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The Transit of Mercury May 9, 2016

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The Transit of Mercury May 9, 2016
By: Brian Ventrudo, Contributing Editor
April 18, 2016

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The planet Mercury transits the face of the Sun on May 9, 2016. It's the first transit of the tiny inner planet in nearly 10 years. The entire event is visible from eastern North America and western Europe, while observers in western North America see the transit in progress as the Sun rises. While Mercury will appear much smaller than Venus during its spectacular transits in 2004 and 2012, with a little preparation and the right gear, you can see for yourself this intriguing celestial event.
Fig. 1: The disk of Mercury, below and right of the center of the Sun's disk, during the transit of November 8, 2006. Credit: Brocken Inaglory through Wikipedia Commons.

1. Introduction

The planet Mercury will appear to pass across the face of the Sun on Monday, May 9, 2016. This event, known as a transit, will be visible in a small telescope with a proper solar filter from much of North and South America, Africa, and western Europe. It's a great opportunity to see the mechanics of the solar system in action and to spot the elusive inner planet as it passes across the blazing solar disk.

Transits of Mercury occur just 13 to 14 times each century, so they are relatively rare. The last transit of Mercury occurred on Nov. 8, 2006. The next two happen on Nov. 11, 2019 and Nov. 13, 2032. Venus, the only other planet to appear to transit the Sun as seen from Earth, does so far less frequently, only twice per century on average. The last two transits of Venus were on June 8, 2004 and June 5, 2012. The next pair occur more than a hundred years from now in 2117 and 2125. So if you want to see a transit of an inner planet in your lifetime, it's going to have to be a transit of Mercury.

2. Visible from Much of North America and Western Europe

The May 9, 2016 transit of Mercury occurs over a 7.5 hour period from 11:12 Universal Time (UT) to 18:42 Universal Time. This handy online calculator converts Universal Time, which is essentially Greenwich Mean Time, to your own time zone. The exact timing depends very slightly on your location and can vary by a minute or two.

Fig. 2: The projected path of Mercury across the disk of the Sun during the transit of May 9, 2016. Image credit: Fred Espenak at www.EclipseWise.com.

The transit begins as the leading edge of Mercury's tiny disk just contacts the face of the Sun. Just 3 minutes and 12 seconds later, near 11:15 UT, the full disk of the planet becomes visible against the Sun. The point of greatest transit, when Mercury lies closest to the center of the Sun's disk, occurs at 14:57 Universal Time. The leading edge of the planet moves off the Sun's disk at 18:39 UT, then the trailing edge exits the disk, and the transit ends, at 18:42 UT. At greatest transit, the center of Mercury's disk will be 318.5” from the center of the Sun's disk. The diagram above from Fred Espenak at EclipseWise.com shows the path of the planet, the timing, and the celestial coordinates of the Sun and Mercury at greatest transit.

The timing of this transit of Mercury favors observers in eastern North America, most of South America, and western Europe. The full transit will be visible in these regions. In western North America, Chile, and Hawaii and much of the Pacific, the transit will be in progress as the Sun rises. In central Europe, western Asia, India, and Africa, the transit will be in progress as the Sun sets. This transit is not visible in Australia and New Zealand.

3. Tips on Observing the Transit of Mercury

The disk of the Mercury is just 12” across, too small to see with your unaided eyes and a pair of eclipse glasses. The planet will have an apparent area just 3% that of Venus during its transit, for example, so it will be hard to distinguish Mercury from a small sunspot. You will need a telescope at a magnification of at least 50x to see or image the transit. And, of course, you will need a good solar filter to keep the brilliant light of the Sun to a safe level. At no time during the transit of Mercury is it safe to look towards the Sun or attempt to image the Sun without a proper solar filter.

Which solar filter works best for observing a transit? Both main types of solar filter-- broadband or “white-light” solar filters and narrowband or “hydrogen-alpha” solar filters work just fine for visual observation and imaging. A white-light solar filter made from Baader AstroSolar film, mounted in a cell and placed over the objective of a telescope, gives an excellent image with most scopes. These filters come in sizes to match most instruments including spotting scopes, camera lenses, refractors, Newtonian reflectors, and Schmidt-Cassegrain scopes.

For refractors of less than 4” aperture, a Herschel wedge (also called a solar wedge) presents another option for observing the transit in white light. These devices take the place of a star diagonal and reduce the Sun's light and heat with a glass prism and integrated neutral density filter. Many experienced solar observers claim these devices produce the best images of the Sun in white light. These devices are available from Lunt Solar Systems and Baader.

Narrowband H-alpha solar filters enable dramatic visual views and images of the transit of Mercury against the seething red-orange chromosphere of the Sun. These solar-filters sets, which include the solar filter itself, a mounting cell, and a special solar blocking filter mounted in a diagonal, are available for many models of astronomical refractors. They also come integrated in dedicated H-alpha solar telescopes from Lunt Solar Systems and Meade/Coronado. Unlike white-light solar filters, H-alpha solar filters show arcing prominences extending from the limb of the Sun. So with a little luck, you might see the disk of Mercury pass through a prominence as it approaches and recedes from the solar disk.

If you find yourself without a solar filter, you can project the image from your telescope with your lowest-power eyepiece onto a sheet of white paper a couple of feet away. This projection method, which is recommended only for scopes with aperture 80mm or less, isn't as effective as direct observation with a solar filter, but it should be sufficient to show Mercury's disk during the transit. Take great care when using this method to ensure no one looks through the unfiltered telescope.

Most backyard astronomers enjoy the transit of Mercury simply for pleasure. But keen observers can make a contribution to science by attempting to measure the precise timing of the transit from their location. ALPO (The Association of Lunar and Planetary Observers) has a program to study the transit. You can learn more at this link.

Whether your interests lie in observing and enjoyment or precise measurement and science, plan to have a look at the May 9, 2016 transit of Mercury. It's the first in nearly 10 years, and there are only two more transits in the next couple of decades. So gear up and get ready to see the solar system in action.

Brian Ventrudo

About the Author

Brian Ventrudo is a writer, scientist, and astronomy educator. He received his first telescope at the age of 5 and completed his first university course in astronomy at the age of 12, eventually receiving a master's degree in the subject. He also holds a Ph.D. in engineering physics from McMaster University. During a twenty-year scientific career, he developed laser systems to detect molecules found in interstellar space and planetary atmospheres, and leveraged his expertise to create laser technology for optical communications networks. Since 2008, Brian has taught astronomy to tens of thousands of stargazers through his websites OneMinuteAstronomer.com and CosmicPursuits.com.


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