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2017 Eclipse Blog / FAQ

Magnitude vs. obscuration?

Whenever eclipse geeks are talking about their calculations, they always need to make sure they are careful to distinguish as to whether they’re talking about “magnitude” or “obscuration”.  In case you’d like to understand this, too, here is a quick explanation.

I’m going to use the word “percentage” below, but if you want to substitute “fraction” or even “amount” (so long as you understand what you mean by that word), that’s OK too:


The percentage of the Sun’s area that is being covered by the Moon at any given moment. Expressed as a number from 0 to 1.

The percentage of the Sun’s diameter that is being covered by the Moon at any given moment.  Expressed as a number from 0 to, uh, well, to possibly a number slightly bigger than 1.

Note that these are NOT the same thing – otherwise, there wouldn’t be any argument.

(Well, to be fair, they are the same if we’re not having an eclipse – they would both be 0 in that case!  But that’s kind of boring, right?)

Why the difference?  Well, look at these two pictures:


 50% magnitude  50% obscuration

You can kind of see that the “bite” the Moon is taking out of the Sun in the picture on the left, comes right to about the center of the Sun’s disk.  In the picture on the right, slightly more of the diameter is covered (about 60%, actually), but right at half the Sun’s area is being covered.  So which one do we mean if we want to say that “half” of the Sun is covered?  They’re both right, in a way, and that’s why we need two terms.

Which one looks to you like “half” of the Sun is being covered?

Our eclipse 2017 app uses magnitude for its display.

What values do we use?

Obscuration has to be equal to 1 if all of the Sun is covered.  It can’t get bigger than 1, because you can’t cover more than 100% of the Sun.  (Well, actually, if you extend the math to imagine that you could do that, then it makes sense to talk about more than 100% coverage.)

Magnitude during totality is always going to be bigger than 1.  I don’t know if it makes any more sense to talk about being able to cover more than 100% of the Sun’s diameter than its area, but that’s what people do.  If the magnitude were exactly 1, and the Moon and the Sun were exactly the same apparent size in the sky, then the Moon would only cover the Sun for 0.0000000… seconds, and the total eclipse would be over before you knew it!

And of course, purists would correct me and say that the eclipse in that case would never be total, because there are valleys around the edge of the Moon that would always be letting a little bit of light through.  This actually has been seen before, with the most famous case being the 1986 eclipse as viewed and described by Glenn Schneider.  We will let his story speak for itself!

Every eclipse has a maximum magnitude that it reaches, both in terms of the spot where the observer happens to be located, and where a person situated in the right spot to see the most coverage would be.

Example:  the 2017 eclipse

This eclipse for someone located in Greenville AL will only achieve about 90% magnitude:

A nice partial, but honestly, this level of eclipse would not cause anyone to become very excited.

However, for those in Greenvile KY or Greenville SC, the eclipse will reach this 90% level, and then will continue on toward totality!  People there WILL experience totality, and will be amazed.

So that’s why we say that for Alabama, this eclipse is only going to be so many % partial – yet the 2017 eclipse can still rightfully be called a “total” eclipse.  It’s total for some people!

But you have to be in the path!

A couple more things to say about this topic:

Who Cares?

Astronomers do, but for most people who just want to see and appreciate a total eclipse, it’s not really important.  If you get nothing else from this discussion, it’s this:

50% magnitude or 50% obscuration: If that’s all you’re going to get for the 2017 eclipse, then you’re not in the path of totality, and you need to move!

And finally, let’s say something about eclipses close to the edge of the path of totality, but not in it.  If you aren’t in the path of totality, you may think “Oh well, I’ve got a 99% eclipse”, or even 99.5%.  Well, look at this:

99% partial eclipse (magnitude)

99.5% partial eclipse (magnitude)

These look pretty impressive, but that little sliver of Sun that is still showing is VERY bright!  At this point of an eclipse, the shadows would look weird and there’d be some noticeable darkening – but if this is the maximum magnitude of the eclipse where you are, then you will not see the dark sky or the corona.  You will HAVE to use your eclipse glasses, even when this small amount of Sun is showing.  And you will not believe the difference between what you see, and what folks just up the road a couple of miles are going to see.

Get to the path!

63 Responses to “Magnitude vs. obscuration?”

  1. anna taft says:

    If I am in Polaris, MT on August 21, at what time should we look for the partial eclipse? Polaris is north of Idaho Falls and west of Dillon,MT.

    • Admin says:

      Anna –

      We have a link on the site to a wonderful Google Interactive map that you can use to find the exact circumstances. Max eclipse for you in Polaris will be about 11:30am.

      You will get about a 97% partial eclipse – which will be interesting but in no way even close to what is in the path just a bit south of you!

      I have to IMPLORE you to take the drive down I-15 to get into the path and see something AMAZING! The difference between total and not total is everything. You are less than 100 miles from a life-changing sight – you are sitting out side the restaurant, smelling the meal. When it is over, you will wonder why you are still hungry… 🙂


    • Al Dabiri says:

      Hello Anna;
      Here is the information you need.

      Good day;
      Al Dabiri
      Max View in Polaris Peak

      Monday, August 21, 2017 at 10:29 am

      Global Type: Total Solar Eclipse

      Polaris Peak: Partial Solar Eclipse

      Begins: Mon, Aug 21, 2017 at 9:14 am

      Maximum: Mon, Aug 21, 2017 at 10:29 am

      Ends: Mon, Aug 21, 2017 at 11:49 am

      Duration: 2 hours, 35 minutes

      Magnitude: 0.92

  2. chris regan says:

    As Mr Creedon says: “totality is everything.” If you experience anything less than 100% coverage of the solar disc you can not view naked eye!!!!!!!! This is a critical safety consideration.

  3. ND says:

    I am going to be little outside of totality..where I’ll have Mag. 1 and Obsc. 99.99%….Do I still need to get to totality path still or should be it okay??

  4. Raquel says:

    Well, it is a bit disappointing to be just out of the path of totality. We think it would be a great learning opportunity for the kids, but unfortunately it is not feasible to put all 700-800 kids from our elementary school on buses and drive 3 hours to the totality path less than a week after school starts. Fundraising, organization, authorizations… So I’m hoping the 98% experience will still amaze them.

    • Admin says:

      Raquel –
      We do what we can do with what we have, where and when we have it. If you have parents who want to ask questions, and possibly take their kids and do this on their own, I will be more than happy to answer their questions.

      I wish you a GREAT eclipse!

      Clear Skies,

  5. David says:

    There is no way to explain totality to anyone that hasn’t seen one. My wife and I were teachers in 1979 when the last eclipse was in the lower 48. We took a “personal necessity” that day (the only one we ever took) and headed for WA state with our two kids (from CA). It is a day none of us will ever forget and our 3rd total eclipse. Our kids got more out of seeing this than they would get in a missed day at school.If you can, GO!

    • Admin says:

      There is no way to convince many people. I can’t believe the number of people who are “OK” with staying somewhere that is only a couple hundred miles from the path, when we veteran eclipse chasers KNOW that it is worth traveling around the world to see totality! There is nothing you can do to convince people, and these are the same people who will come back after the eclipse and say “well, it wasn’t really anything special”. And of course, because they weren’t in the path, they will be completely correct!


      • Gary says:

        I think many people are ok with missing this one because there will be another in a few short years. I’d love to go see this, but I’m in Dallas so it’s not a round trip in a day kind of drive for me. Plus, my house will be in the path of totality in 2024. Plus, it’s the first day of school for my kids, including first day of middle school for my middle one.

        • Admin says:

          There are people who travel to the deserts of Libya, remote Pacific islands, and the frozen plains of Antarctica, just to see totality. I personally went to Kenya for an eclipse with 11 SECONDS of totality – and it was one of the greatest adventures of my life, not exchangeable for ANY other experience. I understand many people are content to stay home, but something life-changing is happening just a bit north of you. I would encourage you to go.


