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Subject: SPACE Digest V13 #346

SPACE Digest                                     Volume 13 : Issue 346

Today's Topics:
              RE: SOLAR ECLIPSES
                <None>
                Re: "Follies"
           Nova: The Dark Side of the Moon
           Re: Solar Eclipse, a few points
              Re: Solar Eclipse

Administrivia:

    Submissions to the SPACE Digest/sci.space should be mailed to
  [e-mail].  Other mail, esp. [un]subscription requests,
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----------------------------------------------------------------------

Date: 2 Apr 91 22:34:33 GMT
From: virga.rap.ucar.edu![e-mail]  (Frank Hage)
Subject: RE: SOLAR ECLIPSES


In <31530@usc> [e-mail] (Steve Robiner) writes:

>Can someone out there who has seen (well, maybe not directly) a total
>solar eclipse describe to me what it and the sky look like at the time?

    I viewed the last total eclipse in the US from the Helena Montana
area and since the people who responded with the factual answers
to your questions didn't include a description of what it looks and
feels like I'll take a stab at a personal testimonial.

We were in an ideal location; a large, flat valley about 50-100 km
wide, ringed by snow covered hills and  mountains. It was a partly
cloudy day. The first thing we noticed when we got up that morning (1
hr before totality) was, although we could see lots of blue sky, it
was only about as bright as an overcast day.  As totality approached,
the "overcast feeling" increased.

At about 5 minutes before totality began, the sky began to turn a
deeper blue, much as it does at dusk. In fact, both before and after
totality, (which lasted about 8-10 minutes, in our location) the effect
was very much like a fast motion sunset or sunrise.  This "fast motion
approach of dusk" gave everyone an eerie feeling, and I personally began
to feel a religious awe as the effect became very noticeable.
About a minute or so before totality, bright stars began to appear as
the sky turned a deep blue/violet and one could begin to see the shape
of the moon in front of the sun with the naked eye. At this point it
was still too bright to look directly at the sun, although we could
glance at it for a second.  (We also monitored the eclipse with
pin-hole devices and fogged film). Soon the moon, which appeared just
slightly larger than the disk of the sun, almost covered the entire
disk, except for thin, extremely bright, arc of light on one side. The
corona was still not visible. The sky then began dimming very rapidly,
losing its early dusk character.

Then, looking to the west, I saw the snow covered peaks across the
valley suddenly begin to turn a sunset orange, then reddish brown, then
dark, all within the span of about 5 seconds. Then a clearly
distinguishable shadow came racing across the valley towards us, the
edges of it diffuse and smoothly shaded in muddy yellows, until it
passed over us.  At this point, a night-like sky appeared, deep violet to
black, with major stars in constellations clearly visible. The sun's
corona became visible, as a hazy, bright, and diffuse ring around the
moon.  I recall seeing a refractive effect you see in the edges of
moon dogs, in the diffuse edges of the corona; blue and red hues were
visible, but the corona was mostly a hazy white.

At this point I understood why people throughout history considered
total eclipses a religious event.  The experience of watching dusk fall
in the span of about 5 minutes, and seeing a shadow move from the
horizon towards you is hard to describe.  I got the feeling that a
giant curtain was sweeping across the valley, taking away the precious
sun, that, up until that point in my life, I had taken for granted
would be there every day.  For me, the most memorable and inspiring
feature of the solar eclipse was not the corona or the color of the
sky, but the quick transition from "day to night" the eclipse brought.

While the sun was totally eclipsed, we could look directly at it,
although, it was bright enough to cause our pupils to constrict such
that one couldn't see anything but the brightest stars unless you
looked away for a few seconds.

The end of totality was much the same as the onset. We could clearly
see the shadow traveling across the valley, the sky brightening. As
soon as the shadow left our location, the corona disappeared, and the
sun became too bright to directly look at.  We had heard that right at
the transition points of totality, people often see a "diamond ring"
effect for a brief instant, where the visible corona appears as the
"ring" while shafts of direct light getting through the craters on the
"forward" or "trailing" edge of the moon appear as a "sparkling
diamond", but did not see this ourselves.

    All in all, it was a very impressive sight. Definitely worth traveling
a long way to see.

-Frank Hage,   National Center for Atmospheric Research
[e-mail].

