to the line from you to the Sun.

d) nothing at all about where the Moon is in space compared to you and the Sun.

The dark side of the moonThe dark side of the moonIs there a dark side of the moon?

Is there a dark side to the moon?Is there a dark side to the moon?Earth exerts tidal forces on the moon’s rocky interior

→ It is rotating with the same period around its axis as it is orbiting Earth (tidally locked)

→ We always see the same side of the moon facing Earth

The Moon is seen to keep one face toward Earth The Moon is seen to keep one face toward Earth at all times. If viewed from a point directly above at all times. If viewed from a point directly above the plane of the planetary system, how does it the plane of the planetary system, how does it

have to rotate to maintain this alignment?have to rotate to maintain this alignment?Q?Q?

a) It must rotate once per year as Earth and Moon orbit the Sun together.

b) It must rotate once per day.c) It must not rotate at all, since we always see the

same face from Earth.d) It must rotate once per month, or once per orbit

around Earth.

Synodic vs Sidereal Synodic vs Sidereal Sidereal period = time for moon to Sidereal period = time for moon to

complete one full orbit of the earthcomplete one full orbit of the earth

Synodic period = time between Synodic period = time between successive New Moons.successive New Moons.

Sidereal Period Sidereal Period • The Moon orbits Earth in a sidereal period of 27.32 days.

27.32 days

EarthMoon

Fixed direction in space

Synodic PeriodSynodic Period

• The Moon’s synodic period (to reach the same position relative to the sun) is 29.53 days (~ 1 month).

Fixed direction in space

Earth

Moon

Earth orbits around Sun => Direction toward Sun

changes!

29.53 days

The size of the moonThe size of the moon

55oo EclipticEclipticplaneplane

Moon’s

Orbital

Plane

Lunar Lunar motionsmotions

Inclination of Inclination of the Moons the Moons orbitorbit

LibrationLibration

http://upload.wikimedia.org/wikipedia/commons/c/c0/Lunar_libration_with_phase2.gif

Earthrise from lunar orbit, Apollo 8

Earth rising, from Japan’s Kaguya spacecraft

EclipsesEclipses

EclipsesEclipsesEclipses are shadowsEclipses are shadowsIf the earth is directly between the moon and If the earth is directly between the moon and

the sun the sun lunar eclipse lunar eclipse the moon is in the the moon is in the EarthEarth’’s shadows shadow

If the moon is directly between the earth and If the moon is directly between the earth and the sun the sun solar eclipse solar eclipse the earth is in the the earth is in the moonmoon’’s shadows shadow

Why donWhy don’’t we have an eclipse every month?t we have an eclipse every month?