Solar Eclipses How, why, where, and when solar an lunar eclipses occur.

Eclipse Basics The Sun, Moon, and Earth line up so a shadow is cast.

Eclipse Path The rotation of the Earth and the orbit of the Moon cause the eclipse shadow to follow a linear path on Earth.

Eclipse Duration  Longest total eclipse – 7:31 m:ss  Longest annular eclipse – 12:30  Longest eclipses in the 20 th century – 3 –June 1937 – 7:04 –June 1955 – 7:08 –June 1973 – 7:04  Number of total eclipses longer than 7 minutes in the 21 st century – 0

Angular Size  Win a few friendly bets – Ask a friend:  What matches the size of the moon at arms length? A pea a grape, an orange or a dime?  What is the size of the sun? Most astronomers know that the sun & moon are about ½ a degree in angular size. Many people, including astronomers are astounded at just how small that is.

Amazing Coincidences  PAST: –The moon was closer to the Earth than it is now. –The moon’s shadow would have been wider and the prominences would not have been visible.  Present: –If the Moon were only 161 miles smaller we could never see a total solar eclipse. –If the Moon had a circular orbit at its current average, we could not have total solar eclipses.  Future: –The moon will continue to move away from the Earth. –In the distant future, no total eclipses will be visible.

Eclipse Types – Total Eclipses

Eclipse Types – Annular Eclipses

Eclipse Types – Combination Eclipse

Eclipse Effects – Baily's beads Named after Francis Baily, the 18th century English amateur astronomer who was the first to draw attention to them. The beads are actually the last few rays of sunlight shining through valleys on the edge of the Moon. Baily's beads make their brief appearance up to 15 seconds before totality.

Eclipse Effects – Shadow Bands As totality approaches, thin wavy lines of shadows appear. These shadow bands are the result of sunlight being distorted by irregularities in the Earth's atmosphere.

Eclipse Effects – Diamond Ring The diamond ring effect is the last bit of surface brilliance showing through a lunar valley. This effect lasts for only a few seconds. Once the diamond ring disappears, it is safe to view the sun without a filter.

Eclipse Effects - Corona As the moon fully covers the sun and blocks the brilliance of the surface, viewers can see the outer atmosphere, called the corona.

Why don’t we have monthly Solar & Lunar eclipses?  The moon’s orbit is inclined 5° to Earth’s orbit.  At two points (nodes) in the Moon’s orbit, the Sun, Moon, and Earth line up for an eclipse.

Eclipse Mechanics - Nodes An eclipse can happen only within 15.33° of the node, for a total of 30.66° of an eclipse season. Traveling at 1° a day, the sun is in the danger zone for about 30 days. Add up to 7 days for differences in angular sizes of the moon and sun.

Eclipse Mechanics - Numbers  One solar eclipse must occur each eclipse season, giving two eclipses per year.*  A lunar eclipse follows a solar eclipse by about 14 days. Most are penumbral and are not visible to the naked eye.  If an eclipse happens one or before Jan 18, you could have five eclipses in a single year – –Solar eclipses in Jan, July, and Dec –Lunar eclipses in Jan and July. * Both may be partial.

Orbital Rhythms – Eclipse Seasons Synodic Month (Lunation) Moon phases from full moon-full moon days Draconic Month Moon – ascending node to asc. node days Tropical YearEarth’s seasons days Eclipse SeasonEclipse is possible – moon at node days long Eclipse Year2 eclipse seasons days

Moving Nodes Because the eclipse year ( days) is shorter than the tropical year ( days), the eclipse season arrives 20 days earlier each year.

What’s a Saros?  Eclipses repeat in patterns, bringing similar sets of eclipses to similar regions. 239 Anomalistic Months days days 223 Synodic Months days6, days 19 Eclipse years days6, days  6,585 days = 18 years and 11 days  The difference in timing (partial days) cause the pattern to move westward 1/3 of the Earth’s surface for each repetition.

20 th Century Saros 136 Eclipses

Saros Development  Each saros starts with a very brief and partial eclipse of the sun.  In 6, day cycles, a repetition of the eclipse (displaced) brings larger partial eclipses until the moon crosses the center.  The pattern repeats, except it’s now fading.  After 13,000 years, the moon no longer eclipses the sun on the predicted date.

Orbital Rhythms – Long period TritosEclipse returns with a different type. (10y, d) 10 years, 334 days InexEclipse returns with different type. (Less accurate than the others) 28 years, 345 days Exeligmos (triple saros) Same longitude, 600 miles north or south 54 years, 34 days

Resources  “Totality: Eclipses of the Sun” 1991, Littman and Willcox  Fred Espenak  Fred Espenak  NASA  Bryan Brewer  NASA