Dimmer Time

May 9, 2016
Mercury transit of 2013Credit Thierry Legault / nasa
Mercury transit of 2013

This week Hal tells us about the not so speedy transit of Mercury.  

Did the Sun look a little dimmer to you today? Well, no. But something did block a tiny portion of the Sun’s light today. Starting just after 5 AM, and finishing about 12:45 PM, the planet Mercury is transiting the Sun’s disk, today! What does that actually mean?

All the eight planets revolve around the Sun in the same flat plane. Because of that, the two planets closest to the Sun, Mercury and Venus, every now and then appear to observers on Earth to be passing in front of the Sun.

Venus last did this in 2012, and it won’t do it again until 2117. But because of various factors of orbital mechanics, the innermost planet, Mercury, passes in front of the Sun, relative to Earth, much more often. The next pass of Mercury we’ll see in front of the sun is November 2019.

So what will you see if you look at the Sun with Mercury transiting? Nothing! You will go blind! Never, ever, ever look at the Sun directly!

But if you have access to a telescope with proper solar filters, you will see the beautiful bright disk of the Sun with a tiny perfectly round, black shadow crossing it. Over the course of about 7 ½ hours, Mercury’s shadow will cut across roughly the bottom one third of the Sun’s disk. It may look a little bit like a sunspot, but you will be able to easily tell the difference through a telescope, because sunspots look blurry, without distinct edges, while Mercury’s shadow will appear to be a dramatic black disk. 

Mercury transit - May 2016Credit M.Procell
Mercury transit - May 2016

   The shadow will not be large - you could put 200 of them across the disk of the Sun, because Mercury is not a very large planet. And Earth would not be much more impressive. An astronaut on Mars, stranded or otherwise, observing Earth transiting the Sun would see that Earth’s shadow was only about 2 ½ times bigger than Mercury’s.

But once again, don’t look directly at the Sun. It’s not a bright idea.

If you’d like to take a closer look at Mercury or the Sun, or any of the other wonderful and amazing things in the sky, please visit KRCC.org or CSASTRO.org for a link to information on our monthly meetings and our free public star parties! 

This is Hal Bidlack for the Colorado Springs Astronomical Society, telling you to keep looking up, Southern Colorado!