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