Today, Venus will pass between the Earth and the Sun. This event only happens twice every 120 years. The transit begins at 3:06 p.m. and lasts for roughly 6.5 hours.
Prof. Adrienne Cool says in an SFSU article:
“It instantaneously gives you this sense of scale,” Cool said. “Venus is about the size of the Earth, and we’re going to see it as this tiny dot crossing the sun. It’s humbling and fun to see that directly.”
The article continues:
First observed in 1639, astronomers realized they could measure the solar system by timing the transit from various locations on Earth, comparing differences in timing at those locations to measure the distance of Venus and Earth from the sun, and then applying those measurements to already-known relative distances of the planets.