See also: 1685 in science, other events of 1686, 1687 in science and the list of years in science.
Astronomy Gottfried Kirch notices that Chi Cygni's brightness varies. Geology Edmund Halley establishes the relationship between barometric pressure and height above sea level. Meteorology Edmund Halley presents a systematic study of the trade winds and monsoons and identifies solar heating as the cause of atmospheric motions. Physics Isaac Newton uses a fixed length pendulum with weights of varying composition to test the weak equivalence principle to 1 part in 1000 Births May 24 - Gabriel Fahrenheit, physicist, inventor (died 1736) Deaths Nicolas Steno: pioneer in early-modern geology (born 1638)
