13
$\begingroup$

There is a total solar eclipse happening in North America in early April. People will be travelling long distances to see it, and towns near where I live are warning residents to buy gasoline in advance so they won't need to go out on the day (due to crowds) and warning visitors that parking and restaurants may all be full. Apparently a million people will visit Niagara Falls, which will have 3 minutes of totality.

This online map tells me I will have 99.55% obscuration at my rural home. This is a location where parking, food and drink etc are not issues, obviously. I now need to evaluate whether travelling an hour or two (due south of me the totality passes over Lake Ontario, so I need to get west or east quite a bit in order to reach a sufficiently southerly spot) will be "worth it" in terms of the experience.

I will do the "worth it" part but I really do not know the difference between 99.55% obscuration and totality. What should I expect? To be clear, while many of the current answers give me opinions on "worth it" I do not want those opinions. I want facts, for example "will it get dark at 99.55%?" and "can we see the corona at 99.55%?" -- the differences. I don't know all the possible differences so I can't ask about all of them specifically. But this is a physics question not a "tell me if I should go or not" question.

$\endgroup$
7

7 Answers 7

22
$\begingroup$

The difference between a deep partial eclipse and a total eclipse is sort of like the difference between a really excellent dinner, the kind of dinner that you'll remember with fondness later, versus your first kiss with your future spouse. Travel to see the totality.

Don't feel like you have to travel all the way to the middle of the path of totality to get a proper eclipse experience. The moon's shadow is a circle, and the slope of a circle goes to infinity at its edge, so the duration of the total eclipse goes up very fast. In 2017, the university where I was teaching hadn't considered holding an event on campus, even though we were in the path of totality, because we were only a mile from the edge of the path and it surely wouldn't be long enough to be interesting. WRONG. Just a mile from the edge of the path, we got about sixty seconds of total eclipse. We had about 5000 visitors for the event, perfect weather, an amazing experience.

The next total eclipse in North America is not for twenty years. You are so close to this one. Don't skip it.


There was a request for a more mathematical answer.

The Sun has apparent magnitude $-26$, while the full Moon has apparent magnitude $-12$. (I couldn't find a visual magnitude for the corona in five minutes of searching, but it is frequently described as "about as bright as the full Moon.") A difference of five magnitudes is a factor of 100 in brightness, so the Sun is nearly a million times brighter than the corona. A 99.9% eclipsed Sun would still be about a thousand times brighter than the corona. It would be very hard to observe the corona safely — you would have to look through your solar filter glasses to see it naked eye, or do some complicated photography. You could take your eclipse glasses out tonight and see if you can spot the Moon through them.

Even a full moon washes out the stars somewhat. A sky with a thousand full Moons would be too bright to spot any but the brightest stars and planets. But that may sound misleadingly optimistic. You can sometimes spot Venus or Jupiter in the full daytime sky, if you already know exactly where to look.

There's another feature of the sky that is worth mentioning, which is impossible to represent in print or on a computer screen, which is the blackness of the Moon within the totality. On a computer screen the darkest you can make the screen is just "off." On paper, you can apply black ink, but unless you have an annoyingly picky artist in the printing process, the black printer ink is nothing special. But the blackness of the totally-eclipsed Sun, set against the corona and the clear midday twilight sky, was like nothing I have ever seen. Blacker than black velvet, blacker than a mineshaft, blacker somehow (perhaps due to contrast) that the part of the tour of Carlsbad Caverns or Mammoth Cave where you get into the belly of the Earth and the tour guide has everyone shut off all of their lights. Like looking through the sky into a light-stealing abyss beyond. The 0.5% Sun you are considering watching will have blue sky within the crescent.

But it's more than the sky. You will see the Moon's shadow rushing at you from the west before the totality, then rushing away into the east. It gets surprisingly cold. Wildlife, like birds and insects, are confused by partial eclipses, but may do very strange things during total eclipses. You may see writhing shadows ("sky snakes") on the ground, which I think are caused by the sunlight passing through the temperature-driven turbulence in the air at the edge of the Moon's shadow, but I don't know whether that is established scientifically. It is a full-body experience, which is very hard to put into words.

