How, Where and When to Watch the Transit of Mercury in the Bay Area

On Nov. 11, a rare and awe-inspiring astronomical event will take place in our skies: The planet Mercury will cross directly between Earth and the sun, appearing in silhouette against the sun’s bright face.

Remember Mercury? That elusive, scarcely seen, rarely talked about and smallest of planets in the solar system? The Nov. 11 event will place the tiny planet in the spotlight for several hours and give us all something to talk about.

The event, called a Transit of Mercury, occurs at intervals measured in years and decades. On average, the planet transits only 13 times a century.

The last one happened on May 9, 2016. The next isn’t until Nov. 13, 2032—and that one won’t be visible from North America. The next one that people in the San Francisco Bay Area can see won’t take place until 2049.

When to Observe

This year’s event begins on Monday Nov. 11 at 4:35 a.m. PST, when Mercury makes first contact with the sun’s limb and begins its crossing. Technically, there are two moments of contact between Mercury and the sun at the beginning of the transit. Contact I happens when the leading edge of Mercury’s disk first coincides with the limb of the sun. Contact II is the moment  Mercury’s trailing edge crosses the solar limb and the planet’s silhouette becomes a complete circle against the sun’s backdrop.

The midpoint of transit is at 7:20 a.m. PST. On the Nov. 11 transit, Mercury’s path slices almost directly through the middle of the sun’s disk, so at midtransit it’ll be possible to find the planet at almost dead center.

Transit ends at 10:04 a.m. PST when Mercury departs the sun and returns to the backdrop of space. The discrete moments that Mercury’s disk begins and ends its crossing of the solar limb are called Contact III and Contact IV.

From the San Francisco Bay Area, the transit will already be in progress when the sun rises at 6:45 a.m. PST. The sun may not be immediately visible if there are buildings, trees or hills on your eastern horizon, but don’t worry. When the sun finally rises above all easterly obstructions, Mercury should still not have reached midtransit, and you’ll have a good three hours to enjoy the show.

Where to Observe

Finding Mercury during the transit couldn’t be easier. Just find the sun. By definition, Mercury will be located somewhere on the sun’s disk throughout the event.

But if you’re not properly equipped to observe the transit and need some seasoned assistance from people who are, you might check on  any observing parties scheduled in your area. Sidewalk astronomers and other amateur enthusiast groups who set up observing camps and invite the public to join them will be equipped with special telescopes, viewing filters, and other methods for sighting Mercury’s solar silhouette.

In the East Bay, Chabot Space & Science Center will open the gates to its observatory complex at 6:30 a.m. PST. You can find details of the event on the Chabot website.

How to Observe

When talking about how to observe the Transit of Mercury, the best place to start is how not to.

Do not look directly at the sun, not even if you’re wearing sunglasses. This point cannot be stressed strongly enough. Looking directly at the sun without proper eye protection or a specialized sun-viewing instrument is dangerous and can damage your eyes, possibly permanently.

You may safely look at the sun if you have the right kind of filter, such as industrial grade welder’s mask glass (#14), or other sun-safe-rated eyewear. Both are specialty items designed to limit direct sunlight exposure to your eyes to safe levels in ways ordinary sunglasses do not.

Even with filters, it’ll be a challenge to see the tiny spot of Mercury’s silhouette without magnification.

Mercury’s angular size will be less than three-thousandths of a degree. That’s equivalent to the size of a nickel seen from a thousand feet away.

So, finding someone who has a telescope equipped with special filters is really the best way to see the Transit of Mercury.

What Can We Learn From Transiting Planets?

Planetary transits are not merely breathtaking spectacles of nature. They are also events of great scientific value that have aided us in exploring the universe.

In the 17th and 18th Centuries, astronomers used observations of transits of the planet Venus to geometrically triangulate the distances to Venus and the sun. Because those scientists already knew the relative proportions of the solar system, this observation revealed the actual distances to all the planets. A Mercury transit may also be used to make this calculation.

By 1992, technology had advanced enough for astronomers to detect transiting planets in other star systems. To date, over 4,000 extra-solar planets, or exoplanets, have been detected and confirmed in more than 3,000 star systems in our neighborhood of the Milky Way galaxy. Transit observations led scientists to discover most of these exoplanets.