It’s more or less clear by now that the non-avian dinosaurs and all other large vertebrates around at the time were wiped out when a five kilometre solid object hit the Gulf of Mexico around 66 million years ago. It has also been argued that the world was in the midst of an ecological crisis at the time and that had the impact occurred in the Jurassic instead, they might have recovered and still be around today. There were also the Deccan Traps, an enormous area of volcanic activity in Laurasia which probably didn’t do life on Earth much good either. Nonetheless, the precipitating event certainly seems to have been this massive great rock or chunk of ice from space smashing into Chicxulub, at which point a whole load of terrible stuff happened, wiping out everything weighing more than twenty-five kilogrammes. This is thought to be because anything larger than that was unable to hide from the conflagration got roasted or possibly starved later.
There’s that then. However, you might be wondering why I’m talking about this now. I’ve had three ideas for a blog post today: this, blue and yellow and the fallacy of personal incredulity. I may well get to those in the end. I’ve committed myself to alternating posts about the Solar System with other topics. In the meantime there’s the looming risk of a world war. In this case, I’d say two things about that. One is that deep time is as good a way of getting things into perspective as deep space, and this provides a respite from the horror of our situation and highlights the current state of affairs as transient and fragile. There will be a time after the war. The other is that the immediate aftermath of the Chicxulub Impact was similar to the immediate aftermath of a nuclear holocaust, which is worth focussing on to highlight the risk being taken, and it edges into the subject of personal incredulity that it might be hard to believe this is really happening, but if it does come to the worst it won’t be the first time and life did go on after it. But of course that’s life rather than humans. So I’ll go on.
The title of this post sounds absurd. How could anyone possibly know what day of the week the dinosaurs were wiped out on? Well in fact there are additional problems with the very phrasing of the question, because I’m asking myself what the date was, what time of day it was and so forth, and this is hard to answer not only because palæontology cannot be that precise, at least for now, but because the day had a different length back then and the continents were in different places. Consequently the idea of time zones and calendars gets rather complicated and it’s probably necessary to reinvent the wheel even to discuss this.
Nonetheless, pinning down the probable time of day and year is surprisingly plausible. Dating in the literal sense is actually impossible because there were about 370 days in the late Cretaceous year, meaning that no calendar used today could be used then. Not even the Julian Day works. I don’t know if other people generally know about the Julian Day, but it’s the number of days which have passed since 1st January 4713 BCE, which is when several cycles coincide and is used to do things like count the number of days between events. I personally use it to help make sense of dates in my diary, which I haven’t always primarily used calendar dates for but instead number them as days elapsed since 2:17 pm GMT on 17th July 1975, which is the date and time of my first diary entry. I actually ignore the time of day in these calculations because it’s very awkward. The Julian Day is actually useful for mundane things, such as working out sell-by and best before dates, which is why, incidentally, the time of day sometimes ends up being included on the labels. The Julian Day doesn’t start at midnight but at noon UT, and UT is only noon for half the year in this time zone alone, so you end up with weird times of day being included on the labels. The entire cycle lasts 7 980 years, after which it repeats. However, it can’t be extended back or forward too far because eventually the calendar we use right now won’t work. Somewhere in this blog there’s a post called “Five and a quarter million Easter Sundays”, which is how many Easters there can ever be before the calculation for them breaks down due to the lengthening of the day.
I’m going to assume, inaccurately, that the day the dinosaurs died lasted exactly 23½ hours. It did, apparently, actually occur a round number of million years ago, at 66.0 million years before the present (BP). BP is actually dated as years before 1950 CE, not the literal present because if it was there would be a rolling timeline which would complicate things too much. This is within two thousand years, although I can’t help thinking that it isn’t really because although it might refer to an event somewhere between 66 001 000 and 65 999 000 BP, it very probably doesn’t because I can’t envisage that it would happen on an absolutely perfectly round million year date. The chances are, after all, a million to one. Hence for that figure to make sense, it probably refers to a rounded off period of time between 66 049 999 and 65 051 000 years BP. I’m going to further assume, again inaccurately, that the length of the day on 1st January 1950 CE was exactly twenty-four hours and that between those two dates the day lengthened in a linear fashion with no other variation. That is, it varied in such a manner that it increased in length by twenty-seven seconds every million years. It definitely did not do this by the way, because earthquakes also influence the length of the day and presumably also the Chicxulub Impact itself. In fact probably the Chicxulub Impact was the single event which had the most influence on the length of the day in that whole period, and we don’t know which way round that was because we don’t know if it struck from a more easterly or westerly direction.
The questions, then, are:
- How long has the Gregorian calendar been accurate?
- How long has the Julian calendar been usable?
- What kind of calendar might be appropriate to date the K-T Event?
