chapter 4.7; eclipses of the Sun and Moon

Sun vs Moon

Sun and Moon appear to be the same size from Earth

• Sun is 400 times larger in diameter than the Moon

• Sun is 400 times farther away

  • so, both the Sun and Moon have the same angular size (about ½ degree)

eclipse

occurs whenever any part of either Earth or the Moon enters the shadow of the other

solar eclipse

when the Moon’s shadow strikes Earth, people within that shadow see the Sun at least partially covered by the Moon

lunar eclipse

when the Moon passes into the shadow of Earth, people on the night side of Earth see the Moon darken

shadows of Earth and the Moon

2 parts:

• umbra: a cone where the shadow is darkest

• penumbra: lighter, more diffuse region of darkness

eclipse season

when the two paths of the Moon and Sun cross

• eclipses are possible during this point

annular eclipse

an eclipse of the sun in which the edge of the sun remains visible as a bright ring around the moon

total solar eclipse

if an eclipse of the Sun occurs when the Moon is somewhat nearer than its average distance, the Moon can completely hide the Sun

• occurs at those times when the umbra of the moon’s shadow reaches the surface of Earth

• if the Sun and Moon are properly aligned, then the Moon’s darkest shadow intersects the group at a small point on Earth’s surface, anyone on Earth within the small area covered by the tip of the Moon’s shadow will, for a few minutes, be unable to see the Sun and will witness a total eclipse

  • at the same time, observers on a larger area of earth’s surface who are in the penumbra will se only a part of the Sun eclipsed by the Moon (partial solar eclipse)

• duration will never exceed approx. 7 minutes

  • tip of the Moon’s shadow sweeps eastward at about 1500km/h

eclipse path

the thin zone across Earth within which a total solar eclipse is visible

• within a region about 3000km on either side of the eclipse path, a partial eclipse is visible

appearance of a total eclipse

• A solar eclipse starts when the Moon just begins to silhouette itself against the edge of the Sun’s disk

• A partial phase follows, during which more and more of the Sun is covered by the Moon

• About an hour after the eclipse begins, the Sun becomes completely hidden behind the Moon

• In the few minutes immediately before this period of totality begins, the sky noticeably darkens, some flowers close up, and chickens may go to roost

• As an eerie twilight suddenly descends during the day, other animals (and people) may get disoriented

• During totality, the sky is dark enough that planets become visible in the sky, and usually the brighter stars do as well

• Gradually, the partial phases of the eclipse repeat themselves, in reverse order, until the Moon has completely uncovered the Sun

corona

the Sun’s outer atmosphere, consisting of sparse gases that extend for millions of miles in all directions from the apparent surface of the Sun

• It is ordinarily not visible because the light of the corona is feeble compared with the light from the underlying layers of the Sun

• Only when the brilliant glare from the Sun’s visible disk is blotted out by the Moon during a total eclipse is the pearly white corona visible

lunar eclipse

occurs when the Moon enters the shadow of Earth

• Earth’s shadow: 1.4 million km long

• visible to everyone who can see the Moon

• An eclipse of the Moon is total only if the Moon’s path carries it though Earth’s umbra

• can take place ONLY when the Sun, Earth, and Moon are in a line

• The total duration of the eclipse depends on how closely the Moon’s path approaches the axis of the shadow

• occur every 2-3 years