NavList:

   

Reply

Fred H, you wrote:
"You appear to have missed the point of both George and Frank Reed's post
that Halley did not need a model; it could be done empirically knowing that
the moon was back in the same spot after 18 years x days later, based upon
data collected beforehand."

I gotta qualify this a little, Fred.

First, George's description of Halley's attempt to determine a set of
empirical tables for the Moon's motion is distinct from what I was saying.
Halley had a specific theoretical hypothesis: that the Moon's position was
primarily subject to the same periodicity that was known from eclipses
(arising from the slow precession of the Moon's orbital plane and its point
of perigee). This theory, really more of a hope than anything else, led
Halley to make painstaking measurements over several decades. In the end, it
didn't work --the Moon's motion is simply more complicated than that. Since
Halley had not accumulated sufficient data (by his standards) by c.1700, I
don't know if he used any of this data for the observations that were at the
center of this discussion, but he may have.

Next, the method I was describing does not depend on any theory of the
Moon's motion at all. Just to make it more clear, let's consider an
imaginary historical scenario. Picture a small comet swinging past the Earth
in 1750. At perigee, a little tidal disruption causes some strong outgassing
which just happens to be aligned forward, in the direction of the comet's
velocity vector. Like a retro-rocket, this jet slows the comet and drops it
into a nearly circular orbit 50,000 miles away from the Earth (let's assume
it ends up in a nearly polar orbit --passing nearly over the Earth's N/S
poles-- so that the Moon's gravity does not kick it out of the system too
quickly). Astronomers of this alternate history marvel at the Earth's tiny
"new moon" and within weeks they realize that they have a tremendous
opportunity to map the Earth. They can observe the position of the
point-like nucleus of the comet-moon relative to bright stars from various
points and thus determine absolute time (GMT) and from that and observations
for local time, they will get fairly exact longitudes. Because the comet is
close to the Earth, it zips across the sky more rapidly than the Moon
resulting in very good longitude measurements. But there's a catch: as low
as it is, the gravitational motion is complicated enough that it is very
difficult to be sure but even worse, the comet continues to fire out jets of
gas and dust. Like random rocket firings from a spacecraft with a drunk
pilot, this outgassing makes the comet speed up, slow down, and in general
change its motion unpredictably. Even today, we would not be able to produce
preditive ephemerides for a sputtering comet like this. So those geographes
in that alternate history cannot hope to determine longitudes from
observations on the spot. Instead, they rely on reference observations by
observers at known locations back home. Each night astronomers in London and
Paris and other cities with good observatories would measure as exactly as
possible the position of the comet-moon relative to the stars. Then by
comparing those observations with measurements made by the travelling
astronomers after their return, they would end up with a detailed and
accurate reference map for future explorations, under the assumption that
some other method for measuring longitude became available later. And in all
of this NO predictive, theoretical model of the comet-moon's motion would be
required! Alas, for our alternate history astronomers, the orbit of the
comet-moon slowly became more eccentric until it eventually intersected the
surface of the Earth, by a tragic coincidence splashing down in the shallow
Atlantic just off western Europe and destroying all of western civilization.
Darn!

 -FER


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply