🚀 go-pugleaf

whit@carson.u.washington.edu (John Whitmore) writes:

[quoting Snarfy]

>>    Throughout the fifties and sixties  ,  the  so  called  "neutral  point"
>>   where  the  gravitational  forces  of  the earth-moon system balance,was
>>   given over and over again as being between 22,100 and 25,200  miles  from
>>   the surface of the moon in the direction of the earth. 

>	Given by who?  It doesn't follow from anything I see in
>the data, nor do you give any attribution.

Judging from calculations I just did, I think this range comes from
there being at least two different "neutral points" in question; the low
number looks about right for the place where gravitational forces
balance, and the high number is probably the Lagrange point where an
object will orbit the earth in step with the moon.

>>    The  July  25th  ,  1969  issue of TIME magazine stated that the Neutral
>>    Point was 43,495 miles from the center of  the moon.  Werner  von  Braun
>>    stated  in  the  1969  edition of "History of Rocketry and Space Travel"
>>    that the neutral point was 43,495 miles the center of  the  moon  .  The
>>    pre-Apollo  distances  were  given  as  20,000  to 25,000 miles from the
>>    center of the moon. 

>	Given by whom?  Either there is a second definition involved, or
>SOMEONE made an error.  Several someones, if the range of 20,000 to 25,000
>is to be believed.  Again, I see no source for the questionable numbers.

Someone who redirected the thread to alt.sci.physics.new-theories
suggested that this was the result of a typo, since the number is
very close to the distance of the Lagrange point from the moon in
*kilometers* rather than miles.  I suspect this may be the case.

Note that the Lagrange point, not the balancing-forces point, is
the spot around which you can actually orbit things; in the rotating
frame, it's a saddle point of the effective potential, with the
stable direction transverse to the earth-moon line.

-- 
Matt McIrvin