------------------------------

Date: 2 Apr 91 22:15:46 GMT
From: pa.dec.com!drbob.tay2.dec.com![e-mail]  (Robert Royal Jaques)
Subject: <None>

Subject:Updated Ariane Launch schedule please
Date: Tue, 2 Apr 91 21:33:44 GMT
Lines: 17

--
_

Could someone kindly post an update Ariane Lunch schedule. I though
there was spose to be a launch of  teh Anik E2 bird in March

thanks
bob
__________________________________________________________________________
|          any relation with reality is purely coincidental               |
|_________________________________________________________________________|
|  Robert R Jaques    !! dr bob !!         |  Mail addresses              |
|  Digital Equipment Corporation           |  [e-mail]  |
|  Cororate Telecommunication Engineering  |  setprv::jaques              |
|  Concord, MA                             |  bob jaques @ vro            |
|  tel [phone]  dnt 273-5162          |                              |
___________________________________________________________________________

------------------------------

Date: 2 Apr 91 16:56:53 GMT
From: agate!bionet!uwm.edu!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!rpi!news-server.csri.toronto.edu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: "Follies"

In article <[e-mail]> [e-mail] ("Allen W. Sherzer") writes:
>>"W> ALS will not achieve an order of magnitude reduction in launch costs.
>
>>The design goal for ALS is $300/lb at 25 launches per year
>>The ALS is a Big-Dumb-Booster concept, far simpler than the shuttle.
>
>...The Shuttle
>was designed by a committee and so is ALS. Nothing has changed and
>therefore ALS will fail for the exact same reason the Shuttle failed.

Amen.  In fact, ALS is *not* a "big dumb booster" concept; it is another
all-things-to-all-users project, struggling to meet half a dozen sets of
conflicting constraints.  For example, it is being pushed as an opportunity
for technology development, which is exactly what a cheap-launcher project
*doesn't* need:  a bunch of wonderful new ideas that don't quite work.  (As
witness the wonderful staged-combustion cycle of the SSME.)

ALS's biggest problem, though, is the same one that bedevils any big-launcher
project right now:  where's the market?  There is about a snowball's chance
in the solar chromosphere that SDI will ever get approval for mass deployment.
And there is no other customer who has both the motive and the money to buy
25 large launches a year.
--
"The stories one hears about putting up | Henry Spencer @ U of Toronto Zoology
SunOS 4.1.1 are all true."  -D. Harrison|  [e-mail]  utzoo!henry

------------------------------

Date: 2 Apr 91 21:54:43 GMT
From: sdd.hp.com!wuarchive!rex!rouge!pc.usl.edu![e-mail]  (Fraering Philip)
Subject: Nova: The Dark Side of the Moon

I was just wondering if anyone here wanted to discuss
the recent Nova episodes on the Soviet space program.

I have only seen the second: "The Dark Side of the Moon,"
and I will see the third one, "The Mission," this Saturday.

Anyone want to start?

If noone does, I have a technical question about the Soyuz.
From what I saw on the program, the Soyuz doesn't use the
same attitude control system the Apollo CSMs did (the 4
thrusters per point, 4 point or whatever it is called).
What sort of system does it use? Is it efficient? I always
thought the one on the CSM looked like an optimum configuration,
but I haven't given it any serious thought.

I suppose a related question would be, where is the location
of the center of gravity on a Soyuz?

Another good question would be: how hard would it have been to
have moved their original two-man lunar mission to the Proton
rocket, using multiple launches? How many Protons would it have
taken?

Is the Soyuz as good a spacecraft as it was made out to be
on the program?

Phil Fraering
[e-mail]
Disclaimer: whatever a disclaimer is, I guess it applies ;-)

------------------------------

Date: 3 Apr 91 02:42:26 GMT
From: sdd.hp.com!spool.mu.edu!munnari.oz.au!yoyo.aarnet.edu.au!sirius.ucs.adelaide.edu.au!hydra![e-mail]  (Francis Vaughan)
Subject: Re: Solar Eclipse, a few points


I won't recover the basics that have been well covered
in a few previous postings, but would like to make a
couple of small points.

Looking at a partial phase of the eclipse without protection
is very dangerous. A major problem is that as the eclipse
progresses the major part of the suns disk is occluded and
when observed the eye does not register pain, however the
remianing cresent has exacly the same power/area of retina
as does the uneclipsed sun. It is quite possible to observe
a cresent sun without feeling any pain, and end up with
a neat cresent shape permanently burnt into your retina.