I started out with an emotional answer, rather than a physics answer, because observing a total eclipse has been an emotional experience for everyone I have know who has seen one. I'm not enough of a poet to get this point across, unfortunately.

$\endgroup$
5
  • 5
    $\begingroup$ Interesting point about duration - just 1 mile from the edge of the 115 mile wide umbra path lets you see the totality for 18% of the maximum duration, indeed about a minute. At 5 miles, it's 40%; at 10 miles, 57%. Over 99% of the area from which the totality can be seen will get at least 30s of totality. $\endgroup$ Mar 20 at 12:15
  • 3
    $\begingroup$ Honestly this answer would be nice on another forum, but not here. It doesn't contain any physics. $\endgroup$
    – Javier
    Mar 20 at 15:31
  • 1
    $\begingroup$ I heard someone describe the difference between a partial and total eclipse as the difference between a handshake and a kiss. $\endgroup$
    – BlackThorn
    Mar 20 at 17:28
  • 1
    $\begingroup$ @KateGregory I expanded the answer. $\endgroup$
    – rob
    Mar 20 at 20:59
  • 1
    $\begingroup$ The "sky snakes" are called shadow bands. "The shadows' detailed structure is due to random patterns of fine air turbulence that refract the collimated sunlight arriving from the narrow eclipse crescent". That article includes a crude video (with audio) from 2017. $\endgroup$
    – PM 2Ring
    Mar 22 at 8:40
11
$\begingroup$

It won't be the same. You will be looking at the dark moon with a really bright spot along one edge. It will make the moon hard to see.

It won't get as dark. If there are hills in the distance, you won't see the moon's shadow rushing over them as the eclipse approaches.

If it was me, I would drive a little ways to get full totality.

On the other hand you will see some interesting things even before near totality. If you have a piece of paper with a pinhole, you see a circular image of the sun in the middle of the shadow of the paper. During an eclipse, the image becomes crescent shape.

If you look at the sun filtering through the leaves of a tree, you have a whole lots of not-quite pinholes. This makes a whole lot of overlapping circular sum images.

During an eclipse, this becomes a whole lot of overlapping crescents. The difference is odd looking.

$\endgroup$
10
$\begingroup$

You are so close! Absolutely go for the 100% experience. It is once in a lifetime, and may be the most incredible thing you will ever see – I say this based on experience! Go for the center of the totality, and make sure you take your goggles OFF and view with the naked eye... but ONLY during totality. For the 99.5% version, it will be unsafe to do this and the result will be much less impressive.

You do not have to be in a city with all the people. Even the side of the road along the path will work. Watch it with your loved ones. It will be an experience you will always share.

$\endgroup$
1
  • 4
    $\begingroup$ I intend to make the "worth it" decision myself. This answer doesn't give me any data for that. Will there be a corona? Will it get dark? What will be different between 99.55 and 100? $\endgroup$ Mar 20 at 11:29
7
$\begingroup$

Things you will miss out:

Hopefully its a sunny non cloudy day. You will see a wall of a gigantic shadow approaching you. From the left of the horizon to the right. Getting closer each part of each second. Just like the shadow of a building during a sunny day, but gigantic. The animals will react to this shadow as well. Birds will rise just before the shadow reaches them into the air. And animals will be silent when the shadow reaches them. And this is a kind of an abrupt silence and jump in sound level you never can experience somewhere else. When the wall of shadow finally passes your body the air will get colder - feelable.

Wow - that was a good day in my life. Go for it as well. I promise this is worth it.

