The Transit of Mercury

On Monday, Mercury crossed in front of the sun, as seen from Earth, a process known as a transit. Although the last transit of Mercury occurred only three years ago, the next won't happen until 2032 and won't be visible from North America. These rare events provide the opportunity for scientists to gather both new scientific observations and re-creations of historical ones. On Monday, scientists were able to replicate Sir Edmund Halley's experiment, using Mercury instead of Venus. Volunteers, mostly students spread across more than a dozen sites in the U.S., will equipment identical to each other to take simultaneous images of the transit, allowing them to replicate the historical experiment.

Mercury's transit also provides an opportunity to probe the thin atmosphere of the planet. When Mercury is in front of the sun, scientists can study the exosphere close to the planet. Sodium in the exosphere absorbs and re-emits a yellow-orange color from sunlight, and by measuring that absorption, we can learn about the density of gas there. In fact, the transit itself echoes how many of today’s exoplanets are found. Instruments like NASA’s Kepler Space Telescope, which discovered thousands of exoplanets, are based on the technique of transits. Kepler detected a dip the star’s brightness as an exoplanet transits, much as Mercury slightly dims the sun as it passes between it and the Earth. So transits of Mercury and Venus provided a foothold into the widespread hunt for other worlds.

Credits:
https://www.space.com/mercury-transit-2019-science.html