  6. Holly Halls says:

    Thanks! This was a GREAT explanation for why it is worth driving the 2 or 3 hours to get to the total eclipse locations.

  7. Lisa says:

    We’ll be at 99% and I’m still not quite sure what will be missing if we stay here rather than drive in the crowds. Other than it being darker, what will the difference be? And isn’t the Corona just when you see a sliver of light all around? Is it really that much neater? Sorry for the questions, I know you say above that you highly recommend going to totality, I’m just having trouble really understanding what the difference will be. Thanks!

    • Admin says:

      Hi –
      If you go to someone who was IN the path the day after the eclipse, and you ask how it was, they will give you an AMAZING story of how they saw the most wonderful thing imaginable. If you go to someone who was at 99%, they will think that first person is crazy. What they saw at 99% was very interesting, but it was not jaw-dropping, knee-buckling, OMG amazing! They will think the first person is overreacting – and of course, that’s because they were not IN THE PATH. That’s it!


  8. Janet says:

    Question: Is 100% the same as anything over 100%? I’m looking at the interactive map for the Oregon path, and the nearest town from us under the absolute direct line (at magnitude 1.01331) is a 1.25 hour drive on a regular traffic day, but there’s a closer town just 40 minutes away that is at the top of the path (at magnitude 1.00009). I might try to fight our way to the 40 minute mark, but the 1.25 hour drive will be impassable — Oregon DOT is predicting carmaggedon. Is the effect the same so long as you are at 1.00009? Or do you really have to be right under the 1.01331 to appreciate the experience of the totality?

    • Admin says:

      Hi Janet –

      More than magnitude, you should consider duration of totality. Inside the path, you see totality. But the closer you get to centerline the LONGER totality lasts. Those seconds are precious as your brain tries to recover from the overload. So yes, you will get totality at the closer spot. And as to carmageddon – it’s truly anybody’s guess!!

      Clear Skies,

  9. L. says:

    I dont get it…do i set my clock ahead an hour or two or what?


  10. Steve says:

    I live in Athens, GA where we will experience 99+% obscuration. I intend to drive as far as 3 hours if i have to to see this eclipse. If people pay thousands of dollars to travel thousands of miles to see these things, then its gotta be worth a half tank of gas and a few dollars for lunch to go see one when its right in your back yard. As for carmageddon, nobody says you have to go to an official event to see this. Just GET IN THE PATH! If your location is like mine, there are plenty of back roads that will get you there that the masses don’t know about and won’t use. I have never seen a total eclipse but i hear that the 99 percent here in Athens is like holding a sparkler in your hand on 5th ave. When the big official NYC fireworks show is just blocks away on the east river. The point is if you can, you SHOULD go see this eclipse. And I will see you there

    • Admin says:

      Steve – I could NOT have said it better myself. I wish you a GREAT ECLIPSE!

      Clear Skies,

    • Janet says:

      Still planning on giving it a try and do know the backroads, and with luck and google maps we could make it into the pathway, but the carmaggedon prediction is likely not overblown…..every rental car in the Portland, Oregon metropolitan area has been reserved for that weekend. Every. Single. Car. And they all have to make it from Portland down to the centerline. Residents in Bend (a nice sized city just over the mountain, and in the path) are being warned to fill their cars with gas and stock up on groceries days in advance because the avalanche of people threaten to overrun supplies. The emergency preparation beats Y2K. Crazy.

      • Admin says:

        Let’s hope that the Carmageddon fears do not materialize! Everyone get an early start on eclipse morning – if you can’t be in the path the night before!


