solar eclipses how, why, where, and when solar an lunar eclipses occur

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:ssLongest annular eclipse – 12:30Longest eclipses in the 20th century – 3

– June 1937 – 7:04

– June 1955 – 7:08

– June 1973 – 7:04

Number of total eclipses longer than 7 minutes in the 21st 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

29.53 days

Draconic Month

Moon – ascending node to asc. node

27.21 days

Tropical Year Earth’s seasons 365.26 days

Eclipse Season Eclipse is possible – moon at node

30-37 days long

Eclipse Year 2 eclipse seasons 346.62 days

Moving Nodes

Because the eclipse year (346.62 days) is shorter than the tropical year (365.26 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 27.55455 days 6585.54 days

223 Synodic Months 29.5306 days 6,585.32 days

19 Eclipse years 346.6201 days 6,585.78 days

6,585 days = 18 years and 11 daysThe difference in timing (partial days)

cause the pattern to move westward 1/3 of the Earth’s surface for each repetition.

20th Century Saros 136 Eclipses

Saros Development

Each saros starts with a very brief and partial eclipse of the sun.

In 6,585.32 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

Tritos Eclipse returns with a different type.(10y, 334.21 d)

10 years, 334 days

Inex Eclipse 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

http://www.mreclipse.com, Fred Espenak http://sunearth.gsfc.nasa.gov, Fred Espenakhttp://www.spaceweather.com NASAhttp://www.earthview.com Bryan Brewerhttp://science.nasa.gov/ NASA