Eclipses
Lining Up
• At new moon the moon can block the sun.
• At full moon the earth can block the sun to the moon.
• This is called an eclipse.– Solar eclipse, lunar eclipse
Orbital Inclination
• The Moon has an orbit tilted at 5.2° compared to the orbit of the Earth around the Sun.
• When the Moon crosses the Sun-Earth plane, it is at a node.
• The period between two ascending nodes is 27.21 days
– This is the draconic month.
Eclipse Seasons
• Conditions for a solar eclipse:
– New Moon (once every 29.5 days)
– Moon at a node (twice every 27.2 days)
• The circumstances happen about every 6 months.
– May 10, 1994: Annular eclipse in N. America
– Nov 3, 1994: Total eclipse in S. America
• Eclipse season lasts about one month.
Kinds of Eclipses
Solar Eclipse– Moon’s shadow on Earth
– Occurs at a new moon
Partial - crescent Sun
Total - all of Sun is blocked
Annular - a ring of the Sun
• Lunar Eclipse– Earth’s shadow on moon
– Occurs at a full moon
Partial - moon partially dark
Total - moon is dark red or copper-colored
Earths atmosphere passes some light.
Viewing Eclipses
• Never look directly at the sun, even at a partial eclipse.
• It’s ok to look at lunar and true total eclipses.
• View sun through #14 welder’s glass or special mylar filters.
Total Solar Eclipse
• August 11, 1999: Szombately, Hungary
Photographs by Michael Fortner
Total Lunar Eclipse
• April 15, 2014: West Chicago, Illinois
Photograph by Michael Fortner
Lunar Orbit
• The Moon has a synodic period of 29.53 days.
– the time from new moon to new moon.
• The Moon’s orbit is an ellipse.
– At the nearest, perigee, the distance is 363,000 km
– At the farthest, apogee, the distance is 406,000 km
• The period between perigees is 27.55 days.
– This is the anomalistic period.
Try It!
• Close one eye.
• Hold a coin out far enough to completely block the circle.
• Move out a little for an annular eclipse.
Annular Eclipse
• May 10, 1994, North Central United States
Photograph by John Chumack
Saros Cycle
• Convergence of cycles
– 223 synodic months is 6585.3 days
– So is 242 draconic months, and 239 anomalistic months
• Every 18 years, 11 days, and 8 hours the Earth, Moon and Sun are in the same position.
• These cycles were known since antiquity, and used to predict eclipses.
Upcoming Eclipses
• 2014– Lunar eclipse
Oct 8• Greatest at
5:54 am CDT
– Partial solar Oct 23
• 4:36 pm to 5:43 pm (sunset)
• August 21, 2017– Total solar eclipse