Mercury is moving between Earth and the Sun today, a relatively rare event known as a solar transit. It began around 7 am today and will continue until 2:42 pm Eastern Time. Whatever you do, don't look directly at the Sun to try watch it. In any case, there's not much to see: just a small black dot (Mercury) on the face of the Sun. If you don't have a solar filter for your camera or binoculars there are plenty of live streams on the Internet, such as the one here. NASA has provided some explanations as to why solar transits matter and what scientists learn from them at their website. One of the most important solar transits in modern history was the 1761 transit of Venus. Astronomers realized that by observing the transit from specific points around the globe, they could gather data that would enable them to calculate the Earth's distance from the Sun. European scientists embarked on dangerous and uncomfortable voyages around the world, from England to Newfoundland and South Africa, from France to India, from Austria to Madagascar, just to watch one planet for six hours, take notes, and do some mathematical calculations. If astronomy or the history of science interests you, check out Andrea Wulf's history of the dangerous, adventurous voyages of these astronomers, Chasing Venus: The Race to Measure the Heavens.