$\endgroup$
7
$\begingroup$

Here is what you will see with a "nearly full" eclipse:

  • Sun's disc having crescent shape like the moon (seen through eclipse glasses or eclipse viewing box)
  • Strange shadows with bokeh (not sure if the right term) in crescent shapes.
  • Twilight in the middle of the day, possibly strange effect on animals/birds

Here's what you will ONLY see with a full eclipse:

  • Ability to take your eclipse glasses off and see the corona!
  • Total darkness

It really feels like a sharp phase transition when the face of the sun gets 100% covered. And in my opinion the corona is an amazing, incredible, magical sight that you won't see anywhere else, whereas just staring at a crescent sun through glasses is far less exciting (we're already used to seeing crescents in the sky - we look at the moon).

Source: Saw the 2017 eclipse

$\endgroup$
1
$\begingroup$

In the end, we went to totality. Some other family members went to a different location for totality, and two stayed at between 99.5 and 100. It was a cloudy day here.

The larger group at Colborne Ontario got no visuals of the sun at all due to clouds, except literally a few seconds of a crescent about 20 minutes after totality. Nevertheless, the wall of darkness approaching was astonishing, as was the cold and the general body sensations (hard to describe) and all agree it was worth the trip. The two who were at work and stepped out for a bit got nice crescent pics and one reported that the building's outdoor lights came on as they do near dusk. Neither said "wow that was so memorable". Meanwhile the person who was least excited about driving to Colborne (about an hour away) is now planning our flights to Australia for 2028.

The answers here definitely informed our decision, so thank you all very much.

$\endgroup$
-4
$\begingroup$

The outdoor light level is approximately 10,000 lux on a clear day. Inside, it may be as low as 25 - 50 lux: on a very dark day, it may be as low 10, and at twilight as low as 1.

0.45% of 10,000 is 45, like what you'd get in a dark corner of a well-lit room, but your eyes will be dark-adjusted, so you'll see better, And better than a dark day or twilight.

Because of the bright edge of the eclipse, I don't think you'll get a good projection of Baily’s Beads as the dark edge eclipses, and because you're off-track, I think the eclipse will not last as long.

Not an expert, theorizing.

My memory, as a teen-ager^, was that a total solar eclipse is sort of over-hyped: it gets dark for a while, then it gets light again. Not totally different to a short very dense cloud cover. We aren't really attuned to the sun unless we work out doors, and having a predicted solar eclipse kind of spoils it. I imagine that a partial solar eclipse would be even less impressive for a lot of people.

I was outside Melbourne, in a rather bare, rather flat area. We did not see the "edge of the shadow" rushing towards us. I don't know how that would work, we were in the penumbra before being in the umbra, but in any case, being in a bare flat area does not give a good view of shadow projection.

^(Melbourne, (South-East Australia), October 23, 1976. Totality:2m 33s. Moon/Sun ratio: 1.047)

$\endgroup$
8
  • 8
    $\begingroup$ I have to disagree with the "over-hyped" part, and my suspicion is you might not have witnessed the total eclipse with the naked eye as a teenager. There's a big difference in that vs a partial eclipse. It is not something you would forget. $\endgroup$
    – RC_23
    Mar 20 at 3:04
  • $\begingroup$ Did you see the corona? $\endgroup$
    – PM 2Ring
    Mar 20 at 3:39
  • 1
    $\begingroup$ If I recall correctly it was cloudy that day (in Essendon anyway). I recall being disappointed, but Melbourne weather being what it is ... $\endgroup$
    – Peter
    Mar 20 at 5:08
  • 2
    $\begingroup$ The outdoor illuminance can be up to 100,000 lux, not 10,000. Blocking 99.5% of it leaves you with lighting comparable to a typical office, which is far dimmer than daylight but perceptually rather unremarkable. Dropping that by another 99% to singe-digit lux is a much bigger perceptual change despite it being a smaller numeric difference in lux. Human vision operates well over a range of thousands of lux, but we only really notice "dark" over a range of tens or at most hundreds of lux. $\endgroup$ Mar 20 at 16:16
  • 2
    $\begingroup$ it looks better in photographs and on TV. I'm going to have to vehemently disagree there. You will never in your life see anything as black as the moon during a total eclipse. And everything else is unbelievably brilliant in contrast. $\endgroup$
    – BlackThorn
    Mar 20 at 17:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.