Mercury takes a trip across the sun

Walter Sprague
Art and Culture Reporter
 
On Monday, Nov. 11, the science department at Newcastle High School set up a couple of telescopes with solar filters to witness a rare event. Jim Stith, Zach Beam and Doug Scribner saw the planet Mercury as it made a transit across the disk of our sun. Siri Karr and Walter Sprague, from the News Letter Journal, came by to view this sight. All five people braved the cold, snow and icy roads to get a glimpse of something that few people ever get to see.
During Sunday’s snowstorm, it became questionable whether or not the transit would be visible. But, even though there were 4 inches or more of snow on the ground, Monday morning dawned bright and cloud-free, offering unobstructed views of the smallest planet in our solar system as it crossed in front of the sun.
“It was like a little freckle, but not many people could see it,” Karr said. “You could only see it with the telescope and equipment they used.” 
She said it was cool that the three science teachers let her look at this event.
A planetary/solar transit happens when one object (a planet) crosses in front of the face of another object (the sun) from our perspective. Because both Venus and Mercury are inside Earth’s orbit, these planets can transit Sol. The other planets orbit outside of Earth and, as such, can never pass in front of the sun. The moon also can pass in front of the sun. But the moon and the sun take up the same amount of area in our sky. That phenomenon is known as an eclipse. An eclipse is where the disk of the moon blocks a part of the sun, called a partial eclipse, or the full face of the sun, called a solar eclipse. These events happen on a reasonably regular occurrence due to the closeness of the moon to Earth, which is roughly 250,000 miles. 
During the 2019 transit, Mercury was 63 million miles sunward from Earth. Add to that the geometry of the different orbital planes of the planets, and you have a rare occurrence. While eclipses happen around the world with a predictable frequency every year, this is not the case with transits. The last transit of the sun by Mercury happened on May 9, 2016. The next one won’t happen until Nov. 11 or 12 (depending on where on Earth you live) 2032.
As rare as the Mercury transit is, Venus is even rarer. The last Venus transit happened on June 6, 2012. The next one won’t happen until 2117. While Venus is considerably closer to Earth and much larger than Mercury, the differences in the orbital planes between Venus and Earth add to the scarcity of this event. So if you missed the 2012 Venus transit, I’m sorry to say the next one won’t happen in our lifetimes.
Astronomers can see other transits, however. Many of the stars in our galaxy have planets orbiting them, and occasionally the planet(s) transit the parent stars. When this happens, the astronomer can measure the dimming effect that transit has on the intensity of the brightness of the star. This measurement, through highly advanced CCD cameras and computers, gives astronomers an enormous amount of information. With advanced math and precise calculations, we can tell the size, mass and even the make-up of the planet in question. The distances between the planet and a star can also be determined. These pieces of information have helped scientists determine a clearer picture of our galaxy. It has become apparent that extrasolar planets are common. There is, however, a bewildering variety of types, sizes and masses of the planets. The sheer diversity is greater than anyone ever expected, with new surprising discoveries made on an almost monthly basis, and the known planets now numbering in the thousands.
As impressive as our ability to discover these planets is, it is more remarkable when one considers how this information first came about hundreds of years ago. The first prediction of transit was in the early 1600s. Johannes Kepler made calculations of the orbits of all our planets and discovered that transits by both Mercury and Venus would happen in the year 1631. The invention of the telescope happened 23 years earlier, and the timing was fortunate. Both planets would not transit in the same year again until the year 13,425. But because Kepler died in 1630, he did not get to see this infrequent occurrence in 1631.
French astronomer Pierre Gassendi became the first person to see the transit of Mercury in 1631. However, a little miscalculation occurred regarding the Venus transit. While it did also happen that same year, Gassendi missed that transit. Since then, the calculations to pinpoint the motions of the planets have improved.
It soon became apparent to scientists why the transit of Mercury was such an important event. In 1677 Edmond Halley realized that the transit of that year could be used to determine how far away the sun was, which was unknown at that time. By having observers at different places on Earth measure the location of Mercury from their viewpoint at the same time, he figured they would know the parallax (the difference of observed position). Using those angles, he could then calculate many characteristics of the sun and the planet. Today we use radar to measure the distance of the sun with better accuracy, which is roughly 92.96 million miles from Earth. But the team that Halley put together got close in their calculations.
Halley also realized that the next transit by Venus could be used to calculate the size of the solar system. Using these calculations, he discovered that the comets of 1531, 1607 and 1682 were the same comet, and predicted the next fly-by of Halley’s Comet accurately, which would happen in 1758. 
With our new technologies and increased accuracy of measurements, the importance of transits is apparent, and each transit is used to make further discoveries about how the solar system, and the universe as a whole, behave. But modern science owes a massive debt to the pioneers of the enlightenment age.