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Eclipse: In a Different Light

Sun-Earth Day Presents: Eclipse, In a Different Light

Questions Frequently Asked by the Public About Eclipses

When can I see the next TOTAL solar eclipse from North America?

The NASA Eclipse site http://sunearth.gsfc.nasa.gov/eclipse/solar.html will show you the eclipse nearest your location. The next TOTAL solar eclipse in North America occurs on August 1, 2008 but it will only be visible from northern Canada (Nunavut). The next TOTAL solar eclipse visible from the United States occurs on August 21, 2017. The track goes from Washington State, and exits on the east coast near the Carolinas. In the mean time there will be lots of LUNAR eclipses to entertain us!

What causes an eclipse to occur?

Eclipses occur due to the special coincidence of the moon and the Sun being the same angular size. The Sun is 400 times wider than the moon, but it is also 400 times farther away, so they coincidentally appear to be the same size in our sky. This is what allows us the phenomenal beauty of the total solar eclipse. (Note: You can give the audience the experience of the change in apparent size of an object close by and the same object farther away. They can use their hands to measure angular size.)

Why don't solar eclipses occur exactly at noon?

Because the geometry required for a total solar eclipse has nothing to do with local noon. It has to do with when the lunar shadow sweeps across your location during the time when the Sun is above the horizon. Even so, it is possible for the Sun to be in full eclipse before it rises at your particular location.

Why don't eclipses occur every new moon?

Eclipses only occur if the Moon is located within 0.5 degrees of the plane of the ecliptic, on a line that passes through the center of the Sun and the Earth. The Moon travels along an orbit that is inclined by 5 degrees to the ecliptic plane, so there are only two opportunities each month when it passes through the plane of the ecliptic. These points are called the ascending and descending nodes. Eclipses of the Sun only occur if new moon occurs when the Moon is within about 18 of one of these nodes.

A similar argument explains why lunar eclipses do not occur every full moon at the node opposite the Sun from the Earth

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When was the last solar eclipse seen from North America, and when will the next one happen?

A partial solar eclipse was seen from the southern United States on April 8, 2005. The next solar eclipse that can be seen in the United States will be on May 20, 2012. It will be an annular solar eclipse. The last annular eclipse visible from the USA was on May 10, 1994. Similar eclipses occur 18 years apart in the Saros Cycle.

Do lunar eclipses only happen at the descending node of the lunar orbit?

There are two locations where eclipses can occur. These are the points in the lunar orbit that intersect the ecliptic plane where the Sun moves in the sky. These are called the ascending node and the descending node. The Sun crosses the ascending node on the Vernal Equinox. It crosses the descending node on the Autumnal Equinox. Lunar eclipses can occur at either node. However, the Moon must be in the full moon phase as it passes the node in order for a lunar eclipse to occur.

Why do eclipse tracks move eastward even though the Earth rotates from west to east?

Because the Moon moves to the east in its orbit at 3,400 km/hour. Earth rotates to the east at 1,670 km/hr at the equator, so the lunar shadow moves to the east at 1,730 km/hr near the equator. You cannot keep up with the shadow of the eclipse unless you traveled at Mach 1.5.

When were solar eclipses first predicted accurately?

The Babylonians knew how to predict lunar eclipses with some accuracy, but solar eclipses are far more difficult because the 'footprint' on the earth is only a few tens of miles across and requires arc minute positional accuracy and forecasting for any specific locale. Apparently Thales, c. 610 B.C., is credited with predicting a solar eclipse from knowledge of a previous eclipse and using the Saros cycle. He predicted the year, but not the month and the day. It wasn't until Ptolemy's time that solar eclipse forecasting became more accurate.

What are "shadow bands?"

These are among the most ephemeral phenomena that observers see during the few minutes before and after a total solar eclipse. They appear as a multitude of faint rapidly moving bands that can be seen by placing a white sheet of paper several feet square on the ground. They look like ripples of sunshine at the bottom of a swimming pool, and their visibility varies from eclipse to eclipse. 19th century observers interpreted them as interference fringes caused by some kind of diffraction phenomenon. The Sun, however, is hardly a "point source" and the patterns are more random than you might expect from diffraction effects.

The simplest explanation is that they arise from atmospheric turbulence. When light rays pass through eddies in the atmosphere, they are refracted. Unresolved distant sources simply "twinkle," but for nearby large objects, the incoming light can be split into interfering bundles that recombine on the ground to give mottled patterns of light and dark bands, or portions of bands. Near totality, the image of the Sun is only a thin crescent a few arc seconds wide, which is about the same size as the atmospheric eddies as seen from the ground. Bands are produced because the Sun's image is longer in one direction than another. The bands move, not at the rate you would expect for the eclipse, but at a speed determined by the motion of the atmospheric eddies.

How long will we continue to be able to see total eclipses of the Sun?

The orbit of the Moon is not stable. Because of tidal friction, the orbit of the Moon is steadily growing larger, so that the angular size of the Moon from the Earth is shrinking.

The Moon's orbit is increasing by about 3.8 cm (1.5 inches) per year. When the Moon's mean distance from Earth has increased an additional 14,600 miles, it will be too far away to completely cover the Sun. This is true even at perigee when its disk will be smaller than the Sun's disk even at perihelion. At the current rate that the Moon's orbit is increasing, it will take over 600 million years for the last total eclipse to occur. A complicating factor is that the size of the Sun itself will grow slightly during this time, which will act to make the time of "no more total eclipses" a bit sooner than 600 million years.

