Changes

The sidereal period of Venus is 224.7 days, and the synodic period is 583.92 days. Remarkably, the Mayans calculated Venus' synodic period at 584 days, a figure accurate to within an incredible three significant digits.<ref name=Kukulcan/> Moreover, the ratio of the synodic year of Venus to the year of Earth is very nearly 13:8, close enough to suggest that Earth and Venus participate in an orbital resonance.

The sidereal period of Venus is 224.7 days, and the synodic period is 583.92 days. Remarkably, the Mayans calculated Venus' synodic period at 584 days, a figure accurate to within an incredible three significant digits.<ref name=Kukulcan/> Moreover, the ratio of the synodic year of Venus to the year of Earth is very nearly 13:8, close enough to suggest that Earth and Venus participate in an orbital resonance.

Venus's orbit is inclined 3.39 degrees from the ecliptic. For this reason, Venus usually passes north or south of the sun in its conjunctions with earth. However, every century or so, Venus makes a pair of transits across the Sun, eight years apart. Indeed the transit of Venus in 1769 allowed astronomers to determine the length of the astronomical unit.<ref name=Hornsby>Hornsby, T. "[http://gallica.bnf.fr/ark:/12148/bpt6k55866b/f617.chemindefer The quantity of the Sun's parallax, as deduced from the observations of the transit of Venus on June 3, 1769]." ''Phil. Trans. R. Soc.'' 61:574-579, December 19, 1771. <doi:10.1098/rstl.1771.0054> Accessed May 22, 2008.</ref>

Venus's orbit is inclined 3.39 degrees from the ecliptic. For this reason, Venus usually passes north or south of the sun in its conjunctions with earth. However, every century or so, Venus makes a pair of transits across the Sun, eight years apart. Indeed the transit of Venus in 1769 allowed astronomers to determine the length of the astronomical unit.<ref name=Hornsby>Hornsby, T. "[http://gallica.bnf.fr/ark:/12148/bpt6k55866b/f617.chemindefer The quantity of the Sun's parallax, as deduced from the observations of the transit of Venus on June 3, 1769]." ''Phil. Trans. R. Soc.'' 61:574-579, December 19, 1771. {{doi|10.1098/rstl.1771.0054}} Accessed May 22, 2008.</ref>

Venus made such a transit on June 8, 2004, and will make another on June 6, 2012. After that, astronomers do not expect another transit until the year 2117.<ref name=transit>Bueter, Chuck. ''[http://www.transitofvenus.org/ Transit of Venus]''. 2003-2008. Accessed May 20, 2008.</ref>

Venus made such a transit on June 8, 2004, and will make another on June 6, 2012. After that, astronomers do not expect another transit until the year 2117.<ref name=transit>Bueter, Chuck. ''[http://www.transitofvenus.org/ Transit of Venus]''. 2003-2008. Accessed May 20, 2008.</ref>

== Rotational characteristics ==

== Rotational characteristics ==

Venus rotates retrograde, or east to west, with a sidereal day of 5832.5 hours, or 243.686 Earth sidereal days. Venus' ''solar'' day is considerably shorter, at 2802.0 hours (117 sidereal days). Oddly enough, the solar day of Venus is almost exactly one-fifth its synodic year with respect to the earth, with the result that at each inferior conjunction (closest approach) of Earth and Venus, the same (night) side of Venus faces the Earth. This has led some astronomers to speculate that Earth and Venus are in some kind of [[tidal lock]]. However, at least two authorities stated that and Earth-Venus gravitational interaction would not produce such a lock unless the sun itself were producing a "tide" in Venus's atmosphere.<ref name=Gold>Gold, Thomas, and Soter, Steven. "[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4731C9M-CY&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8edbfeb87ffb38474d3984472bf1ebdd Atmospheric tides and the resonant rotation of Venus]." ''Icarus'' 11(3):356-366, November 1969. <doi:10.1016/0019-1035(69)90068-2> Accessed May 21, 2008.</ref>

Venus rotates retrograde, or east to west, with a sidereal day of 5832.5 hours, or 243.686 Earth sidereal days. Venus' ''solar'' day is considerably shorter, at 2802.0 hours (117 sidereal days). Oddly enough, the solar day of Venus is almost exactly one-fifth its synodic year with respect to the earth, with the result that at each inferior conjunction (closest approach) of Earth and Venus, the same (night) side of Venus faces the Earth. This has led some astronomers to speculate that Earth and Venus are in some kind of [[tidal lock]]. However, at least two authorities stated that and Earth-Venus gravitational interaction would not produce such a lock unless the sun itself were producing a "tide" in Venus's atmosphere.<ref name=Gold>Gold, Thomas, and Soter, Steven. "[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4731C9M-CY&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8edbfeb87ffb38474d3984472bf1ebdd Atmospheric tides and the resonant rotation of Venus]." ''Icarus'' 11(3):356-366, November 1969. {{doi|10.1016/0019-1035(69)90068-2}} Accessed May 21, 2008.</ref>

== Physical characteristics ==

== Physical characteristics ==