Transit of Mercury 2019

The Transit of Mercury 2019

Are you ready for the Great Mercury Eclipse of 2019?

Mercury passing in front of the Sun May 9, 2016. Image Credit: Solar Dynamics Observatory, NASA

Ok, so it’s not exactly an eclipse, like the solar eclipse that passed over the United States in 2017, but it’s still pretty cool. On November 11, 2019, the planet Mercury will transit the Sun.

“Transit” is the technical name for eclipses; an eclipse is a special form of a transit. Transits occur when a smaller object passes directly in front of a larger astronomical object from your perspective. For example, a transit can happen when an asteroid passes in front of the Sun, or when the space station passes in front of the Moon. (The opposite of this, when a large object passes in front of a smaller one, is called an “occultation”. Frequently the Moon will pass in front of a planet, or an asteroid passes in front of a distant star blocking its light. These are common examples occultations.)

The planets of our solar system all orbit the Sun in nearly the same plane. You can think of them as moving around on an enormous tabletop with the Sun at the center. Two planets orbit the Sun inside the orbit of Earth, and that means that from time to time Mercury and Venus will pass directly between Earth and the Sun. Because the planets are “nearly” in the same plane and not “exactly” in the same plane, these transits occur relatively rarely, only when the geometry of the planets lines up just right.

Venus transiting the Sun on June 5, 2012. Venus appears as a
large black dot in the upper right. Sunspots are visible towards
the center of the Sun's disk. Image Credit: Brian Kennedy

Venus is by far the more impressive of the two transits, blocking enough sunlight to be visible to the naked eye (but you should never look directly at the Sun, because that can quickly damage your eyes permanently). Venus transits are also far rarer of the two. Venus transits occur in a strange pattern which repeats every 243 years. The first transit will occur followed by a second one 8 years later. Then 121 years will pass before the third, with the fourth following 8 years after that. Finally, another 105 years will pass before the pattern repeats. The last Venus transit occurred in 2012, and the next one won’t occur until 2117, so it is unlikely that anyone living today will see another in their lifetime.

Transits of Mercury are the next best thing. Mercury transits the Sun far more frequently than Venus, with transits occurring about 13 or 14 times in a century. This means that most people will have a chance at seeing one. The next transit of Mercury occurs on November 11, 2019. The following transit will occur in 2032, so this is the last chance for a while.

Mercury transiting the Sun on May 9, 2016. Mercury appears as
a small black dot in the lower right. Sunspots are visible toward
the center of the solar disk. Image Credit: Brian Kennedy

Mercury is far smaller and further away from Earth than Venus is, so it blocks much less of the Sun. Transits take many hours. The November 11 transit will last from about 7:35 am EST until about 1:04 pm EST. Throughout this time, Mercury will appear in silhouette, a small black dot against the surface of the Sun.

Of particular interest will be the times when Mercury enters and exits the disk of the Sun. During this time, a phenomenon called the “black drop” effect might be seen, when Mercury could appear less like a perfect circle, and more like an inky droplet. This effect is believed to be more pronounced with Venus, and is not well understood. The leading hypothesis is that the black drop effect is an optical effect that occurs because of some of the surface properties of the Sun. Recent observers of planetary transits have reported less pronounced or entirely absent black drop effects. This may have something to do with the superior telescopes that are widely available today.

Mercury exiting the Sun at the end of the 2016 transit. Image Credit: Brian Kennedy

While astronomers get excited when objects just line up in the sky, transits do have scientific value. Precisely measuring the time length of a transit, and knowing the observer’s location on Earth can allow scientists to accurately measure the distance between Earth and the other planet. From this calculation, we can then accurately measure the distances between the planets and understand the scale of the solar system.

An example of solar eclipse glasses.

How can you view the transit of Mercury? The rules for viewing a transit are very similar for viewing a solar eclipse. The first rule is never look directly at the Sun with your unprotected eyes. Looking at the Sun can damage your eyes quickly and permenantly. The second rule is NEVER EVER look at the Sun through any kind of magnifiying device (like binoculars or a telescope) which does not have a proper solar filter securly installed on the front of it. Looking at the Sun through an unfiltered telescope WILL damage your eyes very quickly and permenantly. If you're not sure if something is a proper filter, do not use it.

For safely viewing the transit you have a couple options. You can use solar eclipse glasses to protect your eyes to look at the Sun. These glasses are frequently made with a thin plastic film that blocks all of the harmful rays from the Sun, and nearly all the optical light from the Sun as well. As a result, the Sun ought to be the only thing that can be seen through these glasses. Before using, inspect the glasses for any scratches or holes in the protective film. If you find any, or any light other than direct sunlight is visible through them, throw those glasses in the garbage. Through eclipse glasses, Mercury should appear as a tiny black dot. But really to see the transit well, you will want to use a telescope.

An example of a solar filter mounted on a telescope.
Image Credit: flickr user Mr.TinDC, CC-BY-ND 2.0

There are a few different kinds of safe telescope options, but the two most common are using a solar filter on a regular telescope, or using a specially made solar telescope. Solar filters are very similar to eclipse glasses, frequently made from the same film, or from glass. These are attached to front the telescope, never the eyepiece, and will protect the telescope and your eyes from the harmful light of the Sun. Solar telescopes are specially designed with filters throughout the telescope to allow one to look directly at the Sun. In fact, the Sun is the only thing you can see through these telescopes. Solar telescopes are usually quite expensive, so your best option is to look for a local astronomy club, school, museum, or planetarium that is hosting a watch party with one.

And it just so happens there is one nearby! We will be observing the Novemeber 11, 2019 transit of Mercury from the Delta College Planetarium in our solar telescope. Provided the weather is clear, we’ll have a telescope set up to safely observe the Sun and Mercury for the duration of the transit. The transit starts at approximately 7:30 am and lasts to about 1pm. We will also have solar eclipse glasses available for visitors to try to spot Mercury with their eyes. It’s the last one until 2032, so don’t miss out.

One last thing before we go: you might have realized that Mars orbits further from the Sun than Earth, and maybe you wonder if Earth ever transits the Sun from Mars. It does! But nobody has ever seen it. The last of these occured in 1984, but there were no operational robots on Mars at the time to photograph it. The next will be in 2084, and there will almost certianly be robots on Mars at that time. Perhaps even people. But if you want to see something really cool, there are times when both Earth (and the Moon) and Venus transit the Sun from Mars at the same time. To see this rare occurance, set your calenders for the year 571,471. You don't want to miss that.