Editing Axial tilt (section)

=== History ===
The ancient Greeks had good measurements of the obliquity since about 350 BCE, when [[Pytheas]] of Marseilles measured the shadow of a [[gnomon]] at the summer solstice.<ref>
{{cite book
 |last=Gore |first=J. E.
 |date=1907
 |title=Astronomical Essays Historical and Descriptive
 |publisher=Chatto & Windus |url=https://archive.org/details/astronomicaless00goregoog
 |page=[https://archive.org/details/astronomicaless00goregoog/page/n78 61]
}}</ref> About 830 CE, the Caliph [[Al-Mamun]] of Baghdad directed his astronomers to measure the obliquity, and the result was used in the Arab world for many years.<ref>
{{cite book
 |last=Marmery |first=J. V.
 |date=1895
 |title=Progress of Science
 |publisher=Chapman and Hall, ld. |url=https://archive.org/details/in.ernet.dli.2015.45033
 |page=[https://archive.org/details/in.ernet.dli.2015.45033/page/n67 33]
}}</ref> In 1437, [[Ulugh Beg]] determined the Earth's axial tilt as 23°30′17″ (23.5047°).<ref>{{cite book |first=L.P.E.A. |last=Sédillot |title=Prolégomènes des tables astronomiques d'OlougBeg: Traduction et commentaire |location=Paris |publisher=Firmin Didot Frères |year=1853 |pages=87 & 253}}</ref>

During the [[Middle Ages]], it was widely believed that both precession and Earth's obliquity oscillated around a mean value, with a period of 672 years, an idea known as ''[[trepidation (astronomy)|trepidation]]'' of the equinoxes. Perhaps the first to realize this was incorrect (during historic time) was [[Ibn al-Shatir]] in the fourteenth century<ref>{{cite book 
 |last=Saliba |first=George 
 |date=1994
 |title= A History of Arabic Astronomy: Planetary Theories During the Golden Age of Islam 
 |page=235
}}</ref> and the first to realize that the obliquity is decreasing at a relatively constant rate was [[Fracastoro]] in 1538.<ref>
{{cite book
 |last=Dreyer |first=J. L. E.
 |date=1890
 |url=https://archive.org/details/tychobraheapict00dreygoog
 |title=Tycho Brahe
 |publisher=A. & C. Black |page=[https://archive.org/details/tychobraheapict00dreygoog/page/n387 355]
}}</ref> The first accurate, modern, western observations of the obliquity were probably those of [[Tycho Brahe]] from [[Denmark]], about 1584,<ref>Dreyer (1890), p. 123</ref> although observations by several others, including [[al-Ma'mun]], [[Sharaf al-Dīn al-Tūsī|al-Tusi]],<ref>
{{cite book 
 |last=Sayili |first=Aydin 
 |date=1981
 |title= The Observatory in Islam 
 |page=78
}}</ref> [[Georg Purbach|Purbach]], [[Regiomontanus]], and [[Bernhard Walther|Walther]], could have provided similar information.