  11. Matthew says:

    Thanks for this article! But, I’m still confused. I’m in an area reportedly to see .96 magnitude, but I can’t find how that translates to obscuration. I get that .96 magnitude does not mean 96% of the sun will be blocked by the moon. But it seems that’s what folks around these parts are going with. Fort Collins, CO

    • Admin says:

      Hi Matthew – The difference is mathematical, academic, and not worth worrying about! If you have a 96% partial eclipse, only 96% of the Sun is going to be covered, and you should make your way into the path to see what the real excitement is about. A 96% eclipse is _somewhat_ interesting, but totality is life-changing!


  12. Tim says:

    To really hammer home the point that totality is where it’s at, can you tell me how much brighter it is at 99% and 99.9% compared to the darkness of totality. I’ve read the sun is 10,000 time brighter at 99.9% eclipse compared to 100%.


    • Admin says:

      Tim –

      That’s the figure I’ve always heard, and I believe it’s due to Jay Pasachoff. 99% is so bright that I’m not sure what that number would be. It’s just better to make sure you’re IN THE PATH! And if you’re not, you always have to use eye protection!


  13. Don says:

    NASA simulation: Compare Corvallis OR (inside the totality) to Portland OR (99.4%) — indistinguishable!

    • Admin says:

      Don –

      I’m sorry, but there is only so much that a simulation can show, regardless of who’s doing it. If you are not in the path, you must use eye protection at all times. If you are in the path, you will see totality. The two things are very different. Simulation is one thing, and reality is different.


  14. Scott Seeley says:

    Can you point me to any info – or provide info on the experience in one is in the path of totality but it is overcast or even raining? Seeing not info on that.


  15. Gretalee Rapp says:

    It’s unclear to me how the size of the eclipsed sun when seen “in person” compares to the pictures every where now, which make it look like a big black ball. My impression is that the sun, seen from earth with the naked eye (or plastic eclipse glasses) is something like the size of a full moon — not all that large. If you don’t have a telescope or binoculars (filtered, of course), how much can you really see? Do you recommend binoculars for everybody? Thank you!

    • Admin says:

      The Sun does appear almost exactly the same size as the full Moon, but the corona stretches out from it and the sight is dazzling beyond description! You do not need binoculars, but if you want to see detail of totality close-up, you can use them – but ONLY during totality! Do NOT EVER use binoculars to look at the Sun if ANY bright part of it (even Baily’s Beads or a small sliver) is visible – you will damage your eyesight. It must be totally eclipsed for you to be able to look at it. It you don’t know when that is FOR SURE, then don’t try it at all.


  16. Petra says:

    So what I gather from this blog is that if I’m in an 81% area, then don’t even concern myself with looking at the sun during the eclipse? There’s nothing to experience?

    I understand that totality is where it’s at and that you need to encourage people to get there if they are close. Clearly though, everyone in the US cannot feasibly do that. So it sounds like there is nothing if you cannot make it to totality? And we should just go about our day as if nothing is happening?

    Btw, I was in elementary school for the 1979 total eclipse. And I remember watching it from the playground.

  17. Bekah says:

    What is the difference between ten feet south of the “line” and 10 feet north? I will be in an area that is just one block north of what all the google maps show as the line of the eclipse. Is the google map that mathematically accurate? Will I actually see totality? And will the people those ten feet south of the line notice that much of a difference? I’m trying to work my head around this…

  18. Tim says:

    Currently where I live the eclipse coverage should be 84%. I could travel to an area where the coverage would be 96%. Would it be just a little more interesting or would it make a really big difference?

    • Admin says:

      It would be slightly more interesting. People go to the ends of the Earth to see totality; I would recommend you go a few more miles and see something you’ll never forget!

  19. Rita says:

    I am driving 4 hours to be in the totality area. However, all motels were booked near the actual center line, so I will be in a small town INSIDE the totality area, but the duration is 1 minute 30 seconds rather than the longer durations near the center line. The small town where our motel is located is NOT having events so I’m hoping we will have quiet and plenty of room where we will be sitting (either in a grassy area I’ve located or a large parking lot.) Other than DURATION, are any of the effects greater near the center line? In other words, I know if we drove to an area closer to the center we’d have 30 more seconds of totality….but where I’ll be (29 miles from the edge) will I still see the corona, diamond ring, and Bailey’s beads as well as if I were nearer the center line?? Will I miss anything other than duration?
    Thank you!

