Gravitation
Aiton's Encyclopedia (1910)
a universal force existing in nature. The law of universal gravitation was enunciated first by Sir Isaac Newton. Every particle of matter in the universe attracts every other particle with a force proportionate to the product of their masses and inversely proportionate to the square of their distance apart. According to this principle, two masses with weights of 3 and 4 at a distance apart of 1 would attract each other with a force of 3 X 4 or 12. The same masses at a distance apart of 2 would attract each other with a force of 12 divided by the square of 2, which would give an attractive force of 3. At a distance apart of 3 they would attract each other with a force of 12 divided by the square of 3, which gives 1 1-3. The earth, like all moving bodies, tends to go in a straight line in whatever direction it happens to be moving. Were it not for gravitation, the path of the earth would be a straight line, and we should in time get so far from the sun that all life would be frozen. The attraction of gravity, however, draws the earth toward the sun. The two forces counteract each other. Instead of falling toward the sun or going off in a straight line, we take an intermediate direction which carries us round and round the sun ceaselessly. The fixed stars , some of which are larger that the sun even, are too far away to affect the earth's course sensibly. The earth draws the sun with a force equal to that with which the sun draws the earth, but the sun is so heavy in comparison with the weight of the earth that the sun scarcely budges, while the earth does the falling. Gravitation refers to the attraction of gravitation between the earth and bodies near its surface. See Weight
What Has Changed in the Last Century
When Aiton's called gravitation "a universal force" obeying Newton's inverse-square law in 1910, that was the whole story; five years later Einstein's general theory of relativity recast gravity not as a force at all but as the curving of space and time by mass. The new theory explained the drift in Mercury's orbit that Newton's law could not, and was confirmed dramatically in 1919 when Arthur Eddington's eclipse expedition measured starlight bending near the sun. Since then relativity has given us gravitational lensing, black holes photographed in 2019, and gravitational waves detected by LIGO in 2015 from two merging black holes; the tiny relativistic corrections are built into every GPS receiver. Newton's arithmetic in the entry above remains perfectly good for spacecraft and tides " gravity is weak and speeds are low " which is why it is still taught first.
Written for this edition, 2026.