Question: How was gravity discovered

Adam Washington answered on 8 Jun 2020:

There’s a fun little bit of the history of gravity that hasn’t been included yet – a geography mistake by King Charles II lead to us being able to MEASURE gravity.

Some of the other scientists have already mentioned Newton’s incredible breakthroughs on the subject. His law of universal gravitation said that gravity depend on the mass of the two items being attracted and another unknown, unchanging value. Mathematically speaking, it’s a simple process to find that other values. Take two things that we know the mass of, measure the force between them, and some simple algebra gives you the missing value FOR THE WHOLE UNIVERSE. The only problem was that the measurement seemed impossible. The force off of most objects we deal with on a daily basis is too small to notice or measure. For most of our lives, the only force of gravity we ever measurably feel is that of the Earth. However, since scientists at the time didn’t know the mass of the earth, it still wasn’t possible to solve.

We then step away from physics for a moment to the UK’s colonisation of the American continents. Briefly put, when awarding Cecil Calvert a charter for the colony of Maryland, he accidentally gave some of the land that has already been assigned to William Penn’s colony of Pennsylvania, including the capital city of Philadelphia. A small war even broke out between the colonies over debated territory. The dispute was finally mediated by King George II, who created a new border that both colonies would agree to. But everyone wanted to make sure that they had the geography right this time.

Surveyors Charles Mason and Jeremiah Dixon were sent to survey the new border and make sure that everything was correct. Students of history might recognize the names Mason-Dixon, as the border they established became the border between the two sides of the American Civil War, but that would be another hundred years hence. At this moment, they were mostly concerned with geography.

As was standard practice, they took measurements as they went along the line and then took the same measurements as they went back to ensure that the results were correct. While the results were within the expected tolerances, they noticed an odd effect – all of the errors were in the exact same direction. To a scientist, that’s like saying that you tossed all the change in your piggy bank and they all came up heads – it’s a sign that something is rigged. But what was rigging it?

One suggestion that came forth was that the the survey instruments weren’t EXACTLY upright, but were being pulled very slightly by the gravitational force of the Allegheny mountain range. The mountains would have been on one side of the instruments as they traveled down the line and the opposite side as they traveled back, leading to the discrepancy.

This turned out to be a breakthrough idea. Previously, gravity had only even been directly measured from the planet Earth. The idea that we could measure the gravity from something smaller, like a mountain, meant that the force of gravity was much stronger than many scientists had expected (since they’d greatly over-estimated the weight of the planet). Of course, it was only a hypothesis at this point. So, seven years after the measurement of the Mason-Dixon line, Mason chose the Schiehallion mountain in Perthshire, Scotland to perform a repeat of the experiment (though the actual experiment was performed by Royal Astronomer Nevil Maskelyne and the mathematicians Charles Hutton and Reuben Burrow). At the end, they became the first people to measure both the gravitational constant of the universe and the mass of the earth, which they got to within about 20%.

Knowing the approximate size of the answer let scientists then optimise their experiments and, about twenty years later, Henry Cavendish performed his eponymous experiment that found the gravitational constant to within a percent.