LIST OF QUESTIONS

What are 'shadow bands'?

These are among the most ephemeral phenomena that observers see during the few minutes before a total solar eclipse. They appear as a multitude of faint 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 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 simplist explanation is that they arise from atmospheric turbulence. When light rays pass through eddies in the atmosphere, they get refracted. Unresolved distant sources simply 'twinkle', but for nearby large objects, the incoming light can get 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 Earth-Moon system is unique in the solar system, because only for this system at the present time, does the angular size of the Moon match the angular size of the sun as seen from the surface of the Earth. This means that sometime during its orbit, the Moon can exactly cover the Sun, causing an observer to be thrown into an eery night time in the middle of the day!

But, 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 growing smaller. When we get to the point where the Moon only covers 98 percent of the Sun's disk, enough of the Sun will still be visible at totality, that you will not experience night time during a total eclipse.

The Sun has a diameter of 870,000 miles. At the present time, the Sun's angular diameter varies from 32.7 minutes of arc when the Earth is at its farthest point in its orbit (aphelion), and 31.6 arc minutes when it is at its closest (perihelion). The Moon on the other hand has a diameter of 3,476 kilometers, and varies in distance between 356,000 (perigee) and 406,000 kilometers (apogee). This means its angular size changes from 33.5 to 29.43 arc minutes. So, there is plenty of opportunity for the angular sizes of the Moon and Sun to be equal for a total eclipse.

But, the Moon's orbit is increasing by about a centimeter per year, so that when the Moon drifts about 20,200 kilometers further out from the Earth, the Moon will be so far away even at perigee, that its disk will be smaller than the Sun's disk even at perihelion. At a generous speed of 2 centimeters per year, it will take about 1 billion 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 earlier than 1 billion years hence.

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 eclipse covers about 1/2 the surface of the Earth. At any given location, you can have up to 3 lunar eclipses per year, but some years there may be none. In any one calendar year, the maximum number of eclipses is 4 solar and 3 lunar.

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

My guess would be that it would be equal to the typical daytime minus nighttime temperature difference at that time of the 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 foot print 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, just how large the actual temperature swing actually is.

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

Have a look at the Eclipse Paths page and find the eclipse nearest your location. My impression is that there will not be any total SOLAR eclipses visible from North America for a good many years. On August 21, 2017 there will be one whose track goes from Washingtion state, and exits on the east coast near the Carolinas. There will, however, be lots of LUNAR eclipses to entertain us!

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

I am not an expert in this area, but 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?

I have found three books that may be helpful if you can find them at your library. The research library at the Goddard Space Flight Center shows the following books:

"Atlas of historical eclipse maps for east asia 1500BC to 1900 AD" by F.R. Stephenson and M.A. Houlden, (Cambridge University Press) 1986. This is a book of maps of China showing the eclipse tracks, but includes no other geographic locations.

"Canon of Eclipses" by Theodor Oppolzer, translated by Owen Gingerich in 1962. (Dover Books, New York). This book is one long table showing where all of the solar eclipses are from 1207 BC to 2161 AD. You have to look up the month and year, and it gives the information you can use to plot the track of totality.

"Canon of Solar Eclipses" by Jean Meeus and Hermann Mucke, (Astronomiches Buro, 1983) Vienna Austria, second edition. I haven't had a chance to look at this book yet.

Other places to look are back issues of Sky and Telescope. There have been articles on solar eclipses showing their ground tracks, but I don't recall if they go back too far in time. If I come across any other references, I will post them in a revised version of this response in the future.

Oh yes, there is a website that shows plots of eclipses. See my answer to a previous question for its location.

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 millenia, 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.

Why don't eclipses occur every new moon?

Eclipses only occur if the satellite of a planet is located within 0.5 degrees of the plane of the Ecliptic, on a line which passes through the center of the Sun and the Earth. The Moon travels along an orbit which 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...called the ascending and decending nodes. These two points connected to the barycenter of the Earth- Moon system ( roughly the center of the Earth ) define a 'line of nodes', and eclipses of the Sun and Moon will occur if this line of nodes coincides with the line drawn between the center of the Earth and Sun. Again, the Moon also has to be within 0.5 degrees of one or the other of the nodes so that the disk of the Sun is partially or totally covered in a solar eclipse. A similar argument explains why lunar eclipses do not happen every full moon at the node opposite the Sun from the Earth.

How do computers predict eclipses?

Astronomers first have to work out the orbital 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 which can be identified in the computer runs and captured. Current eclipse forecasts are accurate to less than a minute in time over a time span of hundreds of years.

When was the last time a lunar or solar eclipse happened on the equinoxes?

I couldn't find any exact matches, but did find several close calls:

September 20 1960 partial solar
September 23 1987 total solar
September 22 2006 total solar
September 23 2033 partial solar
September 19 2043 lunar eclipse

March 23 1951 lunar eclipse
March 19 2007 partial solar
March 20 2015 total solar
March 25 2024 partial lunar
March 20 2034 total solar

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

May 10, 1994 was annular and seen in the United States. The next eclipse that can be seen in the United States will be on May 20, 2012, and will also be annular. Similar eclipses occur 18 years apart in the Saros Cycle.

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

I have not thought about this very much, but there are in principle 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 Ascending Node is the one located opposite of the Earth from the Sun and it is here that lunar eclipses occur. The Descending Node is between the Sun and the Earth and it is here that solar eclipses can occur. Because the orbital plane of the Moon, and so the so-called line of nodes, rotates once every 18 years, the above nodes, can switch places. So, the more complete answer is that lunar eclipses can also happen at the Ascending Node.

Why do eclipse tracks move eastwards 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 high 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 ca 610 BC is credited with predicting a solar eclipse using knowledge of a previous eclipse 47 years before and adding 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 precise and useful.

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!