The 2017 Eclipse

The Moon’s Cycle of Phases• 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!)

Basics of Eclipses• Lunar Eclipse

• When the Moon passes into the Earth's shadow

• Sun – Earth – MoonFull Moon

• Solar Eclipse• When the Earth crosses

the Moon's shadow• Sun – Moon – Earth

New Moon• Why don’t eclipses happen

every Full and New Moon?

The Moon’s Orbit is Tilted• The Moon’s orbit is tilted by 5º to the Earth’s

orbital plane • The Earth’s orbital plane is called the ecliptic

• 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

Solar Eclipses• Earth passes into

the Moon’s shadow• Any type of solar

eclipse occurs roughly twice a year• Total eclipses occur

only once every 1.5years (somewhere on the Earth)

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

• A solar eclipse will only be visible on a small part of the Earth

The path of a Total Eclipse is only about 170

miles wide!

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

Total vs. Partial Solar Eclipses

WY Total Solar Eclipse 2017

perigee apogee

More on the Lunar Orbit• The Moon's orbit

around the Earth is not a perfect circle• Distance from the

Earth to the Moon varies by ~10%

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

• This causes annular (i.e. ring) eclipses• This makes total eclipses even rarer • So, Casper in 2017 is pretty special

What’s so Special About the 2017 Eclipse?

• First solar eclipse visible in the continental US since 1979• It will Only Last About 2.5 Minutes!• Moon’s Shadow Will Sweep Across the Entire Continental US

(Oregon to S. Carolina) in 90 min.• Eclipse path crosses homes of 10 million people in the US

• Several million expected to travel to see the event first-hand• 300 - 500 million people expected to see it either first hand or on

the internet• Potentially the most-viewed astronomical event in history

• Not Counting: Sunrises and Sunsets, Moon Phases etc.• Last Wyoming eclipses were on Jan. 1, 1889 and July 3,

1878

The 2017 Eclipse