What happens more often, solar or lunar eclipses?

According to Fred Whipple's book "Earth, Moon and Planets," page 102-104, solar eclipses are fairly numerous, about 2-5 per year, but the area on the ground covered by totality is only a few miles wide. In any given location on Earth, a total eclipse happens only once every 360 years. Eclipses of the Moon by the Earth's shadow are actually less numerous than solar eclipses; however, each lunar eclipse is visible from over half the Earth. At any given location, you can have up to three lunar eclipses per year, but some years there may be none. In any one calendar year, the maximum number of eclipses is four solar and three lunar.

Typically, how big a temperature drop do you get during a total solar eclipse?

It would probably be equal to the typical daytime minus nighttime temperature difference at that time of year and location on the Earth. It would be modified a bit by the fact that it only lasts a few minutes, which means the environment would not have had much time to thermally respond to its lowest temperature, so it would probably only be 3/4 or 1/2 the maximum day-night temperature difference. Because the patch of the shadow travels faster than the speed of sound, weather systems will only be affected very locally directly under the instantaneous footprint of the eclipse. The main effect is in the "radiant heating" component which goes away suddenly at the moment of eclipse and produces a very fast temperature decrease. If the wind is blowing, your body probably exaggerates, by evaporative cooling, how large the actual temperature swing actually is.

How well are the ground tracks for solar eclipses known in advance of the event?

The positions of the Sun and Moon are known to better than 1 arc second accuracy. This means that on the Earth, the location of the track of totality is probably known to about (1.0/206265.0) x 2 x pi x 6400 km = 0.19 kilometers or a few hundred meters at the Earth's equator.

Is there a book that shows the solar eclipse tracks going back a few hundred years?

  • "Atlas of Historical Eclipse Maps for East Asia 1500BC to 1900 AD" by F.R. Stephenson and M.A. Houlden, (Cambridge University Press) 1986.
  • "Canon of Eclipses" by Theodor Oppolzer, translated by Owen Gingerich in 1962. (Dover Books, New York).
  • "Canon of Solar Eclipses" by Jean Meeus and Hermann Mucke, (Astronomiches Buro, 1983) Vienna Austria, second edition.

How do computers predict eclipses?

Astronomers first have to work out the geometry and mechanics of how the Earth and Moon orbit the Sun under the influences of the gravitational fields of these three bodies. From Newton's laws of motion, they mathematically work out the motions of these bodies in three-dimensional space, taking into account the fact that these bodies have finite size and are not perfect spheres, and that the Earth and Moon are not homogeneous bodies. From careful observation, they then feed into these complex equations the current positions and speeds of the Earth and Moon, and then program the computer to "integrate" these equations forward or backward in time to construct ephemerides of the relative positions of the Moon and Sun as seen from the vantage point of the Earth. Eclipses are specific configurations of these bodies that can be identified by the computer. Current eclipse forecasts are accurate to less than a minute in time over a span of hundreds of years.

Do lunar and solar eclipses have any noticeable effect on humans?

There is no evidence that eclipses have any physical effect on humans. However, eclipses have always been capable of producing profound psychological effects. For millennia, solar eclipses have been interpreted as portents of doom by virtually every known civilization. These have stimulated responses that run the gamut from human sacrifices to feelings of awe and bewilderment. Although there are no direct physical effects involving known forces, the consequences of the induced human psychological states have led to physical effects.

Other facts of interest:

  • The longest duration for a total solar eclipse is about 7.5 minutes.
  • A total solar eclipse is not noticable until the Sun is more than 90 percent covered by the Moon. At 99 percent coverage, daytime lighting resembles local twilight.
  • Eclipse shadows travel at 1,100 miles per hour at the equator and up to 5,000 miles per hour near the poles.
  • The width of the Moon's shadow is at most 170 miles wide.
  • The maximum number of solar eclipses (partial, annular, or total) is 5 per year.
  • There are at least 2 solar eclipses per year somewhere on the Earth.
  • A total eclipse can only happen during a new moon.
  • Total solar eclipses happen about once every year or two.
  • Nearly identical eclipses (total, annual, or partial) occur after 18 years and 11 days, or every 6,585.32 days (Saros Cycle).
  • From the Earth's surface, the Sun's corona ("crown") can ONLY be seen during a total eclipse.
  • Every eclipse begins at sunrise at some point in its track and ends at sunset about half way around the world from the start point.
  • Partial solar eclipses can be seen 2,000 to 3,000 miles from the track of totality.
  • Before the advent of modern atomic clocks, studies of ancient records of solar eclipses allowed astronomers to detect a 0.001 second per century slowing down in Earth's rotation.
  • Total solar eclipses happen because the Sun is near one of the nodes of the lunar orbit, and the Moon is near perigee at this node at the same time.
  • Annular solar eclipses happen because the Sun is near one of the nodes of the lunar orbit, and the Moon is near apogee at this node at the same time.
  • Shadow bands are often seen on the ground as totality approaches.
  • Light filtering through leaves on trees casts crescent shadows as totality approaches.
  • Local animals and birds often prepare for sleep or behave confusedly during totality.
  • Local temperatures can drop as much as 20 degrees during a total solar eclipse.
  • During totality, the horizon is illuminated in a narrow band of light, because an observer is seeing distant localities not under the direct umbra of the Moon's shadow.
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