May 9, 2016, Mercury Solar Transit Takes Seven and One-Half Hours

Summary: The Monday, May 9, 2016, Mercury solar transit, in which Mercury crosses between Earth and the sun, lasts seven and one-half hours.

orientation of Earth during transit of Mercury, May 9, 2016: Tomruen, CC BY SA 4.0, via Wikimedia Commons

The Monday, May 9, 2016, Mercury solar transit, in which inner planet Mercury transits, or crosses, the face of the sun, takes seven and one-half hours, from 11:12 to 18:42 Coordinated Universal Time (7:12 a.m. to 2:42 p.m. Eastern Daylight Time).

The astronomical event is visible, via telescopes with solar filters, from Earth. Mercury will appear as a small black disk moving from west to east, across the face of the sun. The May 9, 2016, Mercury solar transit is visible, wholly or partially, from most of Earth. Australia, Eastern Asia, Indonesia and New Zealand lie outside of the viewing area.

Times of five critical instants during the seven and one-half hour transit pinpoint Mercury's placement with respect to the sun's face. Four contact times are paired as frames around a central point, known as the greatest transit.

Contact I, the instant that Mercury appears externally tangent to the sun, marks the beginning of the transit. Mercury's leading edge is adjacent to the sun's outer limb, or edge. Contact I occurs at 11:12 UTC (7:12 a.m. EDT).

Contact II, the instant that Mercury's trailing edge is internally tangent to the inner solar limb, marks the beginning of the small planet's passage across the sun's face. Contact II occurs at 11:15 UTC (7:15 a.m. EDT).

Greatest transit describes the instant of Mercury's closest passage to the sun's center as seen from Earth's center. Greatest transit is the midpoint in Mercury's transit. Greatest transit takes place at 14:57 UTC (10:57 a.m. EDT). Greatest transit occurs 3 hours 45 minutes after Contact I, the instant of external tangency with the sun, and 3 hours 42 minutes after Contact II, the instant of internal tangency with the sun.

Contact III marks the instant of Mercury's reach of the opposite limb of the sun. Mercury's leading edge is internally tangent to the sun's opposite, inner limb. Contact III occurs at 18:39 UTC (2:39 p.m. EDT). Contact III happens 3 hours 42 minutes after greatest transit.

Contact IV, the instant that Mercury appears externally tangent to the sun, signals the end of Mercury's transit. Mercury's trailing edge is adjacent to the sun's outer limb. Contact IV takes place at 18:42 UTC (2:42 p.m. EDT). Contact IV occurs 3 hours 45 minutes after greatest transit.

From the perspective of Earth, transits by only two planets are viewable. As the two closest planets to the sun, Mercury and Venus have orbits that allow for occasional passages between Earth and the sun.

NASA astrophysicist Fred Espenak, known popularly as “Mr. Eclipse,” notes on NASA's Eclipse Web Site the relative rarity of transits. Mercury averages 13 transits in a century.

A Mercury solar transit requires two astronomical events. An inferior conjunction with the sun places Mercury between Earth and the sun. Mercury orbit also crosses through the ecliptic, the Earth's orbital plane.

Mercury has an orbital tip of 7 degrees to Earth's orbit around the sun. Currently, Mercury's orbit crosses the plane of the ecliptic twice a year, in May and in November. May transits occur at the descending node, the point where Mercury's orbit crosses Earth's orbital plane from north to south. November transits occur at the ascending node, the point where Mercury's orbit crosses the ecliptic from south to north.

May transits happen about a month after Mercury's reach of aphelion (Ancient Greek: ἀπό, apó, “from” + ἥλιος, hḗlios, “sun”), or farthest center-to-center orbital distance from the sun. Planetary velocity decreases at greatest distance from the sun. At aphelion, Mercury has an orbital velocity of 38.9 kilometers per second.

November transits precede Mercury's reach of perihelion, or closest center-to-center orbital distance to the sun, by only a few days. At perihelion, Mercury has an orbital velocity of 59.0 kilometers per second.

According to NASA's Eclipse Web Site, the seven centuries between 1601 and 2300 CE (Common Era) will yield 94 Mercury solar transits visible from Earth. November transits are more common, with 63 occurrences. May transits claim 31 occurrences.

The 21st century, spanning Jan. 1, 2001, to Dec. 31, 2100, opens with a May transit and closes with a November transit. May dates account for only five of the century's 14 transits. The first happened May 7, 2003, from 05:13 to 10:32 UTC (1:13 a.m. to 6:32 a.m. EDT). The three occurring after the 2016 May transit take place Friday, May 7, 2049, from 11:03 to 17:44 UTC (7:03 a.m. to 1:44 p.m. EDT); Wednesday, May 10, 2062, from 18:16 to 00:57 UTC (2:16 p.m. to 8:57 p.m. EDT); Sunday, May 8, 2095, from 17:20 to 00:50 UTC (1:20 p.m. to 8:50 p.m. EDT).

