See the rare transit of Mercury on Monday

On Monday, Nov. 11, Mercury will pass in front of the Sun. Unlike solar eclipses which are measured in minutes, transits of inner planets like Mercury are measured in hours, so there will be plenty of time to experience this.

Posted 8:55 p.m. Nov 8, 2019 — Updated 3:54 p.m. Nov 11, 2019

Tony Rice

, WRAL contributor/NASA ambassador

The show begins shortly after sunrise, Mercury will appear at around the 7 o’clock position at 7:35 a.m. EST. It will reach nearly the middle of the Sun’s disk at 10:19 a.m. The show ends at 1:03 p.m., as the tiny dot moves off at nearly the 3 o’clock position.

Transits are essentially partial eclipses. Instead of the Moon coming between us and the Sun, it is Mercury. The same safety precautions apply. Never look directly at the Sun without protection, especially with a telescope. If you still have eclipse glasses from 2017, they’ll work great. Be sure to check them for damage and don't use if there are scratches or even small pinholes.

Other methods that worked well for viewing an eclipse, such as creating a pinhole projector, won't be as effective in seeing the transit. Mercury will appear as a tiny dot on the face of the nearly 200 times larger Sun.

Mercury passes between us and the Sun pretty often. Its path around the Sun is much shorter than Earth’s. A year on Mercury lasts only 88 days.

The May 9, 2016, transit of Mercury as seen by the Solar Dynamics Observatory mission. Mercury's path appears straight when viewed from space while its more curved when viewed from Earth because of the Earth's tilt (image: NASA/SDO)

Like a speedy car on the inner lane of a racetrack, Mercury laps Earth about 7 to 8 times each year. So why doesn’t Mercury transit the Sun 7 or 8 times each year? The answer is the same as why don’t we see a solar eclipses each month: the orbit of the planets, Mercury and Venus included, as well as the orbit of the Moon, are a little tilted.

99% of those passes are a little above or a little below the Sun because of that tilted orbit. During the remaining 1%, which always occurs in either May or November, we can see this innermost planet moving across the face of the Sun.

Transits have a long history as important scientific events.

While observing the transit of Mercury in November 1677, Sir Edmund Halley came up with the idea to use transits of inferior planets like Mercury or Venus to measure Earth’s distance to the Sun.

Halley used the fact that each transit looks a little different to observers spread widely across the Earth. Each observer measured the times the transit started and stopped from their location. Using the principles of parallax, this information can be used to triangulate the distance to the Sun with a little high-school-level geometry.

By measuring the light from distant stars, astronomers can discover planets around those stars by noting when and how much the star dims (Image: NASA)

Today, transits are used by astronomers to discover planets outside of our solar system. In a process called transit photometry, astronomers watch distant stars for changes in brightness. If they dim at regular intervals and for a regular length of time, there is probably a planet passing in front of that star. You can determine a lot from these observations including orbital period (year length), size of the planet and sometimes what makes up its atmosphere,

The Morehead Planetarium in Chapel Hill is hosting a viewing event from 11:00 a.m. Monday through the end of the transit just after 1:00 p.m. Volunteers from local astronomy clubs will have solar telescopes available to provide a closeup view. Weather should be good. Skies are expected to be mostly clear for the start with only a slight increase in cloudiness through the early afternoon. There are also several online sources you can see the transit through solar telescopes including one from the Royal Observatory in Greenwich England.

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