Motion of the Moon

Lecture 5

3-1 Why we see the Moon go through phases

3-2 Why we always see the same side of the Moon

3-3 The differences between lunar and solar eclipses

3-4 Why not all lunar eclipses are total eclipses

lunar phases = different appearances of the Moon

What is causing the phase of the Moon?

Common mis-conception = shadow of the Earth

Phase of the Moon

On its way to Jupiter, the Galileo spacecraft took a picture of Earth + Moon. In this image, Earth and Moon showed the same phase.

if the shadow of the Earth was causing the phase of the Moon, why the Earth also showed the same phase?

Image taken by Galileo spacecraft in 1992

Why the Moon goes through phases

relationship b/w lunar phases and Moon’s positions in its orbit

Have you ever noticed that, on a clear dark night, you can not only see the bright-side of the Moon but also the dark-side of the Moon?

What is this?

Because the Earth shines the un-illuminated side of the Moon known as Earthshine.

Earthshine

Moon’s synchronous rotation

Moon’s synchronous rotation

sidereal month : time takes the Moon to complete one full orbit of Earth w.r.t. distant stars (27.32 days)

synodic month : time takes the Moon to complete one cycle of phases (29.53 days)

Sidereal and Synodic Months

When Sun, Earth, and Moon are all aligned to a straight line, an eclipse happens.

Earth-Moon-Sun solar eclipse Moon blocks the Sun

(always new Moon)

Moon-Earth-Sun lunar eclipse Shadow of Earth blocks the Moon

(always full Moon)

Why aren’t there lunar/solar eclipses every synodic months?

Eclipses

Due to the changing apparent sizes of the Sun and Moon, we see different types of eclipses (especially solar eclipses). these changing angular diameters are due to elliptical orbits of the Earth and Moon.

Sun’s angular diameter : 31.’6—32.’7Moon’s angular diameter : 29.’3—34.’1

Different eclipses

Conditions for Eclipses

Total Lunar Eclipse Nine photographs over 3 hours during the total lunar eclipse of Jan 20, 2000. Lunar eclipses can be seen nearly all across the world (why?)

Three types of lunar eclipses

In summary…

Important ConceptsMoon’s phase and time if its

visibilities relative to the SunSynchronous rotation of the MoonMechanism of eclipse

Important Termslunar phasesidereal monthsynodic monthEarthshineline of nodesumbra and penumbra

Chapter/sections covered in this lecture : sections 3-1 through 3-4