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Eclipses. Edward M. Murphy Space Science for Teachers 2005. Eclipses. Although the Sun is about 400 times larger than the Moon, it is also about 400 times further away. Both appear to be about ½ degree across.

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Edward M. Murphy

Space Science for Teachers



  • Although the Sun is about 400 times larger than the Moon, it is also about 400 times further away.
    • Both appear to be about ½ degree across.
    • As a result of this coincidence, the Moon can completely cover the Sun, producing a total solar eclipse.


  • The shadow of any body consists of two parts:
    • The umbra is where the Sun is completely blocked from view.
    • The penumbra is where the Sun is only partially blocked.


solar eclipse
Solar Eclipse
  • Under the most favorable conditions, the diameter of the shadow cone is 269 km at the Earth’s surface.
    • At the equator, the shadow moves at 1730 km/hr.
    • Totality can last as long as 7½ minutes.
  • A total solar eclipse occurs about once every 18 months somewhere in the world.
  • At any given location, a total solar eclipse occurs once every 360 years.
    • The next total solar eclipse in the U.S. is on Aug. 21 2017.


total solar eclipse2
Total Solar Eclipse
  • As the Moon orbits the Earth, its apparent size varies.
    • Only when the Moon is closest to the Earth can it completely block the Sun.
    • If the Moon does not completely cover the Sun we get an annular eclipse.


frequency of solar eclipses
Frequency of Solar Eclipses
  • We do not get solar eclipses at every new moon because the orbit of the Moon is tilted by about 5 degrees with respect to the ecliptic.
    • At most new moons the Moon is either above or below the Sun.
    • The tilted orbit of the Moon crosses the ecliptic at two points. When the Sun approaches these points, eclipses are possible. This will happen twice a year.


lunar eclipses
Lunar Eclipses
  • A lunar eclipse occurs when the Moon passes into the shadow of the Earth.
  • The lunar eclipse is visible to everyone on the night side of the Earth.
  • Aristotle (384-322 B.C.) used the apparent shape of the Earth’s shadow to show that the Earth is a sphere.


nasa eclipse page
NASA Eclipse Page
  • The best web page on eclipses is: