This Day in History: 1687-07-05
In 1687 Sir Isaac Newton’s Philosophiæ Naturalis Principia Mathematica, (Latin for Mathematical Principles of Natural Philosophy) was first published.
The book includes Newton’s laws of motion and his Laws of Universal Gravitation. Sir Isaac Newton is considered one of the greatest Physicists, Mathematicians and inventors who’s ever lived. He is famous for the claim that his scientific contributions were only possible “by standing on the shoulders of giants”, but his modesty should not overshadow his genius. His improvement to the original telescope, known as the Galilean telescope, is far superior and the design is still used for the most powerful modern-day telescopes. His observations of the cosmos using that telescope (the Newtonian or reflective telescope) aided the English Astronomer in calculating the laws of Universal Gravity to amazing accuracy that was used by physicists for almost 300 years. It also proved once and for all the Heliocentric (with the sun at the centre) theory of planetary motion which was first described by Kepler. Kepler died before his theory became widely known but it was highly ridiculed by many physicists and the church. Galileo strongly supported the theory but it would cause much anger from the church and led to his house arrest right up until his death (in 1642 the same year as Newton’s birth), but with Newton’s book the theory became undeniable.
While under house arrest Galileo worked tirelessly (even when his sight began to fail) on the motion of objects on Earth, making detailed sketches and observations. Newton expanded on this work and created his Laws of Motion.
His first law said that an object would remain still or continue at the same speed unless another force acts upon it (known as inertia). His second law shows the relationship between force mass and acceleration as described in his equation F=ma. His third law states that if object 1 applies a force to another object 2 then that object also exerts a force of the same magnitude on object 1.
Newton then applied these laws of motion to much larger objects in motion, the planets, moons, comets and the sun.
The book explains how the motions of large bodies such as the sun and planets can be explained by the effects of his theoretical force of Gravity. He also explains the laws of gravity and how to calculate the force of gravity on all objects mathematically. He stated that the gravitational force between 2 objects is equal to the mass of 1 object multiplied by the mass of the second object and then divided by the square of the distance between the centres of the two objects.
Newton was unable to calculate the value of G (the gravitational constant) but some years later Sir Henry Cavendish was able to calculate its value as 6.67 x10-11 Nm2 kg-2.
With the value of G and Newton’s equation, it was possible to calculate the sun’s mass and the distance between each planet as the mass of any planet with a satellite.
Newtonian gravity was used by scientists for centuries until it was improved upon by Albert Einstein in 1916 with his General Theory of Relativity which he calculated using both general relativity and Newton’s Universal Laws of Gravity.
