Editing Cosmic distance ladder (section)

=== Astronomical unit ===
{{main|Astronomical unit}}

Direct distance measurements are based upon the astronomical unit (AU), which is equal to {{val|149597870700|u=metres}}<ref>{{cite conference |id=Resolution B2 |title=On the re-definition of the astronomical unit of length |publisher=International Astronomical Union |conference=XXVIII&nbsp;General Assembly of International Astronomical Union |place=Beijing |date=31 August 2012 |url=http://www.iau.org/static/resolutions/IAU2012_English.pdf |url-status=dead |archive-url=https://web.archive.org/web/20250305215811/http://www.iau.org/static/resolutions/IAU2012_English.pdf |archive-date=5 March 2025|quote=... recommends [adopted] that the astronomical unit be re-defined to be a conventional unit of length equal to exactly {{val|149,597,870,700}} metres, in agreement with the value adopted in IAU&nbsp;2009 Resolution&nbsp;B2}}</ref>  and historically was conceived as the mean distance between the [[Earth]] and the [[Sun]].
[[Kepler's laws of planetary motion|Kepler's laws]] provide precise [[ratio]]s of the orbit sizes of objects orbiting the Sun, but provide no measurement of the overall scale of the orbit system. [[Radar]] is used to measure the distance between the orbits of the Earth and of a second body. From that measurement and the ratio of the two orbit sizes, the size of Earth's orbit is calculated. The Earth's orbit is known with an absolute precision of a few meters and a relative precision of a few parts in 100 billion ({{val|1|e=-11}}).

Historically, observations of [[Transit of Venus#Scientific interest in transits|Venus transits]] were crucial in determining the AU; in the first half of the 20th century, observations of [[asteroids]] were also important. Presently the orbit of Earth is determined with high precision using [[radar]] measurements of distances to Venus and other nearby planets and asteroids,<ref>{{cite journal |last1=Ash |first1=M. E. |last2=Shapiro |first2=I. I. |last3=Smith |first3=W. B. |year=1967 |title=Astronomical constants and planetary ephemerides deduced from radar and optical observations |journal=[[The Astronomical Journal]] |volume=72 |page=338 |bibcode=1967AJ.....72..338A |doi=10.1086/110230 |doi-access=free}}</ref> and by tracking interplanetary spacecraft in their orbits around the Sun through the [[Solar System]].