It is quite safe to observe the totality. It is really quite
something. I don't think anything could have prepared me for
one I observed (Oct '76). The corona is quite beautiful,
but the magenta flares, nobody had warned me about. When
totality ended there was silence, until one wit said "Wow
I want to do this again!"

NEVER EVER use fogged film as a filter. I am not sure about
B&W film but colour negative film is TRANPARENT to Infra-Red.
If you ever need to make an IR ftransmission ilter, fogged
colour film is very good. I would avoid B&W since from memory
silver halides are transparnet to UV.

Purchase a cheap aluminised mylar filter. These are very good
and provide good protection. Be careful not to scratch it.

Things to look for:

1st contact, only seconds after the limb of the moon first
intersects the sun a noticable notch is visible. It is fun
to keep time.

( things get boring for a while as totality approaches )

countdown to totality...

Baileys Beads. In the last few (<10) seconds before
totality the last vestiges of the cresent turn into a fast
moving group of dots of light as light from the sun passes through
lunar valleys.

Totality. Stunning, will live with you for the rest of your life.

End of totality...

Baileys Beads again.

Diamond ring, mostly due to dark adaption. The first full blast
of the sun on the revealed side looks like a shining jewel against
a still visible corona. A fantastic sight.


Photoghraphy. Obviously you need a telescope or telophoto lens.
For 35mm 1000mm is the maximum useful focal length (and probably
the best, although fine work is poossible with 500mm or less.) Use slow
fine grained film. Kodak Tech Pan (2415) for B&W and Kodachrome 25
for colour transaprency. (This assumes you have some sort of mount,
an equitoral is nice but I suspect that a reasonable tripod would work.)

Always keep the solar filter (a mylar one is perfect) on the lens,
I have seen some very neat cresent shaped holes in camera shutters.

The moment totality starts ( and oyu have captured the Baileys Beads)
whip it off. Be ready to put it back.

Make a plan, and write it out, things happen awfully fast at 2nd
and 3rd contact. A motor drive is great for Baileys Beads. Take
lots of bracketed shots from underexposure to gross over exposure
during totality. There is fine inner detail in the corona and wispy
detail in the outer reaches. No one exposure can capture it all.
My best shot was 1 second at f5 on 25 asa. I wished it had taken
2 and 4 second shots.

As the end of totality approaches (3rd contact) fire off another
round of shots to try and capture the diamond ring. Expose as
for totality. One shot will work, the one before everything is
blasted out by the emergent sun.

Put the filter back on!! Quick.

I anyone is interested I can dig out some more info and advice
about photographing the event. Otherwise find a library and
trek through Sky and Telescope.

                    Francis Vaughan

------------------------------

Date: Tue, 2 Apr 91 19:26:44 EST
From: John Roberts <[e-mail]>
Disclaimer: Opinions expressed are those of the sender
    and do not reflect NIST policy or agreement.
Subject: Re: Solar Eclipse


>From: mips!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!ra![e-mail]  (Terry Matula)
>Subject: Re: Solar Eclipse

>In reply to: Steve Robiner
>>Is it truly more dangerous to look at an eclipse than looking at the
>>normal sun?  It can't possibly be any brighter.  The only danger I can
>>see is that while viewing the relatively dark eclipsed sun, the moon
>>moves out of the way, and one's dialated eye is caught off gaurd and
>>exposed to the sun's full brightness.  Is there any other danger
>>beyond that?

>Well, what you say is true.  Also, during a partial eclipse (before and
>after totality) the sun is a good bit dimmer than normal and a person
>can comfortably look directly at the sun for long periods of time without
>experiencing any discomfort.  But the ultraviolet rays are still present
>and can do permanent damage to the delicate structures in the eye during
>this time.

