Watch Mercury Skip Across The Sun On May 9

The May 7, 2003 transit of Mercury captured by the Solar and Heliospheric Observatory (SOHO). Credit: NASA/ESA
The May 7, 2003 transit of Mercury captured in multiple images by the Solar and Heliospheric Observatory (SOHO). Credit: NASA/ESA

On Monday May 9th, we have a rare opportunity to see the solar system’s innermost planet pass directly in front of the sun. Maybe you saw one or both Venus transits back in 2004 and 2012. All you needed then was a handheld solar filter to that planet’s perfectly black, perfectly circular self silhouetted against the brilliant sun. Mesmerizing.

Mercury photographed with a sunspot during its last transit on Nov. 8, 2006. Mercury looks different from a sunspot because it's exactly round and pure black. Sunspots typically have two parts: a darker, inner umbra and lighter, outer penumbra. Credit: Edhiker / Wiki Commons
Mercury photographed with a sunspot during its last transit on Nov. 8, 2006. Mercury looks different from a sunspot because it’s exactly round and pure black. Sunspots typically have two parts: a darker, inner umbra and lighter, outer penumbra. Credit: Edhiker / Wiki Commons

Mercury transits aren’t quite as rare as the Venusian variety, but the last occurred nearly 10 years ago, so they’re uncommon. You’re going to need two things to enjoy the upcoming event: a telescope and safe solar filter. Mercury will only appear one-sixth as large as Venus, so a telescope magnifying around 30 times will do the trick.

Not only is Mercury physically smaller than Venus, it’s also farther away. Through a telescope it will look like a small, perfectly round “sunspot” creeping across the sun’s face.

If you don’t already have a solar filter, you can pick one up at  Orion Telescopes, Thousand Oaks Optical, Kendrick Astro Instruments and Amazon.com. Or consider making your own by purchasing a sheet of Baader AstroSolar aluminized polyester film and cutting it to size. Despite the film’s flimsy appearance, it provides a sharp image, excellent contrast and a pleasant, neutral-toned solar image. You can purchase any of several different-sized films to suit your needs either from Astro-Physics or on Amazon.com.  Prices range from $40-90.

Two basic types of safe solar filters for telescopes: an aluminized polymer such as Baader film and a dedicated glass solar filter for a particular make and model. Credit: Bob King
Two basic types of safe solar filters for telescopes: an aluminized polymer such as Baader film and a dedicated glass solar filter for a particular make and model. With the Baader material, resist the temptation to smooth it out. Credit: Bob King

When the material arrives, follow these instructions to make a snug-fitting telescopic solar filter. If for whatever reason, you can’t get a hold of a filter but have a telescope, use a low power eyepiece to project the sun’s image onto a white sheet or piece of paper. Remember to NEVER look directly at the sun at any time during the transit.

Time Zone Eastern (EDT) Central (CDT) Mountain (MDT) Pacific (PDT)
Transit start 7:12 a.m. 6:12 a.m. 5:12 a.m. Not visible
Mid-transit 10:57 a.m. 9:57 a.m. 8:57 a.m. 7:57 a.m.
Transit end 2:42 p.m. 1:42 p.m. 12:42 p.m. 11:42 a.m.
Map showing Mercury’s path across the Sun at three key points on May 9: transit start or ingress (left); midpoint and transit end or egress (right). Credit: Tom Ruen with additions by author
Map showing Mercury’s path across the Sun at three key points on May 9: transit start, also called ingress (left); midpoint and transit end or egress (right). Times shown are CDT and UT (Universal Time) Credit: Tom Ruen with additions by author

As you can see from the table above, the transit begins in the early morning on May 9 for much of the U.S., Canada and Central and South America and last for 7 1/2 hours. That’s how long it will take for Mercury, buzzing along its orbit at an average speed of 105,947 mph (170,505 kph), to cross from one end of the sun’s disk to the other. Such a lengthy passage is great news for skywatchers. Even if the weather forecast is uncertain, the event lasts so long it’s bound to partially clear at least for a short spell.

World map showing where the May 9-10 Mercury transit will be visible. Universal times of the four key contacts (see below for details), mid-transit time and position angle on the Sun's limb where the planet will first appear and disappear are given at upper left. To find Credit: Xavier M. Jubier
World map showing where the May 9-10 Mercury transit will be visible. Universal times of the four key contacts, mid-transit time and position angle on the Sun’s limb where the planet will first appear and disappear are given at upper left. Credit: Xavier M. Jubier

I plan on taking the day off, but if you have to work, consider bringing a portable telescope and taking a peek during lunch hour. Wherever you are, make the transit an opportunity to share this cosmic dance with friends, co-workers or even strangers.

Just watching Mercury crossing this sun from east to west is a pleasure in itself, but there are several key events more ardent observers may want to look out for:

First contact (11:12 UT/6:12 a.m. CDT): Watch for the planet to first “notch” the sun just south of the due east point.

Second contact (11:15 UT/ 6:15 a.m. CDT): About 3 minutes later, Mercury’s trailing edge touches the sun’s inner limb. Does the planet make a clean break from the limb or seem to “stick” to it for a few moments with a stretchy, drop-like appearance?

 

The "black drop effect" can be reproduced by slowly bringing your thumb and index finger together. It's caused by diffraction combined with blurring from the atmosphere. Credit: Bob King
The “black drop effect” (left) — visible when Mercury is very close to the sun’s inner edge —  can be reproduced by slowly bringing your thumb and index finger together. It’s caused by diffraction combined with blurring from the atmosphere. Credit: Bob King

This “black drop effect” is caused primarily by the bending and interfering of light waves (called diffraction) when they pass through the narrow gap between Mercury and the sun’s edge. You can replicate the effect by holding your hand up against a bright backdrop and slowly bringing your thumb and index finger closer and closer together. Immediately before they touch, a black, stretchy arc will flitter in the gap between them.

Third contact (18:39 UT/ 1:39 p.m. CDT): Just before Mercury’s leading edge touches the opposite limb of the sun, watch for the black drop effect to return.

Fourth contact (18:42 UT/ 1:42 p.m. CDT): Mercury exits the Sun. The next transit occurs on November 11, 2019 and favors the the western hemisphere and Europe.

Let’s hope for good weather on May 9, but if you’re blocked by clouds, Italian astrophysicist Gianluca Masi will stream the transit live on his Virtual Telescope website starting at  11:00 UT (6 a.m CDT).