Exact Eclipse Timings
On eclipse day, the most important question is to know where you need to be in order to see totality. Of course, by now you know that you have to be in the path of totality! But there is another important question that everyone wants to know: When will the eclipse happen? Maybe by now you know that that will be different for everyone, because the Moon’s shadow moves across the face of the Earth as it is creating the eclipse. This is why we have to know your location in order to tell you what time the eclipse will happen.
Astronomers are able to calculate eclipse timings VERY accurately! You will see times given on the internet that are published to within a tenth of a second, and for astronomers, this type of accuracy is very important when they are programming their telescopes and cameras to capture all the data they need for their experiments. For the rest of us, not so much. In general, it’s good enough for us to say something like “a little after 2:30pm”, and that’s all we need. We don’t need to know tenths of a second, or even seconds. In fact, we really don’t need to know the eclipse times to the minute, if we’re just casual observers. We just want to make sure we show up before then, and are there in plenty of time to get a good spot, to socialize a bit before first contact, and then to see first contact and participate in the anticipation and excitement of following the Moon’s progress as it makes its way to the main event – totality!
A good analogy might be going to see a baseball game. You want to know what time the game starts, so you can plan your drive to the stadium accordingly. Once you get to the stadium, though, you really don’t care about “world time” so much, because you’re on “game time” at that point. Who cares if the 5th inning starts at 2:37pm? You just want to make sure you made it into your seat in time for the 1:05 start, and the 5th inning will happen when it happens – namely, after the 4th inning. After the game is over, then you leave “game mode”; you look at the clock and re-synchronize your life back to world time as you make your way on to your next thing.
Now, back at the eclipse site, if you happen to not have very good weather, and you’re chasing a hole in the clouds to even SEE totality, THEN you might very well need to know that there are only 45 seconds to go before second contact! But even in that case, you still don’t need to know the time to the tenth of a second.
Once you are on “eclipse time”, you may care about seeing that first little bite at first contact. Someone with a telescope will see it, will announce it, and then you can hold your eclipse glasses up to your eyes and try to make it out. (Yep, there it is – right on schedule!) But as to when second contact happens – well, it’s going to happen when it happens, and that means whenever the Moon has covered up the last little bit of the Sun’s disk. You see, it is THAT action of the Moon, and not any specific time that you might read on a clock or a website, that determines the most important thing of all: when exactly it is that you can look at the totally eclipsed Sun without any eye protection at all. You can ONLY do that when the Sun is totally eclipsed, and the transition from “not covered” to “covered” is something that happens right before your (protected) eyes. So that’s one reason why it’s not really necessary to get all worked up about a few tenths of a second here or there.
“One” reason? Yes, because there is something else going on that you should be aware of. Totality begins the moment that all of the Sun’s bright disk has been covered by the Moon. Just like the partial phase starts when the first bite is taken out of the Sun’s disk (and we are all very relieved that yes, we will indeed be having an eclipse today), the start of totality is defined by the time when ALL the Sun’s light has been blocked from getting to us.
Well, that should be easy enough to calculate (relatively speaking) – right? We know all this stuff about the position and motion of the Moon, and we can do all this math, so surely we should be able to be REALLY accurate about when that last bit of Sun is covered up? Right?
Well, since the question has been brought up, you can probably guess that there’s a bit more to it than that. IF the Moon’s disk were an absolutely perfect circle (or ellipse, if you’re going to account for its VERY negligible oblateness), then yes this would all be correct, and you could set your watch literally by that exact moment of second contact when the last sliver of Sun was covered up by the Moon. It would have been forecasted perfectly, and that would in fact be the actual instant when you wouldn’t be able to see anything through your eclipse glasses. You could then take them off and look directly at totality, safe in the knowledge that you wouldn’t be hurting your eyes. Math would have defined the entire experience for you.
But the Moon is not a perfect circle. Around the edge of that round disk you’re looking at, there are mountains and valleys that are impossible to make out from Earth except with the most powerful telescopes. But if you imagined a lunar disk with those mountains and valleys greatly exaggerated, you would see something like this:
You can see that it depends very much on what this “lunar limb profile” looks like, that will determine EXACTLY WHEN that last little bit of Sun gets covered up by that very last lunar valley. When astronomers do the basic calculations, they count on a Moon’s disk that is a perfect circle. That means that those very accurate calculations (due to the geometry of the Moon’s movement) are basically correct, but not EXACTLY correct because the limb profile hasn’t been taken into account. Those extra lunar limb calculations are extraordinaily difficult, because the limb profile varies depending on where you are located on Earth! That’s right – the correction that has to be applied to the calculations for the “perfectly round” Moon’s disk will be different depending on what eclipse you’re talking about (due to libration of the Moon), the spot on the Moon’s disk that is the last to cover the Sun (which depends on your location on Earth), AND the lunar limb profile at that point on the Moon’s disk (again unique for your exact location on Earth)! The calculations are incredibly complicated and processor-intensive, and there are only a small handful of individuals on our planet who are doing them.
(Note to purists: We are not even bringing up the difficulty in defining how to determine the EXACT average lunar disk to use when doing the standard calculations. That debate is ongoing, and is of interest only to the true eclipse/math/physics geek. The average observer only needs to understand that different values of the mean lunar radius can cause the timings to vary by a second or two either way.)
How much of a difference can such a far-off mountain or valley on the Moon make to the times of second and third contact? Well, remember that all the eclipse events occur due to the Moon’s orbital motion around the Earth. If you do some VERY rough math, you’ll find that the Moon moves in its orbit about 500m or so each second – give or take. 500 meters is a pretty tall mountain or deep valley, but remember that the correction has to be made from the center of figure of the Average, perfectly round Moon. The mountain doesn’t have to be 500m tall from some imaginary lunar “sea level” to affect the calculation – it only has to be that much (or more) removed from the perimeter of the center of figure of the Moon.
The upshot to all this is that those times that are perfectly calculated down to the tenth of a second might need to be adjusted up to as much as 5 seconds either before or after the “published” time, to give us the perfect timings for a specific location. That’s not much, but it is enough of a difference that we simply cannot tell you EXACTLY what time the eclipse starts. Well, we COULD – using those calculations that are done for us by Xavier Jubier or NASA, for example – but even then, we would still make the statement that the ONLY determining factor for when you can safely look at totality without eye protection is when the Sun has been COMPLETELY covered by the Moon, and NO MORE light from the Sun’s disk can be seen.
The visual upshot of this is that, at the time when the textbook is saying the Moon should be covering that final bit of Sun, there might still be some little beads of light coming through the lunar valleys. These “beads” grow, shrink and change in real time, as you watch them through your eclipse glasses. The valleys may be small, but the Sun is bright – and any amount of its light that gets through is going to be visible! Against a darkening sky, and the corona that begins to shimmer into view, this dance and play of light on the edge of the Moon’s disk is mesmerizing. It is one of the stars of the show, to be sure! These beads are knowns as “Baily’s Beads” after the astronomer Francis Baily who received naming credit after describing them in 1836. (He was not the first to observe them, but as president of the Royal Astronomical Society he likely had dibs on most important topics like this!)
The author has a distinct memory of the total eclipse of 11 July 2010, observed from Tatakoto atoll, where the very last Baily’s Bead at second contact hung on for what seemed like 10 seconds or more. It was really remarkable and memorable, and reminds us that while every eclipse is the same, each is also each very unique in its own way. And because Baily’s Beads are just slightly different for each observer, you can be sure that your experience in the shadow on eclipse day will be uniquely your own!