Psyched For Tomorrow’s Mercury Transit? Me Too — Come Join Us

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 or ingress (left); midpoint and transit end or egress (right). Credit: Tom Ruen with additions by author

On Monday May 9, the planet Mercury will be out in front of the sun nearly all day. Instead of hiding in its glare, the innermost planet will pass directly across the sun’s face in an event called a transit. Our sun is big and Mercury is both small and far away (52 million miles / 83.6 million kilometers), so it takes 7 1/2 hours for the little orb to travel from one side of the solar disk to the other.

Mercury is too small to see silhouetted against the sun with the naked eye and sun filter alone, so you’ll need a scope. Anything that magnifies around 30x that’s equipped with a safe solar filter will do the trick. The planet will look like a small black dot similar to a sunspot but perfectly circular and black as midnight. No scope? If you happen to be in the Duluth, Minn. area maybe we can help.

I along with my colleagues from the Arrowhead Astronomical Society will be out in front of the Marshall W. Alworth planetarium on the University of Minnesota-Duluth campus for the transit duration. The forecast is perfect — mostly sunny skies with the temperature in the mid-50s. Some of us will be there as early as 6:15 a.m., but most, including myself, will set up shop between 9 and 10 a.m. Mercury enters the solar stage around 6:15 a.m. CDT and exits at 1:42 p.m.

I’d love it if you stopped by. With all those scopes, you won’t be able to miss us. Stay for 5 minutes or stay for an hour. We’ll be there till transit end.

Watching a Mercury transit will give you a feel for how astronomers discover planets orbiting around other star. Much can be gleaned about a planet's properties by careful measurements made during its stellar passage. Credit: NASA/Ames
Watching a Mercury transit will give you a feel for how astronomers discover planets orbiting around other star. Much can be gleaned about a planet’s properties by careful measurements made during its stellar passage. Credit: NASA/Ames

When you watch the transit, you’re witnessing in real time how professional astronomers discover the majority of extrasolar planets: they measure the drop in light as a planet passes in front of its host star. Measuring the depth in the dip in brightness, scientists can determine the object’s diameter. From the time between successive transits, they measure its orbital period or how long it takes to go around the star. With the orbital period known, Kepler’s Third Law of Planetary motion yields the average distance between planet and star.

The table below lists the times of the event across the four time zones of the continental U.S. If clouds prevent you from viewing this cool and rare event, Italian astronomer Gianluca Masi will live stream it from his telescope starting at 6 a.m. CDT (11:00 UT) Monday. For more detailed information, check out this earlier blog on the topic. And though I’ve said this before, let me caution again never to look directly at the sun without a safe, secure solar filter.

I hope to see you tomorrow!

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.

6 Responses

  1. caralex

    Bob, if Mercury and the sun were in fact, an exosystem, how much of the sun’s light would really be dimmed, given the tiny size of Mercury?

    1. astrobob

      Hi Carol,
      Now there’s a great question. I’m so bad at math, but let’s see if we can figure at least half this out. Maybe we’ll get some help. Let’s assume the sun’s 30 arc minutes (1800 arc seconds across). Mercury is 10 arc seconds, so it would block 0.55% of the solar disk. Exactly how much that would dim the sun’s magnitude is where I need help. Anyone?

  2. Steve Moline

    I’m no math expert, but I will take a shot at it. The magnitude scale is a 5th root of 100 (Pogson), so not quite a logarithm, but close at 2.51 roughly.

    Just guessing here, without running the numbers, and being much the novice, but 0.55% in direct luminosity would be 1 – .0055 =.9945 to 2.51 power = .986, so roughly a 1.4% reduction in magnitude. Sound right?

    Steve (Oshkosh)

    1. astrobob

      Thanks Steve. The sun’s magnitude is -27, so a 1.4% reduction would be … 0.38 mag reduction?

        1. astrobob

          Carol,
          Yes, you’d need an instrument to detect that small difference. But something tells me 0.38 is still too high.

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