earth-moon motion

Shara Alyssa A. Tolentino

IV-24 BSE Mathematics

Michael Jackson's signature’ moonwalk’ is

the most recognized dance in the world.

Why is the same side of

the moon always faces

Earth?

At any position, half

of the Moon is

illuminated by the

Sun (the light side of

the Moon) and half

is not (the dark

side).

The Sun is off in the

distance, lighting the

Earth-Moon system.

The Sun is off in the

distance, lighting the

Earth-Moon system.

One side or “face” of

the Moon is ever visible

from Earth.

The “far side” of the

Moon is permanently

turned away.

Also, half of the Moon

is visible to the Earth

(the near side of the

Moon) and half is not

(the far side).

As the Moon moves

around the Earth, we

can see different

fractions of the

illuminated half of the

Moon.

In other words, it takes the Moon the

same amount of time to rotate around

once as it does for the Moon to go around

the Earth once.

Because of the effect on the Moon of tidal

forces due to the Earth, the same side of

the moon always faces the Earth.

Therefore, Earth-bound observers can

never see the 'far-side' of the Moon.

Tidal forces cause many of the moons of

our solar system to have this type of

orbit.

<This image shows the Earth and the

moon, and the bulges produced on

each side from the tides.>

<This image shows the Earth and the

moon, and the bulges produced on

each side from the tides.>

<This image shows the Earth and the

moon, and the bulges produced on

each side from the tides.>

The crescent Moon, Venus, Jupiter and Spica

as seen near Quebec on September 6, 2005.

(Image Credits: Quebec, Jay Ouellet, apod050909; Los

Angeles, Sheri Seligman)

The crescent Moon, Venus, and Jupiter as

seen near Los Angeles on September 6,

2005

(Image Credits: Quebec, Jay Ouellet, apod050909; Los

Angeles, Sheri Seligman)

During the half orbit that it is

approaching us, our mutual gravitational

pull accelerates the Moon,

its angular speed among the stars will appear to decrease by about 12% of its average speed, half of that change being due to its greater distance, and half being due to an actual decrease in speed.

Since 12% of 13.2 degrees per day is

1.6 degrees per day, the daily motion

of the Moon to the east can vary from

as little as 11.6 degrees per day near

apogee to as much as 14.8 degrees

per day near perigee.

the apparent

size of the

Moon at

apogee (on

the left).

the apparent

size of the

Moon at

perigee (on

the right).

(The change in size is not so obvious

when the time between the extremes is

two weeks, as it is when images are

placed side by side.)

the apparent

size of the

Moon at

apogee (on

the left).

the apparent

size of the

Moon at

perigee (on

the right).

When at apogee the Moon will appear to

move less than 12 degrees per day to the

East among the stars, whereas at

perigee it will appear to move nearly 15

degrees per day.