Editing Halley's Comet (section)

===1531-1759===
[[File:Astronomicum Caesareum (1540), p.89v (comet illustration).jpg|thumb|Illustration of the 1531 appearance in Petrus Apianus' [[Astronomicum Caesareum]], noting that a comet's tail always points away from the sun]]

[[Petrus Apianus]] and [[Girolamo Fracastoro]] described the comet's visit in 1531, with the former even including graphics in his publication. Through his observations, Apianus was able to prove that a comet's tail always points away from the Sun.<ref name="Barker2008"/>

In the [[Sikh]] scriptures of the [[Guru Granth Sahib]], the founder of the faith [[Guru Nanak]] makes reference to "''a long star that has risen''" at Ang 1110, and it is believed by some Sikh scholars to be a reference to Halley's appearance in 1531.<ref name="Kapoor2017"/>

Halley's periodic returns have been subject to scientific investigation since the 16th century. The three apparitions from 1531 to 1682 were noted by [[Edmond Halley]], enabling him to predict it would return.{{sfn|Grier|2005|pp=11–25}} One key breakthrough occurred when Halley talked with Newton about his ideas of the laws of motion. Newton also helped Halley get John Flamsteed's data on the 1682 apparition.<ref name="1985JHA"/> By studying data on the 1531, 1607, and 1682 comets, he came to the conclusion these were the same comet, and presented his findings in 1696.<ref name="1985JHA"/>

One difficulty was accounting for variations in the comet's orbital period, which was over a year longer between 1531 and 1607 than it was between 1607 and 1682.{{sfn|Sagan|Druyan|1985|p=57}} Newton had theorised that such delays were caused by the gravity of other comets, but Halley found that Jupiter and Saturn would cause the appropriate delays.{{sfn|Sagan|Druyan|1985|p=57}} In the decades that followed, more refined mathematics would be worked on, notable by Paris Observatory; the work on Halley also provided a boost to Newton and Kepler's rules for celestial motions.<ref name="1985JHA"/> (See also [[#Computation of orbit|computation of orbit]].)