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.
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.
long will we continue to be able to see total eclipses of the Sun?
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.
more often, solar or lunar eclipses?
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.
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.
can I see the next solar eclipse from North America?
Have a look
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!
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
a book that shows the solar eclipse tracks going back a few hundred
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
"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
"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.
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.
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.
don't eclipses occur every new moon?
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.
computers predict eclipses?
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.
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:
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
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.
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.
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!
were solar eclipses first predicted accurately?
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
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