Phys141 Principles of Physical Science

Chapter 17 The Moon

Instructor: Li Ma

Office: NBC 126Phone: (713) 313-7028Email: malx@tsu.edu

Webpage: http://itscience.tsu.edu/ma

Department of Computer Science & PhysicsTexas Southern University, Houston

Nov. 22, 2004

Topics To Be Discussed

General Features Lunar Motions Phases of the Moon Skip

– §17.2 Composition and History of the Moon– §17.5 Eclipses– §17.6 Ocean Tides

About the Moon

The Moon is related to the time measurement– The length of our present month is based on the

motion and phases of the Moon

The Moon appears as the second-brightest object in the sky because it is very close to us– The average distance between the Earth and the

Moon is 384,000 km

General Features of the Moon

The Moon revolves around the Earth in approximately 29.5 solar days

Because it rotates at the same rate as it revolves around the Earth, we see only one side of the Moon

So on the Moon– You can always see the Earth if you stand on the

side facing the Earth– The Sun appears to rise and set once every 29.5

days

General Features of the Moon (cont)

The nearly spherical Moon has a diameter of 3476 km, slightly greater than one-fourth of the Earth’s diameter

The surface gravity of the Moon is only one-sixth that of the Earth

The Moon does not possess a detectible magnetic field

Its phases and the appearance of its surface are the Moon’s predominant features

General Features of the Moon (cont)

The surface of the Moon is marked with craters, basins, plains, rays, rills, mountain ranges, and faults

These features vary in size, shape, and structure

The most outstanding of them are the craters that are clearly visible to an Earth’s observer with even a telescope

Lunar Motions

The Moon revolves eastward around the Earth in an elliptical orbit

There are two different months– The period of the Moon with respect to the Sun is

a little over 29.5 days – synodic month, or the month of the phases

– The period of the Moon with respect to a star other than the Sun is approximately 27.33 days – sidereal month, the actual time it takes for the Moon to revolve 360°

Lunar Motions (cont)

To an observer on the Earth, the Moon appears to rise in the east and set in the west each day.

This apparent motion of the Moon is due to the Earth’s completing one revolution each day

Phases of the Moon

The most outstanding feature is the periodic change in its appearance– A new moon: the illuminated half is all hidden– A full moon: the illuminated half is all presented– A crescent moon: less than one-quarter of the

Moon’s surface appears illuminated– A gibbous moon: more than one-quarter of the

Moon’s surface illuminated

Phases of the Moon (cont)

The new phase of the Moon occurs when the Earth, Sun, and Moon are in the same plane, with the Moon positioned between the Sun and Earth

The Moon is in first-quarter phase when it is 90° east of the Sun

When the Moon is 270° east of the Sun, it will be in the last-quarter phase

Phases of the Moon (cont)

When the Moon is 180° east of the Sun, it will be in full phase and will appear as a full moon to the Earthbound observer

The waxing phase means that the illuminated portion of the Moon is getting larger

The waning phase means that the illuminated portion of the Moon is getting smaller

Assignment

Homework Assignment– Review Questions (page 455 – 456):

2,5,6,24,28,29,32,36

– Exercises (page 457): 2

– It’s due Monday, 12/6/04

Reading Assignment– Chapter 18