A transit occurs when a smaller celestial object passes directly between an observer and a larger, more distant object, temporarily blocking a portion of the larger object's light. This astronomical phenomenon requires precise alignment—the three objects must lie nearly in a straight line for the transit to be visible.</p><p>The most famous transits involve planets crossing the face of the Sun as viewed from Earth. Venus transits are particularly rare and spectacular, occurring in pairs roughly every 120 years. The last Venus transit was in 2012, with the next not occurring until 2117. Mercury transits happen more frequently, about 13 times per century, with the most recent in November 2019.</p><p>Historically, transits revolutionized astronomy. In 1761 and 1769, international expeditions observed Venus transits to calculate Earth's distance from the Sun—the first accurate measurement of our solar system's scale. Captain James Cook's famous voyage to Tahiti was partly to observe the 1769 transit.</p><p>Today, transit photometry is the primary method for discovering exoplanets. When a planet transits its host star, it causes a tiny but measurable dimming—typically 0.01% to 1% depending on the planet's size. NASA's Kepler Space Telescope used this technique to discover over 2,600 confirmed exoplanets, fundamentally changing our understanding of planetary systems throughout the galaxy.