Our Moon

Goals of this Lecture

• Understand the Lunar Phases

• Introduce tides and tidal forces

• Understand Lunar Eclipses

• Understand Solar Eclipses

Assigned Readingand Announcements

Chapter 3, all of it

Homework #1 will be posted today

Due in class on Thursday 18

Lunar Phases

Earth & MoonLike planets, the moon does not

emit its own right.

The Moon is ~1/4 of the Earth in diameter and ~1 percent in mass.

The rotation of the Moon and its revolution around the Earth are synchronized and take 27 days,7 hours, and 43 minutes.

The length of the Lunar orbit around Earth (Sidereal Period) is 27.32 days.

The length of the Lunar Phase (Synodic Period) is 29.53 days.

We only see one side of the moon.

The Phases of the Moon (1)• The Moon orbits Earth in a sidereal period of

27.32 days.

27.32 days

Earth

Moon

Fixed direction in space

The Phases of the Moon (2)

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

Fixed direction in space

EarthMoon

Earth orbits around Sun => Direction toward Sun changes!

29.53 days

One orbit in 27.32 daysOne rotation in 24 hours

Earth Rising on the Moon

Apollo 11

Survey QuestionIn this picture of the Earth rising, what is the

predominant direction of the sun in relation to the observer?

1) above the observer

2) below the observer

3) behind the observer

4) behind the Earth

How long does it take for the moon to rotate once?

• One day

• About 27 days

• Half a year

• One year

How long does it take for the moon to rotate once?

• One day

• About 27 days

• Half a year

• One year

Lunar and Solar Eclipses

Lunar, from “Luna”Solar, from “Sol”

Lunar Eclipse

Why is the shadow curved?Whose shadow is this?

Reminder: angular (apparent) diameter

• Angular diam / 206,265” = linear diam./distance

• NOTE: linear diam. and distance MUST be in the same units

• It so happens that both the Moon and the Sun have angular diameter of about 0.5 degree, the exact value at a given time depending on the exact distance.

Shadows

A Total Lunar Eclipse (1)

Motion of the moon against the background of stars

Motion of the moon against the horizon

A Total Lunar Eclipse (2)A total lunar eclipse can last up to 1 hour and 40 min.

During a total eclipse, the moon has a faint, red glow, reflecting sun light scattered in Earth’s atmosphere.

Partial lunar eclipse

During a lunar eclipse, what would you see from the moon?

Geometry of Solar Eclipses

Umbra (shadow): sun is completely blocked out.Penumbra (almost shadow): sun is partially blocked out.

Earth and Moon’s Orbits Are Slightly Elliptical

Sun

Earth

Moon

(Eccentricities greatly exaggerated!)

Perihelion = position closest to the sun

Aphelion = position furthest away from the sun

Perigee = position closest to Earth

Apogee = position furthest away from Earth

Annular Solar Eclipses

The angular sizes of the moon and the sun vary, depending on their distance from Earth.

When Earth is near perihelion and the moon is near apogee, we see an annular solar eclipse.Perig

eeApogee

Perihelion

Aphelion

Annular eclipse

Looking Back on an Eclipsed Earth; 1999 August 11 Credit: Mir Space Station Crew

Total Eclipse of June 21st 2001

The umbra is never more than 270 km in diameter and sweeps across Earth's surface at 1600 km/hr (1000 mph)!

Survey Question

The phase of the moon at a solar eclipse is?

a) new

b) 1st quarter

c) full

d) it depends on the year

How come there isn't an eclipse every full moon and new moon?

• The main reason is that the moon’s orbit is tilted from the earth’s orbit.

• A total eclipse from a given point on the surface of the Earth is not a common occurrence. For example, it will be two decades before the next total solar eclipse visible in North America occurs.

To sun

Effects of Moon on Earth: Tides

Spring Tides

Occur at every new and full moon

Neap tides

Occur at every first- and third-quarter moon

Earth's rotation slows down by 0.0023 s/100 years.Only 900 million years ago, Earth' day was 18 hrs long.The moon's orbit is growing larger by about 4 cm/yr.

Friction drags the tidal bulges eastward out of the direct earth-moon line

We see only one side of the moon, because the tidal friction has slowed the rotation of the moon so that its rotation and

revolution are synchronized

The Tidally-Locked Orbit of the Moon

The 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.

Solar day and Sidereal day

• The apparent solar day = the time from high noon to high noon.

• The average length of the apparent solar day = 24 hours

• Sidereal day = the time from a star to the same position.

• Because the sun moves in respect to stars, the sidereal day is different from the solar day.

The Length of the Solar Day

Earth revolves about 360o/365 ~ 1o around Sun

Earth rotates at a rate of 360o/(24x60)=0.25o/minute

So need an extra 1o/0.25o =4 minutes to complete a solar day