(Editor’s Note: The following was written by Bill Cloutier with the John J. McCarthy Observatory in New Milford.)

In the early 1600s, shortly after the telescope was invented, astronomers recognized that solar transits (the crossing of inferior planets in front of the sun) could be used to more accurately measure the apparent diameter of Mercury and Venus.

French astronomer Pierre Gassendi was one of the first people to witness a transit of Mercury in 1631, which had been predicted by mathematician Johannes Kepler.

In 1677, Edmond Halley observed another transit of Mercury (they can occur up to 14 times a century), during which he realized that the parallax shift of the planet — the difference in the planet’s apparent position during the transit, as viewed from different latitudes on Earth — could be used to determine a more accurate distance to the sSun (the astronomical unit or a.u.), which was poorly known at the time.

Recognizing that he wouldn’t live to see the 1761 Venus transit, Halley urged “curious astronomers” to take up the challenge.

Expeditions were launched from Great Britain and France in 1761 and again in 1769 (Venus transits occur in pairs, eight years apart), in an attempt to define the a.u. Results were mixed and while the there was some improvement in the calculation, it wasn’t to the precision that Halley had hoped.

Today, transits are used to detect worlds orbiting around other stars (exoplanets).

NASA’s Exoplanet Archive lists 4,057 known exoplanets (as of the end of September), 3,137 of the alien worlds discovered by the transit method. The method detects planets orbiting distant stars by measuring the minute dimming of the star’s light as a planet crosses in front of the star, as viewed from Earth.

This month’s “Seconday Saturday Stars” program will explore the world of exoplanets and the implications for life on other worlds. The program will be held Oct. 12 from 7 to 9 p.m. at the observatory behind New Milford High School.

In November, the planet Mercury will transit the sun. The entire transit, lasting approximately five and a half hours, will be visible from New Milford. Weather permitting, the McCarthy Observatory will be open to visitors for viewing the transit.

Another transit won’t be visible from our area for 30 years.