Explanation: The small, dark, round spot in this solar close up is planet Mercury. In the high resolution telescopic image, a colorized stack of 61 sharp video frames, a turbulent array of photospheric convection cells tile the bright solar surface. Mercury's more regular silhouette still stands out though. Of course, only inner planets Mercury and Venus can transit the Sun to appear in silhouette when viewed from planet Earth. For this November 11, 2019 transit of Mercury, the innermost planet's silhouette was a mere 1/200th the solar diameter. So even under clear daytime skies it was difficult to see without the aid of a safe solar telescope. Following its transit in 2016, this was Mercury's 4th of 14 transits across the solar disk in the 21st century. The next transit of Mercury will be on November 13, 2032.
<<A transit of Earth across the Sun as seen from Mars takes place when the planet Earth passes directly between the Sun and Mars, obscuring a small part of the Sun's disc for an observer on Mars. During a transit, Earth would be visible from Mars as a small black disc moving across the face of the Sun. During the event, the Moon could almost always also be seen in transit [at ~63% the relative Mercury to Sun transit size as seen in today's APOD], although due to the distance between Earth and Moon, sometimes one body completes the transit before the other begins (this last occurred in the 1800 transit, and will happen again in 2394).
No one has ever seen a transit of Earth from Mars, but the next transit will take place on November 10, 2084. The last such transit took place on May 11, 1984. A [Chesley Bonestell illustrated] science fiction short story published in 1971 by Arthur C. Clarke, called "Transit of Earth", depicts a doomed astronaut on Mars observing the transit in 1984. This short story was first published in the January 1971 issue of Playboy magazine.
A transit of Earth from Mars corresponds to Mars being perfectly uniformly illuminated at opposition from Earth, its phase being 180.0° without any defect of illumination. During the 1879 event, this permitted Charles Augustus Young to attempt a careful measurement of the oblateness (polar compression) of Mars. He obtained the value 1/219, or 0.0046. This is close to the modern value of 1/154 (many sources will cite somewhat different values, such as 1/193, because even a difference of only a couple of kilometers in the values of Mars' polar and equatorial radii gives a considerably different result).>>
Re: APOD: Mercury in Silhouette (2019 Nov 13)
Posted: Wed Nov 13, 2019 1:30 pm
by neufer
Click to play embedded YouTube video.
Re: APOD: Mercury in Silhouette (2019 Nov 13)
Posted: Wed Nov 13, 2019 1:46 pm
by diffeq
What font is that? It's beautiful.
MERCURY*SOLAR TRANSIT & CONVECTION CELLS | D*200 0.3”/PIXEL 610NM | 11.11.19
I used my binocleers, but couldn't see anything. Still can't; something's wrong.
Re: APOD: Mercury in Silhouette (2019 Nov 13)
Posted: Wed Nov 13, 2019 3:56 pm
by SteveMiall
When I look at the sun, just behind Mercury, it looks like the convection cells form a circle around the image of Mercury. Is anyone else seeing this effect? Is it real, or computer-generated?
Re: APOD: Mercury in Silhouette (2019 Nov 13)
Posted: Wed Nov 13, 2019 4:30 pm
by Chris Peterson
SteveMiall wrote: ↑Wed Nov 13, 2019 3:56 pm
When I look at the sun, just behind Mercury, it looks like the convection cells form a circle around the image of Mercury. Is anyone else seeing this effect? Is it real, or computer-generated?
You're seeing about two arcseconds of "fuzz" around Mercury's limb, which is pretty consistent with what you'd expect from a stack of lucky imaging frames shot in the daytime with the equipment used. In other words, it's a combination of seeing effects and imaging artifacts. You don't see it on transit images made from space.
Re: APOD: Mercury in Silhouette (2019 Nov 13)
Posted: Wed Nov 13, 2019 4:39 pm
by SteveMiall
Thanks, Chris.
Re: APOD: Mercury in Silhouette (2019 Nov 13)
Posted: Wed Nov 13, 2019 5:10 pm
by aildoux
Oh, thank you. Did not see that a Wiki page existed on the subject. Future astronauts on Mars (if there by 2084) should bring a small telescope with sun filter to see that. Presently, I presume that robots and satellites on and around Mars cannot point and zoom in at the sun without damage.
Re: APOD: Mercury in Silhouette (2019 Nov 13)
Posted: Wed Nov 13, 2019 5:28 pm
by Chris Peterson
aildoux wrote: ↑Wed Nov 13, 2019 5:10 pm
Oh, thank you. Did not see that a Wiki page existed on the subject. Future astronauts on Mars (if there by 2084) should bring a small telescope with sun filter to see that. Presently, I presume that robots and satellites on and around Mars cannot point and zoom in at the sun without damage.
Curiosity imaged a Mercury transit from Mars in 2014.
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aildoux wrote: ↑Wed Nov 13, 2019 5:10 pm
Oh, thank you. Did not see that a Wiki page existed on the subject. Future astronauts on Mars (if there by 2084) should bring a small telescope with sun filter to see that. Presently, I presume that robots and satellites on and around Mars cannot point and zoom in at the sun without damage.
Curiosity imaged a Mercury transit from Mars in 2014.
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Mercury in Silhouette
As hot as Mercury is; the shady side pretty cool; like ice was found on Mercury! 
