Eclipses and the Lunar Cycle

! How much of the Moon’s surface is illuminated by the Sun at any time of the month? Why do we see Moon phases? What are waxing, waning, gibbous, crescent, first and third quarter?

! Does the Moon have a dark side? A far side?! Can you draw the geometry of lunar and solar eclipses?

When would we see annular, penumbral, partial and total eclipses? Why don’t eclipses occur every month?! Can you draw a diagram that shows at what time the

different Moon phases rise, cross meridian and set?! Note the [xtra] beside lectures 1-7 on the schedule.

These may be helpful to learn difficult concepts

Learning Objectives

! As the Moon orbits the Earth, we see a cycle of phases

! The Moon’s phases depend on how much of the sun-lit Moon we see

! Not caused by the shadow of the Earth (Not to scale)

The Cycle of Phases

Lunar Phases - Terms! Waxing: Becoming Fuller! Waning: Becoming Newer (less Full)

! Crescent: croissant-shaped (the Moon appears to be less than half lit)

! Gibbous: “rounded” (the Moon appears to be more than half lit)

! First Quarter: The Moon’s orbit is one-quarter complete (the right-half of the Moon appears to be lit as viewed from the Earth’s Northern Hemisphere)

! Third Quarter: The Moon’s orbit is three-quarters complete (the left-half of the Moon appears to be lit as viewed from the Earth’s Northern Hemisphere)

Waxing crescent

The Face of the Moon! From the Earth, we only see one face of the Moon! Does this mean the Moon doesn’t rotate?! No…the Moon rotates so that the same face is

always pointed towards the Earth! A lunar day equals a lunar orbit! There is no dark side of the Moon!

No rotation Rotational period= Orbital period

Basics of Eclipses! Lunar Eclipse

!When the Moon passes into the Earth's shadow

!Sun – Earth – Moon happen at Full Moon

! Solar Eclipse!When the Earth crosses

the Moon's shadow!Sun – Moon – Earth

happen at New Moon! Why don’t eclipses

happen every Full and New moon?

The Moon’s Orbit is Tilted!! The Moon’s orbit is tilted to the ecliptic by 5º

! The Moon appears to be about ½o in angular size in the sky, so 5o is ~10 Moon diameters

! The Moon must also be near the ecliptic for an eclipse to occur

Lunar Eclipses

Umbra? Penumbra? Latin for “complete

shadow” and “partial shadow”

Red Moon! During a total lunar eclipse, the Moon

turns a blood-red/burnt orange color

! The red color is caused by sunlight diffused through the Earth's atmosphere

! The atmosphere scatters blue light more, so the Moon is dimly illuminated in red

Solar Eclipses! The Earth

passes into the Moon’s shadow

! Occur roughly twice a year

! The Moon’s shadow does not cover the entire Earth

! A solar eclipse will only be visible on part of the Earth

Solar Eclipses! Three types

!Partial – when the Moon only partially blocks the Sun

!Total – when the Moon completely blocks the Sun

!Annular – when the Moon appears too small to completely block the Sun

WY Solar Eclipse: 8/21/2017

perigee apogee

More on the Lunar Orbit

!The Moon's orbit around the Earth is not a perfect circle!The distance

from the Earth to the Moon varies by about 10%

!This makes a difference to the Moon’s size in the sky

!This causes annular eclipses

Important bad drawingMoon phases, eclipses, what Moon

phase is up at what time of day

Next Time

Kepler’s Laws, or Ne Frustra Vixisse Videar