The takeaway for the May 9, 2016, Mercury solar transit that takes seven and one-half hours is the excitement of the astronomical event's relative rarity, especially as a May happening.

“Astronomers get excited when any two things come close to each other in the heavens,” explains Louis Mayo, program manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland. “This is a big deal for us.”

graphic of Mercury's east-to-west solar transit May 2016; horizontal yellow line represents the ecliptic: Tomruen, CC BY SA 4.0, via Wikimedia Commons

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

orientation of Earth during transit of Mercury, May 9, 2016: Tomruen, CC BY SA 4.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Transit_of_Mercury_May_9_2016_Orientation_of_Earth.png

graphic of Mercury's east-to-west solar transit May 2016; horizontal yellow line represents the ecliptic: Tomruen, CC BY SA 4.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Transit_of_Mercury_May_9_2016_path_across_sun.png

For further information:

Byrd, Deborah. “See It! Monday's Transit of Mercury.” EarthSky > Today's Image. May 9, 2016.
Available @ http://earthsky.org/todays-image/see-it-mondays-transit-of-mercury

Espenak, Fred. “2016 Transit of Mercury.” Eclipse Wise > Observer's Handbook.
Available @ http://eclipsewise.com/oh/tm2016.html

Espenak, Fred. “Seven Century Catalog of Mercury Transits: 1601 CE to 2300 CE.” NASA Eclipse Web Site > Planetary Transits Across the Sun.
Available @ https://eclipse.gsfc.nasa.gov/transit/catalog/MercuryCatalog.html

Espenak, Patricia Totten. “Get Ready for May 9th's Transit of Mercury.” Sky & Telescope > Astronomy News > Observing News. April 14, 2014.
Available @ http://www.skyandtelescope.com/astronomy-news/observing-news/get-ready-for-may-9th-transit-of-mercury-041320162/

King, Bob. "How to Safely Watch Mercury Transit the Sun on May 9." Universe Today. April 27, 2016.
Available @ http://www.universetoday.com/128321/guide-upcoming-transit-mercury/#

MacRobert, Alan. “Mercury Transit: Everything You Need to Know.” Sky & Telescope > Astronomy News > Observing News. May 6, 2016.
Available @ http://www.skyandtelescope.com/astronomy-news/observing-news/may-9th-transit-of-mercury-everything-you-need-to-know/

MacRobert, Alan. “This Week's Sky at a Glance, May 6-14.” Sky & Telescope > Observing > Sky at a Glance. May 6, 2016.
Available @ http://www.skyandtelescope.com/observing/sky-at-a-glance/this-weeks-sky-at-a-glance-may-6-14/

“May 9 / May 10, 2016 -- Mercury Transit.” Time And Date > Sun & Moon > Eclipses.
Available @ http://www.timeanddate.com/eclipse/transit/2016-may-9

McClure, Bruce. “Transit of Mercury on May 9, 2016.” EarthSky > Tonight. May 7, 2016.
Available @ http://earthsky.org/tonight/mercury-transit-may-9-2016-how-to-watch

NASA.gov Video. “2016 Mercury Transit Path.” YouTube. May 3, 2016.
Available @ https://www.youtube.com/watch?v=IEkkCaBTgZ8

NASA SunEarth @NASASunEarth. “On Monday, May 9, several NASA satellites will see Mercury enter the spotlight.” Twitter. May 3, 2016.
Available @ https://twitter.com/NASASunEarth/status/727547440560279553

“November 11 / November 12, 2019 -- Mercury Transit.” Time And Date > Sun & Moon > Eclipses.
Available @ http://www.timeanddate.com/eclipse/transit/2019-november-11

Schlieder, Sarah. “Satellites to See Mercury Enter Spotlight on May 9.” NASA > Features > Eclipses and Transits. May 3, 2016.
Available @ http://www.nasa.gov/feature/goddard/2016/satellites-to-see-mercury-enter-spotlight-on-may-9

Walker, John. “Quarter Million Year Canon of Solar System Transits.” Fourmilab Switzerland > Canon Transits. Nov. 27, 2004.
Available @ http://fourmilab.ch/documents/canon_transits/

Webb, Brian. “Coordinated Universal Time (UTC)." Space Archive. March 27, 2016.
Available @ http://www.spacearchive.info/utc.htm