    • Admin says:

      You will be fine so long as your location is getting 1m 30s. The only thing you’re sacrificing by not being closer to centerline is duration.

      Clear Skies!

  20. Phyllis says:

    My Sister and I are traveling to Northeast Arkansas to see the Eclipse which has a magnitude of 97%. Do you think we will enjoy the view??

    • Admin says:

      I would recommend you go one state north and make it 100%. I would not recommend traveling anywhere to see a partial eclipse, unless you were traveling for another reason in the first place.

  21. M Sterling says:

    I live north of San Diego. We can’t travel north of Portland to the path. I think we only get 50%, right?. What does this then mean? Nothing, just looks like an overcast day, or what? Do we even need solar viewer glasses?

    • Admin says:

      Pretty much not even an overcast day. You CANNOT look at the Sun without eye protection!! You will be left wondering what all the excitement was about, when in Oregon people are going nuts with the coolest thing they will ever see in their lives. No such word as “can’t” when it comes to totality!

  22. Matt wade says:

    How dark would it be at 96.5% obscursion.
    How would it compare to the darkness when the sun sets?

    • Admin says:

      There will be some darkening. The effect will be interesting, to be sure. But the real show is totality, and the corona! If you can get to the path, please find a way! Where you are, please remember to use certified eye protection AT ALL TIMES.


  23. Devin says:

    So what I get from this discussion is that if I am not in the “totality”… even if circumstances dictate that the closest I will be is around the 98% mark, I really should just sod myself and stay inside with the lights off, because that’s not even worth witnessing.

  24. Jess says:

    Dear Admin – I just wanted you to know that after reading your blog, my husband and I agreed to take our 2.5 year old and make the trek to the path… while we’re Portland residents, motels were booked and cars rented and roads already crazy by mid-week. So insteaad, we made a 6+ hour drive in the middle of the night (hoping our little one would sleep – she did, a bit) and landed at a friend’s property near John Day, OR at around 2am on Saturday morning. We were greeted with plenty of hands to set up camp and beer, so all was good! We joked all day Sunday that had taken off work, sacrificed sleep, over-paid for pet care at home + travel expenses en route, all for our toddler to “not remember a mid-day sunset…”, lol! But the weekend was great, we were surrounded by good people and good vibes, so it was only well-intentioned humor. As this morning began, we mixed cold brew coffee, filled out aspirin for peoples’ headaches, and started digging through our hatchback for the glasses. Nothing much seemed different from any other weekend with friends. But around 9:30, there was a bit of a “buzz” the started to rise – it’s almost impossible to describe – the only thing tangible was the sensation of cooling coming over the hills. Around 10:00, a few of our friends’ dogs started up – suddenly playing and chasing and going somewhat nuts. By 10:15, we were witnessing more noticeable things – the darkening horizon, the chill in the air, the unbelievable shadows and the millions of little eclipses cascading through trees. Moments later, I was frantically ensuring that my toddler had her glasses aligned, and that cameras were snapping and video was recording, and we all buzzed with who-knows-what excitement for whatever this would be like. And then it happened: totality. Amidst gasps and cheers and plenty of dumbfounded profanity, I felt my husband remove my little one’s glasses by my side and heard her yell with true shock and utter delight, “Mommy, my yellow moon!”

    …..I shared this in support of your remarks over the past few months: suggesting and urging and pleading. To the many folks that asked with obvious frustration about the *real difference* 99.999999999% vs. Totality (a frustration with which I empathized), your responses were not condescending, but kind. “JUST GO,” was perhaps the simplest way to push for others to experience what we experienced today. It was truly, entirely – I’ll say it: totally – unbelievable.

