<p><strong>The black dot of <a href="http://science.nationalgeographic.com/science/space/solar-system/venus-article/">Venus</a> punctuates the setting sun in a picture of the <a href="http://news.nationalgeographic.com/news/2012/06/120604-transit-of-venus-2012-sun-planet-hubble-space-science-how-when/">2012 transit of Venus</a> taken near Salt Lake City, Utah, on Tuesday.</strong></p><p>Transits happen when a planet crosses between Earth and the sun. Only Mercury and Venus, which are closer to the sun than Earth, undergo this unusual alignment.</p><p>Due to the planet's tilted orbit, Venus transits are so rare that only six have been observed since the invention of the telescope more than 400 years ago. (See a <a href="http://ngm.nationalgeographic.com/2009/07/telescopes/telescopes-interactive">telescope time line</a>.)</p><p>The 2012 transit of Venus saw the planet glide across the sun's face for the last time for 105 years. Some countries saw the transit on Tuesday, while others saw it Wednesday morning.</p><p><em>—With reporting by Andrew Fazekas</em></p>