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I don't have a telescope but I'm interested in seeing events like eclipses and transits.

I'll use the atmosphere as my big natural lens. So I'll watch the upcoming mercury transit at the sunset time where the sun looks bigger than usual. Will this make the black dot of mercury bigger or big enough to be obviously seen by the naked eye? I've never seen it before.

Event: https://www.timeanddate.com/eclipse/in/egypt/cairo

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    $\begingroup$ It's probably a bad idea to stare directly at the Sun, even at sunset. Mercury is about 1/285 of the size of the Sun (relative diameters). It's going to be difficult to see without some magnification (Earth's atmosphere doesn't make that much of a difference). $\endgroup$ – Anthony X Nov 2 '19 at 22:44
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    $\begingroup$ Yep, attempt to observe Mercury transit with naked eye will very likely land you in hospital... $\endgroup$ – SF. Nov 2 '19 at 23:02
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    $\begingroup$ I've removed your request for medical or safety advice. Those kinds of things can't be reliably answered here because in Stack Exchange anyone can post an answer. There's no guarantee that an upvoted answer won't have incorrect statements. Never look directly at the Sun! $\endgroup$ – uhoh Nov 3 '19 at 0:26
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    $\begingroup$ Pardon my french, but the only accurate answer is : "Fuck no". $\endgroup$ – Eric Duminil Nov 3 '19 at 9:40
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    $\begingroup$ I've never seen it before. If you approach it the way you describe, you'll never see it again, either. In fact, you'll never see anything again. $\endgroup$ – Oscar Bravo Nov 4 '19 at 12:28
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I'll use the atmosphere as my big natural lens. So I'll watch the upcoming mercury transit at the sunset time where the sun looks bigger than usual.

While the Sun and Moon might seem larger at the horizon, their angular size doesn't get larger. It's an optical illusion.

Will this make the black dot of mercury bigger or big enough to be obviously seen by the naked eye?

Therefore, no it won't, and never ever look directly at the Sun unless you are using appropriate Eclipse-viewing glasses that are specifically designed for this purpose. Sunglasses are not the correct thing to use, they can block some wavelengths more than others, causing your pupils to dilate while still passing some damaging wavelengths.

Properly protected human vision - No.

At closest approach, mercury is only 11 arcseconds wide, well below what we can resolve with unaided vision. We can still notice stars because they are on a black background, but it doesn't work the other way around. Mercury's sub-resolution black spot would not be noticeable against a bright disk.

Pinhole projector (aka camera obscura) - No.

In this Astronomy SE answer I've said that even using a pinhole projector, you won't be able to see Mercury's transit since they provide very low resolution.

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    $\begingroup$ Maybe you could fix the typo in the "pinhole protector" subheading, to prevent possible confusion around using pinholes to protect one's vision? $\endgroup$ – TheGreatCabbage Nov 4 '19 at 14:30
  • $\begingroup$ @CarlWitthoft thanks for the edit; for some reason thought I looked right at it, I could not see it. $\endgroup$ – uhoh Nov 5 '19 at 1:05
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    $\begingroup$ @uhoh NP - we've all done that sort of think :-) #iseewhatididthere $\endgroup$ – Carl Witthoft Nov 5 '19 at 12:23
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I did see the Venus transit before the Sun in 2004.

I used no telescope, but of course I used proper solar eclipse glasses to protect my eyes. The black circle was small but clearly visible.

But Mercury is much smaller, the transit is not visible without a telescope. See Wikipedia.

Here an image of a Venus transit:

enter image description here

Here the much smaller Mercury:

enter image description here

The dark spots to the right and left are sunspots, Mercury is the very tiny black spot below the center of the Sun.

Images from Wikipedia.

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    $\begingroup$ @OscarLanzi readers, exercise caution! Never point a telescope anywhere near the Sun unless it is equipped with a special solar filter designed explicitly to work with your telescope, obtained from a recognized company who supports solar filter for telescopes. Even then, don't use it without reading both the instructions and several articles on the topic. Familiarize yourself with the technique. "And proper filtering" should not be a separate afterthought! $\endgroup$ – uhoh Nov 3 '19 at 23:30
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    $\begingroup$ Luckily we're at a sunspot minimum, so any spots are most likely transits. apod.nasa.gov/apod/ap191028.html $\endgroup$ – Barmar Nov 4 '19 at 6:44
  • $\begingroup$ Even Venus' transit is hard to eyeball through dark filters. Doable, unless you are too old, but a bit iffy, especially with the spots in your eyes that you pock up from looking anywhere near the sun. It helps to know which solar quadrant you want to look in before starting. Camera with a moderate zoom will pick it out nicely. Filters are still a good idea, as the sun can melt holes in plastic camera optics. $\endgroup$ – Wayfaring Stranger Nov 5 '19 at 15:26
  • $\begingroup$ @WayfaringStranger For me it was not hard at all to see. Very easy to find Venus before Sun. I am near-sighted but perfectly corrected. $\endgroup$ – Uwe Nov 7 '19 at 12:38
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There is even more to @uhoh's "no" answer. Even if the planet were larger in the sky, as it would be with Venus, we still could not see it through clear air without a filter. The light from the remaining visible part of the Sun, scattered by the atmosphere, effectively cloaks the transiting planet in brightness, an effect we also see when we try to view a lunar eclipse in a dawn or dusk sky. (#)

Actually I did catch the 2004 transit of Venus by "cheating" -- the Sun was rising that morning through a heavy pollution layer, which killed the scattered light and allowed the transit to be very briefly seen with momentary glances. But -- once the Sun rose above that layer into the clear sky, no dice.

So unless you're lucky with pollution, you won't see anything at all without a filter, even with a large planet. And then you might as well use one that gives proper protection so you can have a good, long, and at least in the case of Mercury, telescopic look.

(#) Updated in response to a comment.

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    $\begingroup$ I would have thought that a heavy pollution layer would scatter even more light, not less. $\endgroup$ – Michael Seifert Nov 4 '19 at 1:56
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    $\begingroup$ This one was really bad. Scattered light was (temporarily) blocked, the Sun itself reduced to a less bright orange-red ball. Venus appeared dark gray matching (cloaked by) the pollution. Temperature inversion formed over Lake Michigan beneath warm air in Chicago. $\endgroup$ – Oscar Lanzi Nov 4 '19 at 2:05
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    $\begingroup$ Huh, I don't recall that (I was observing the 2004 transit from Promontory Point). But I got there a fair bit after sunrise and I was using binoculars to project the image, rather than trying to "sneak a peek" with the naked eye. Plus, it was 15 years ago. $\endgroup$ – Michael Seifert Nov 4 '19 at 2:08
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    $\begingroup$ It was only a few minutes that the Sun was in this layer and like that description. "A fair bit" later was likely too late, Sun was in clear sky. For me this was really all by accident. Purposeful viewing requires a filter, period. $\endgroup$ – Oscar Lanzi Nov 4 '19 at 2:20

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