May 9th Transit of Mercury: Everything You Need to Know

The littlest planet will cross the enormous Sun for viewers in most of the world.

As the Sun crosses the sky on May 9th, Mercury will cross the face of the Sun for the first time since 2006.

Mercury in transit appears tiny, but unlike sunspots, it’s round and has no penumbra. It moves noticeably in 15 minutes, the average time between these exposures taken by Dominique Dierick during the transit of May 7, 2003.
Dominique Dierick

If you’re in western North America, the rising Sun will already display Mercury’s telltale black dot, as indicated on the world map at right. Easterners and many Western Europeans will be able to watch the entire transit, weather permitting, from Mercury’s first nudge onto the Sun’s face to its final slide-away 7½ hours later.

For the rest of Europe, Africa, and most of Asia, the transit also begins in the daytime but will still be underway when the Sun sets. Folks in Australia and eastern Asia will just have to watch online.

Most of the inhabited world will be able to see at least part of the transit, weather permitting. Observers in eastern North America and western Europe can watch both the entry and exit of Mercury across the Sun’s edges.
Sky & Telescope diagram

Looking for Sky & Telescope's live webcast? Go to Livestream to watch the full transit of Mercury, with commentary at the top of every hour between 7 a.m. and 3 p.m. EDT:
http://livestream.com/SkyandTelescope/Mercury

Transit Timetable

The timetable below tells when the first edge of Mercury enters the Sun and the last edge leaves (first and last contact), in Universal Time and in civil daylight-saving times for North America. Times of the events will differ by a few minutes as seen from various locations on Earth.

TIME ZONE	TRANSIT BEGINS	TRANSIT MIDPOINT	TRANSIT ENDS
Universal (GMT)	11:12	14:57	18:42
Eastern (EDT)	7:12 a.m.	10:57 a.m.	2:42 p.m.
Central (CDT	6:12 a.m.	9:57 a.m.	1:42 p.m.
Mountain (MDT)	5:12 a.m.	8:57 a.m.	12:42 p.m.
Pacific (PDT)	*	7:57 a.m.	11:42 a.m.
Alaskan (AKDT)	*	6:57 a.m.	10:42 a.m.
Hawai‘ian (HST)	*	*	8:42 a.m.

Times for your location may differ by several minutes. (*Transit begins before sunrise.)

Safe Solar Viewing

If you don’t have a safe white-light solar filter that mounts over the front of your telescope, now’s the time to get one. They’re available from astronomy dealers in many sizes and fits. When we reviewed them, we liked the ones made with Baader Astro-Solar aluminized polyester the best (February 2005 issue, page 102, and July 1999 issue, page 63). This material is optically superb despite its wrinkly appearance, and it leaves the Sun a fairly natural color.

Alternatively, you can use an unfiltered telescope with your lowest-power eyepiece to project an image of the Sun’s disk onto white paper a foot or two behind the eyepiece, and watch the events transpire on the paper. But a direct view through a solar filter shows the scene better. (Of course, never look directly at the Sun without a proper filter.)

What to Watch For

Observers in the eastern half of North or South America get to see the entire transit on May 9th. At locations farther west, the event begins before sunrise. This diagram shows celestial north as straight up. When the Sun is in the morning sky, celestial north is tilted some tens of degrees counterclockwise from up, depending on the time and your latitude. In the afternoon, the Sun will appear rotated clockwise from this view.
Sky & Telescope diagram

Mercury is the smallest planet, and you'll need a telescope to observe its transit. The black silhouette will appear only 12 arcseconds wide even though Mercury is at inferior conjunction. That’s about 1⁄160 of the Sun’s width, and only a fifth the diameter (and 4% of the area) of Venus’s dramatic black disk during the rare transits of Venus. At first glance you might mistake Mercury for a small sunspot — but look again. It’s precisely round and lacks a gray penumbra.

And it moves! The most interesting aspects to watch will be Mercury making its entrance and/or exit across the Sun’s limb. The planet will take 3 minutes and 12 seconds to do so. If you can watch at the time of ingress, keep your eye on the limb barely south of due east for the first detectable sign of a tiny dent. Use high power. You can tell which limb is celestial east by turning off your telescope’s drive if it has one; the Sun will drift across the eyepiece view from east to west.

Don't miss the chance to watch Mercury slip off the solar disk. Start looking by about 18:35 Universal Time (2:35 p.m. Eastern Daylight Time). Be alert for any hint of the "black drop effect" as the silhouette nears contact with the edge.
Fred Espenak

At second contact — when Mercury’s trailing edge comes onto the Sun — watch for any sign of the black drop effect: the appearance of a tiny black line still connecting the planet to the outer darkness. And does the black disk show a central point of light? Read more about such anomalous appearances on page 38 of the May issue.

As Mercury travels across the Sun’s vast expanse, how readily can you see its motion? If it passes near a sunspot, can you see that it’s darker than even the sunspot’s umbra? When Mercury departs at egress, the sequence of phenomena at ingress unwinds in reverse order.

The Next Transit of Mercury

Although transits of Mercury are less dramatic than those of Venus, they come more often. The last two Venus transits happened in 2004 and 2012 after a gap since 1882, and not until 2117 will the world see another. But Mercury passes between the Earth and Sun about 13 or 14 times every century. It will next do so on November 11, 2019 — again visible from the Americas and Europe.

There are a couple of reasons for the difference. Mercury rounds the Sun more frequently than Venus does and passes through inferior conjunction five times as often. And Mercury is also closer to the Sun, so from Mercury, the Sun presents a larger target for a line of sight from Earth through the planet to hit.

As you’re watching the transit, imagine a copy of Earth replacing Mercury. It would look only 2.6 times wider than the tiny dot — a reminder of how insignificant the terrestrial worlds appear next to the awesome scale of our home star.