The Gregorian calendar works by skipping leap years if the number of the year ends in two zeroes and is not divisible by four hundred. This yields a year length of 365.2425 years, unlike the actual length of the year, which is 365.2422 years, rounded up from 365.242199 (I think). It should also be pointed out that this is the tropical year, which is only one of several types of year. The sidereal year has a different length, as does the sidereal day, because the Sun doesn’t stay in the same place in the sky but gradually moves around it throughout the year, but the other stars do, almost. There is therefore a slight discrepancy every year even with the Gregorian calendar and ignoring the decelerating rotation, amounting to a mean difference of twenty-six seconds, which adds up to a day over 3 323 years, which since it was first adopted over much of Europe in 1582 gets us to the year 4 905 before anything drastic needs to be done, and in the parts of the world including the former British Empire a century and a bit later, in theory. Oddly, this seems to mean that the calendar we use in Britain is slightly different than the one in France, by just over fifty-five minutes, but this clearly isn’t so, so not only did we famously lose eleven days but also by now almost another hour.
At some point, the Julian calendar would actually be perfectly accurate, and it’s possible to work out roughly when this is. The mean length of the year is currently eleven minutes and fourteen seconds shorter than exactly 365¼ days. Hence the day is 1.8453 seconds shorter than it would need to be to allow that exact figure, which, with spurious accuracy, happened 68 348 years BP, which is near the beginning of the last Ice Age. Further back than that would be a time when leap years would have been unnecessary because the year was exactly 366 days long, which is a fifth of the way back to the extinction of the non-avian dinosaurs, making it 13.2 million years ago, in the Serravalian Age of the mid-Miocene. Possibly our ancestors wouldn’t have had much use for such a calendar since they were living in a tropical rainforest and eating fruit, which would probably have ripened at intervals not corresponding to an astronomically-based calendar.
But this raises a problem. It means that the calendar would have been different. A minimal but sufficient difference would be simply to make every year a leap year by giving February twenty-nine days. This would’ve been fine for quite a while, and also back further into the past by intermittently lengthening the month of February, introducing and removing leap years or shortening the month by a day instead of lengthening it. However, at some point February would have had to have had 32 days, which is just unnatural, and this would happen some time in the early Cenozoic. There is, however, a solution to this: use a lunar calendar.
It is also true that Cynthia is edging very slowly away from Earth and therefore that the length of the lunar month is also gradually increasing, and in fact this is for the same reason that the length of the day is slowly increasing: tides. Consequently the length of the month would grow, and it would’ve been shorter at the time of Chicxulub. However, this doesn’t make any difference to the number of days if the year itself is ignored and a day is seen as the interval between two sunrises on the Equator, or two sunsets or whatever. Such a calendar could in fact have been useful for our simian and prosimian ancestors, since predators would have been able to see them more easily on a bright night. On the other hand, the kind of intelligence used to devise a calendar in the first place would probably have been better employed finding ways to avoid such predators in other ways. The obvious next question is how long was a lunar month 66 million years ago?
200 million years ago, Cynthia was 4 400 kilometres closer to Earth, meaning that the lunar month was 28.998 days (our days today, that is) long. However, it wasn’t, because days were shorter back then. Consequently, 66 million years ago, assuming a linear change in month length, the lunar month was 29.33305 days long, assuming a day is 86 400 seconds, which it wasn’t at the time. Assuming the exact 23½ hour length gets us to 29.95725745 days, which is usefully close to exactly thirty days. This is interesting because it means that in the late Cretaceous waxing and waning would’ve been almost in sync with intervals of exact days, meaning that there are effectively four “weeks” of 7½ days each corresponding to waxing crescent, waxing gibbous, waning gibbous and waning crescent. This is also roughly true today, except not so close due to the 29.503-day lunar month. This almost provides a calendar on its own. It also makes blue moons, the second full moons in a calendar month, even less frequent, down to about once every three years.
The names of the moons can still be used in such a calendar: Wolf, Snow, Worm, Pink, Flower, Strawberry, Buck, Sturgeon, Full Corn (presumably the same as Harvest), Hunter’s, Beaver and Cold. These are, I think, Native American names, although presumably not all over North America. The Anglo-Saxon equivalents are: Æftera Geola, Sólmónaþ, Hreðmónaþ, Eostremónaþ, Þrímilce, Ærraliða, Æfterliða, Ƿeodmónaþ, Háligmónaþ, Ƿinterfylleþ, Blótmónaþ and Ærra Geola. However, there is another version of these with Ƿolfmónaþ in the same place as in the Native American version. Several of these would not have been applicable at the time either because for example there were no bovids, wolves or in all probability strawberries. There were, however, sturgeon, and this turns out to be crucial.