    Thank you for your earnest persuasion.

    (And my 2.5 year old that obviously won’t remember it… well, she just might.)

    • Admin says:

      What a story!! You now know what it is that drives us “eclipse chasers” to the ends of the Earth to see this remarkable event, and create our own life stories that can only be understood by people who have taken the chance and ventured out for themselves. Each one of these stories makes all the work we spent on the site worth it!! I’m so glad you did it – would you please share your story and files on our Eclipse Memories section of the site?

      I hope she remembers it! You can replay the videos for her in preparation for the next USA eclipse in 2024!


  25. Matt Beeman says:

    I traveled from Raleigh, NC to Wedgewood, SC on the day of the eclipse so I could be in the path of totality. BTW, Best decision ever! Raleigh area was 94%, and I heard many people say it was actually quite boring and they didn’t notice a huge difference in sunlight. My mom even called it a “nothing burger” 😀

    I was wondering if there was some sort of chart relating the percent obscuration to the strength of the sunlight, I feel like it was at 60% before we saw any difference, and really above 95% before it was dramatically different, my mom thought it was going to be directly linear where a 94% was going to be a 94% reduction in light, but it obviously wasn’t. Being the data / analytical guy I am, I figured someone would have a chart.

    I got a fair video during totality –

    • Admin says:

      Matt –

      During this eclipse, I made it a point to have alarms go off on my phone at various %ages of coverage, so I would be able to look around and mentally note 90%, 93%, 95%, etc. And, just as we tried to tell people beforehand, anything less that 99% is a “nothing burger” IMHO. I even commented to my group at one point, “This is what 99.5% looks like – this is all you’d get!” And they were in agreement that they wanted more.

      In my opinion, things don’t even get more than mildly interesting until you’re over 99.7% – and if that’s all I got, I’d feel feel a bit cheated.

      I hope people will heed this advice, and plan to be IN THE PATH for the 2024 eclipse!


  26. We went up from the SF bay area to a friend’s house in Lincoln City. Warnings of gridlock and gas shortages proved groundless. When the fog seemed unpredictable, we headed east to the small town of Dallas. Most of the town was either watching from their front yards or gathered in a supermarket parking lot. We headed back out of town to a place we’d passed that had clear horizons all the way to some distant hills, as I was hoping to catch the shadow approaching. (Couldn’t see it.) Only one other couple was parked along this part of the road. They were in a shallow turnout on the east side of the road…. we were in another on the west side. As we were waiting, as it was getting darker, a sheriff drove by and stopped to make sure we weren’t standing in the road. A tiny black dot near the horizon turned out to be a hot air balloon. Several jets were leaving contrails — one of them making a lazy circle in the sky. I got a shot of one appearing to fly right next to Venus, which was already bright in the sky. Then totality hit. Noticeably cooler — we put our coats on. Expecting dark skies and stars and planets, I was quite surprised to only be able to see Venus, which had become visible a good 15 minutes before totality. There was considerable smoke in the air from the Oregon fires, but still I had expected a darker sky. I’d say it looked rather like any other dusk, when only Venus is visible, but quite bright. The wispy extremities of the corona extended between a half degree and a degree from the moon, but my camera only caught perhaps a quarter of it. The moon itself was the only black thing in the sky, which was quite striking. I snapped a few other shots of the surroundings, and a couple of short videos, one of which, for a moment, almost caught the proper lighting. And then Bailey’s Beads again and back to the bright sun, even the tiniest sliver of which was extremely bright compared to the eclipse.

    On the way home we found the traffic we’d been warned about.

    The corona, faint as it is, was still bright enough to fool my camera into stopping down enough that the sky beyond it came out black. My question is, what range of magnitudes would a corona typically be during a total eclipse? It seemed fainter to me than a full moon, but then again it was much larger in area than the moon. Has anyone taken any measurements of corona magnitudes? perhaps between -5 and -10? Just wondering.

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