The event will begin at 7:36 a.m. EST as the planet appears to touch the southwest edge of the sun. By 7:38 a.m., it will appear completely within the disk of the sun (it takes 101 seconds for it to cross the edge). The planet will then be a tiny dot crossing the face until 1:03 p.m. when it will begin to leave the sun. By 1:04 p.m., the entire event will be over.

Figure 1 (at right, top) shows the path of Mercury across the sun from west to east and notes where it will be over the six-hour event. Since Mercury will be 1/194th the size of the sun, it will require a telescope to see the planet.

Transits of Mercury are somewhat rare, occurring only 13 to 14 times each century, in spite of the fact that it passes between the Earth and sun about 315 times per century. This is because its orbit is tilted about 7 degrees with respect to Earth’s. Most of the time, Mercury passes above or below the half-degree disk of the sun. In order for it to be seen in transit, it has to not only be between the Earth and sun, but be very near the plane of Earth’s orbit. Figure 2 (at right, bottom) shows the orbits of Earth and Mercury from above Earth’s north pole. Both planets move eastward, counterclockwise, in their orbits and Earth is only in a position allowing us to see Mercury transit in early May and early November.

Mercury carries the name of the swift messenger of Roman mythology because it moves faster than any other planet. Taking only 88 days to orbit the sun (with respect to the stars), it experiences more than four Mercurian years for every year we experience on Earth. Since Earth is also moving in its orbit, the time between consecutive alignments of Mercury and the sun from Earth’s point of view is 116 days on average, but can vary from 105 to 129 days because Mercury’s orbit is quite elliptical, unlike Earth’s, which is nearly circular. At its closest approach to the sun, perihelion, it’s 28.6 million miles from the sun (Earth’s perihelion distance will be 91.4 million miles on Jan. 5, 2020). At aphelion, farthest from the sun, Mercury is 1.5 times farther than at perihelion, 43.4 million miles (Earth will be 94.5 million miles from the sun at aphelion on July 4, 2020 — about 5% farther than its perihelion distance.)

Figure 2: The orbit of Mercury is tilted 7 degrees with respect to Earth’s, so transits are only possible when it is passing between the Earth and sun as it crosses the plane of Earth’s orbit. Due to the orientation of the orbits, these only occur in early May or early November. (Provided image)

Isaac Newton taught us that a planet’s orbital speed depends on its distance from the sun, so the ellipticity of Mercury’s orbit means it varies in speed from 96,700 mph to an extremely zippy 121,200 mph. Earth is no slouch, though, as it varies from 65,500 to 67,800 mph, but those are only a bit more than half of Mercury’s astounding speed.

The ancients were able to infer these speeds from the fact that Mercury occurs as a morning star, rising before the sun, and evening star, setting after the sun three to four times per year. Though it was mostly lost in the dusk of sunset, Mercury has been in our evening sky since the beginning of September. After the transit, it will become visible in the morning sky through November and December. It will be farthest from the sun (and easiest to spot) on Nov. 28. Look for it around 6:30 a.m. about 10 degrees (approximately the width of your fist at arm’s length) above the southeastern horizon.

It will be close to Zubenelgenubi in Libra and Mars will be another 10 degrees above and right of it.

The observers of the Adirondack Sky Center invite you to view the transit of Mercury through our telescopes if it is clear on Monday, Nov. 11 at the Roll Off Roof Observatory (RORO).

If it is clear, we plan to be open with solar-filter equipped telescopes and binoculars, allowing you to view this rare event. Please check our website at https://www.adirondackskycenter.org and our Facebook page for updates on the weather and whether we will open. The RORO is also open to the public on the first and third Fridays of each month and we invite you to come explore the sky with us.

On our public observing days you can call the RORO at 518-359-6317 to talk with one of our astronomers.