Many fish scales show growth rings, one example of which is the sturgeon. Sturgeons are members of a particularly basal order of fish, the acipenseriformes, who haven’t changed much since they first appeared in the early Jurassic. It’s possible to look at fossilised sturgeon scales from the Mesozoic and work out what season they died in because their scales grow faster in spring and summer than in autumn and winter. There are a lot of dead fish in the Western Interior Seaway, an ancient stretch of shallow sea reaching into North America up to Canadian latitudes from the Gulf of Mexico, whose demise is marked by their remains’ entanglement with small specks of iridium-rich dust. Iridium is the second-heaviest element, and in any substantial planet or moon will have sunk to the core. In comets and asteroids, however, it’s still near the surface and the distribution of elements is generally more even because it hasn’t got so far to sink and the gravity is in any case negligible. Therefore, these specks, found all over the planet at this point, are associated with the Chicxulub Impact. In the case of the Western Interior Seaway, there is a direct route for the water from where the impactor hit. A very large number of the fish scales have their growth arrested in late spring. Moreover, isotopic analysis of the carbon in the scales has been used to detect the diet of the sturgeon at the time, which is in the form of insects emerging from the water during the spring and more aquatic animals in the winter. Therefore, it’s possible to say almost for sure that the dinosaurs were wiped out when the Northern Hemisphere was experiencing late spring. This is a particularly bad time of year for it to happen because it’s when juvenile organisms are just emerging and not yet able to reproduce, meaning that although the impact was globally severe, it would’ve been worse in the Northern Hemisphere. It occurs to me that all ratites, i.e. flightless or almost flightless birds such as the rhea and tinamou, are from the Southern Hemisphere and are the closest species to non-avian dinosaurs still alive today, but that’s just my speculation.
The most precise estimate for the K-T Event is currently 66 038 000 years BP, with eleven millennia leeway each way. This corresponds to the year 66 036 051 BCE, because there is no year zero – 1 BCE is followed by 1 CE. Late spring is from one eighth to two-eighths of the way through the year from the Vernal Equinox. Let’s take an average and call it 0.1875 of the way through the year. Now estimate the average length of a day between those two points in time at 23.75 hours. The number of seconds between the Chicxulub Impact and 1st January 1950 is 31 559 639.9747 multiplied by 66 038 000, which is 2084135504649238.6 seconds. The average length of a day over that time is 85 500 seconds, so that is equivalent to 24375853855.54664327485 days. Now subtract 0.1875 of a 370-day year. That gives 69.375 days less, which is 24375853786.17164327485 days. Divide that by seven and you get 3482264826.595949039265 days, which is off from an exact multiple of seven by four days. 1st January 1950 was a Sunday, so that would make it a Wednesday, nine days into the Flower Month. Proportionately, on our own calendar that would make it 28th May, bearing in mind that this is a “scaled” calendar where each day actually represents two percent longer than a day as it actually would’ve been at the time.
But maybe Wednesday 28th May, 66 036 051 BCE isn’t precise enough. There’s also the question of time of day. Although I realise I’ve been a bit silly here, it’s even possible to draw conclusions about this which make it more likely. It’ll be local time for now, because there’s another complication or two regarding time zones. This estimate is somewhat more sensible than the spurious precision of the other one. On average, the direction of a meteor impact is 45° and it’s more likely to happen in the morning of the four possibilities. Given a 24-hour day, that would make four times the maximum risk: 3 am, 9 am, 3 pm and 9 pm, with the peak at 9 am. This is because in the morning the rotation of this planet is in the same direction as its orbit, and most objects in the Solar System orbit in approximately the same direction. If it happened at midday, it would be moving directly away from the Sun, which is unlikely. In fact, statistics do bear this out and in two hour periods the most likely time is between 8 and 10 am and the least between 12 noon and 2 pm. However, these are not the right times because, as has been repeated ad nauseam in this post, the late Cretaceous day only lasted 23½ hours, so the local time of impact would be 37.5% of the way through that day, which is 31 725 seconds into the day, which is 8:48:45 am.
Only trouble is, well actually one of several, this is local time in Chicxulub. That would be fine if the continents were in the same place back then. Yucatán is six hours behind GMT but Greenwich and it were not in the relative same places back then. If I, weirdly, insist on still going on Greenwich Mean Time on a planet with a 23½-hour day whose continents are in different position and Great Britain doesn’t really exist, Chicxulub would only be four hours behind us due to North America and Eurasia, or rather the future Eurasia, being that much closer. Actually it wouldn’t be four hours but three hours and fifty-five minutes. That makes it kind of 12:43:45 pm, but taking BST into account, 1:43:45 pm. For the sake of simplicity, I’m going to assume the day ends at 11:30 pm and then starts again at midnight.
So, with spurious accuracy and on the balance of probabilities, the Chicxulub Impact occurred at 1:43:45 pm BST on Wednesday 28th May 66 036 051 BCE. This is quite a messy way of working the whole thing out because the time of day is not scaled but the time of year is. Although the chances of that date and year being correct are small, they are about two million to one. The time of day, such as it is, is much more likely to be right, although the precision makes it less likely. Now I say “much more likely”, but that doesn’t actually make it probable. The chances, though, are much better than one in 84 600. There’s a twelve percent chance that it happened between 8 and 10 am using scaled-down hours and that’s higher than at any other time, and the probability does peak at 8:48 am local time.
There you go then. Rather reminiscent of Bishop Ussher’s idea that Earth was created at 9 am on Monday 23rd November 4004 BC.
Back to reality, sadly.
A Box Of Chocolates 