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Q: science in the 1700-1800's ( Answered,   1 Comment )
Subject: science in the 1700-1800's
Category: Science
Asked by: lawnmowerman1067-ga
List Price: $2.00
Posted: 14 Jan 2003 14:02 PST
Expires: 13 Feb 2003 14:02 PST
Question ID: 142670
in the 1700's and 1800's how did they find the distance to the
moon,and the mass of the earth
Subject: Re: science in the 1700-1800's
Answered By: bio-ga on 14 Jan 2003 18:15 PST

You don't have to wait until 16th century to estimate the distance to
moon. According to a NASA site:

Aristarchus around 270 BC derived the Moon's distance from the
duration of a lunar eclipse (Hipparchus later improved that method).
His argument, in a nutshell: if the Moon circles the Earth, then in
about a month it completes a full orbit, the length of which (assuming
it is a circle) is about 6 times the distance of the Moon.

For the question of the mass of earth, we have to come to the Newton's
age. Isaac Newton showed that the gravitational acceleration "g"
experienced by an object caused by the gravitational attraction of a
second body, is directly proportional to the mass "M" of the
attracting body, and inversely proportional to the square of the
distance "R" between the two bodies.

g = (G * M) / (R * R)

Here, G denotes the gravitational constant. once G is known the mass
of the Earth can be obtained from the 9.8 m/s2 gravitational
acceleration on the Earth surface.

G was first measured in the laboratory; in 1798 by Cavendish and
co-workers accurate to about 1%. More information can be found at:

Bonus: Using this information, the Sun's mass can also be obtained
from the size and period of the Earth orbit around the sun.

Hope this helps
Google Answers Researcher
Subject: Re: science in the 1700-1800's
From: iang-ga on 15 Jan 2003 06:14 PST
Bio's right, but if you're interested in 16th century astronomy the
answer's parallax - you measure the position of the moon against the
stars, simultaneously, from widely separated positions on Earth.
Knowing the distance between your observing points you can work out
the distance to the moon.

Ian G.

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