Supposedly the lens of the eye filters out "ultraviolet". It is thought
that UV from the sun can contribute to cataracts, but only over the
course of decades. I have never seen a description of the exact risk of
looking at the sun that seems significantly more plausible than "touching
frogs causes warts". Just from personal observation I would guess that the
primary effect is localized heating, and that moreover it really only the
fovea (the point of sharpest and color vision) that is highly susceptible -
people routinely spend long periods of time with the sun in their peripheral
vision, without losing the peripheral vision, and yet there are stories of
people who looked directly at the sun for less than a minute and lost
nearly all their central vision. I don't think instantaneous intensity at
anywhere near the level of sunlight is the problem - otherwise, xenon
strobe lights would be illegal. I've accidentally had the sun cross the
center of my field of vision very briefly any number of times, with no worse
effect than an afterimage lasting several minutes.

So for eclipses, I imagine it would be a slightly lesser degree of the same
risk. Viewing the sun even during a partial eclipse is never recommended
unless you know what you're doing and use a projection system or a filter
approved for your particular application. And *NEVER* look through unfiltered
binoculars or telescope at the sun for even an instant - it can blind you
very quickly! I have used an approved solar filter with a telescope with
some success. It fits across the aperture (front) of the telescope and
reduces the incoming light by a factor of 100000 (as well as filtering out
UV, etc.)

A year or two back I posted a set of instructions on how to build a very
superior pinhole projector (large, detailed image) for a dollar or two.
(Build it for the eclipse, then throw it away so you don't have to store it.)

>>How big is the Moon's shadow on the Earth?  I would guess it's nearly the
>>size of the moon, but there must be some refraction of light around the
>>edges of the moon, making the totally dark shadow of the moon somewhat
>>smaller.

>The part of the moon's shadow where a total eclipse is visible is know as
>the umbra.  The part where a partial eclipse is visible is know as the
>penumbra.  The penumbra is pretty much the same size as the moon itself.
>The umbra, however, barely reaches Earth and is never larger than 269 km
>(168 miles) in diameter.

>[e-mail]

I tried to calculate the shadow characteristics. Here is an attempt at a
diagram using ASCII characters. The lines tracing the outer boundary of the
penumbra are shown with periods, and the lines tracing the outer boundary
of the umbra are shown with colons:

                           -
                      /         \

                    |             |  SUN

                      \         /
                     :.    -    .:

                        .     .
                      :         :
                          . .

                       :  . .  :

                        .  -  .
                        /     \   MOON
                      . \     / .
                           -
                    .    :   :    .

                  .   P    U    P   .
                          : :
                .                     .

              .            :            .
                           ^farthest point of umbra

Use the following variables and values to calculate an approximate (good to
a few digits) value for the diameters of the umbra and penumbra at Earth:

R = diameter of sun = ~1390600 km (don't confuse with radius)
r = diameter of moon = ~3476 km
D = distance from sun to earth = (min 1.471E8, avg 1.496E8, max 1.522E8) km
d = distance from moon to earth = (min 3.63E5, avg 3.8E5, max 4.06E5) km
    (center to center - dia. earth = ~12760 km)

The distance the umbra extends past the moon is only about
 r x (D - d) / (R - r), which with these numbers gives
 (min 3.68E5, "avg" 3.74E5, max 3.81E5) km.

If the eclipse moves past the point on the earth directly facing the moon,
then the equivalent for d may be decreased by the radius of the earth. Still,
it can be seen that sometimes the umbra will be long enough to reach the earth,
and sometimes it will not. In the latter case, the best one can hope for is
an annular eclipse. Sometimes if the umbra reaches just short of earth, or at
the onset of totality of a full eclipse, observers will notice Baily's beads,
which are a beadlike pattern of bright spots around the limb of the moon
caused by the sunlight shining through the valleys and being blocked by the
mountains on the moon.

If the umbra is long enough and the earth-moon distance short enough for a
total eclipse, then the width of the projection of the umbra on the earth
is about  r - (d x (R - r) / (D - d)), so I get a maximum width of about
218 km. (A book reference gives 270 km, which is pretty close. I suspect
inaccuracies caused by my initial numbers are greater than the inaccuracies
caused by approximations in the calculations.)

Similarly, the width of the penumbra should be about
 r x (1 + (d x (R + r) / (D x r))),
so a typical penumbral shadow on the earth would be about 2 times the
diameter of the moon, or roughly 7000 km. Viewed from space, this shadow
would be very faint at the outside edges, and darker toward the center,
with no sharp boundaries between the different parts of the eclipse shadow
or the unshaded area.

        John Roberts
        [e-mail]

------------------------------

End of SPACE Digest V13 #346
*******************