June 21st is an important day this year. Not only is it the summer solstice (that is to say, the longest day of the year in the northern hemisphere), but it is also one of the longest days ever in the history of the Earth. Not only is it one of the longest days ever, but it’s Father’s day!
My dad inspired me to become a scientist and astronomer. He is one of the most curious people I know; in fact, I guarantee that he will be one of the first people to read this article. Back when I lived in a suburb of Seattle filled with light pollution, he would enthusiastically break out his refracting telescope. From the end of our driveway, pointing away from that damn streetlight that would never turn off, we’d gaze upon Saturn and Jupiter.
Fast forward a decade or so and I’m an over-caffeinated grad student anxiously awaiting the setting of the sun. Last week, I had an observing night on a 3.5 meter telescope down in New Mexico. I observe remotely from my office in Colorado (while blasting Taylor Swift of course), and where usually I would have at least five of six hours of darkness in a half-night to observe my favorite galaxies and black holes, now I was reduced to less than three. Why was I experiencing this time crunch? Astronomers have to wait for the Sun to set to see these extremely faint objects, and since we’re approaching the summer solstice, I’m losing my coveted nighttime rapidly.
However, it turns out that if I were observing centuries ago, I would actually have a couple fewer milliseconds of time to observe my galaxies. The Earth’s rotation is slowing down ever so gradually, contributing to the fact that this Father’s day is one of the longest days ever. Whoa – what’s going on?
How could Sunday be one of the longest days in the 4.5 billion year history of Earth?
There’s a couple of factors in this game.
First, the seasons. Sunday is the longest day of the year for the northern hemisphere. This happens because of the tilt of the Earth as it orbits the Sun. A very common misconception is that the seasons are caused by Earth moving closer to the Sun in summer and farther away in winter. Not only is this incorrect, but in my opinion it excludes everyone who lives south of the equator. Due to the tilt of the Earth, our friends down south are actually experiencing summer right now. Check out this infographic that demonstrates the illumination of the Earth during various seasons:
Second, gravity. Alright, so yeah it makes sense that this is the longest day of the year for the northern hemisphere. But why is this one of the longest days ever? The Moon’s gravity is tugging on the Earth, slowing down its rotation. It turns out that Newton was right – for every action, there’s an equal and opposite reaction. This means that yes, the Earth is more massive than the Moon, but the Moon exerts an equal force back on Earth. This force is evident in the way the Moon drags the water on Earth, a phenomenon we know as tides.
A great way to make this concept more intuitive is to imagine the massive amount of water the Moon is dragging to create the tides. The Earth is still rotating about its axis, so there’s this gigantic tug of war between the Earth attempting to drag its oceans along while the Moon tries to hold them back. This competition works to slow down the Earth’s rotation very gradually; between 15 millionths and 25 millionths of a second are added to the standard day yearly. But this is still a measurable difference.
Third, climate change. It turns out that releasing a bunch of warming agents into our atmosphere melts the ice on the poles. And when you melt ice on the poles this water is redistributed around the equator. So the Earth has gained some love handles. Meanwhile, the crust under the vacated ice actually springs back, adding some mass under the poles. Having more water mass around the equator but comparably more land mass in the poles actually makes the Earth rotate slightly faster.
This explains why the longest recorded day was not this year but instead back in 1912 before we started melting the ice at the poles.
Fourth, earthquakes. Earthquakes and other natural events such as temporary shifts in the polar ice caps can alter the rotation time of Earth on millisecond scales over yearly periods. This is an additional reason that this current solstice isn’t the longest period of daylight ever. This is also why it’s not a good idea to blindly claim that the current or upcoming year will set a new record for the longest days ever.
So all in all, these four main factors influence why today is a VERY long day, but not the longest. For me, I’m excited that everyone gets that millisecond or two of extra daylight to BBQ with their dad. I’ll just have to wait for a couple months until my coveted nighttime hours get longer as the seasons change. But don’t worry – from here on out the night will only get longer as we march on towards winter.
“Due to the tilt of the Earth, our friends down south are actually experiencing summer right now.” Uh, with the north getting the summer solstice and starting summer, the south is getting the winter solstice, leaving fall/autumn, and starting winter. To the extent that places like Rio de Janeiro get winter, of course, lucky sods.
” Having more mass around the equator actually makes the Earth rotate slightly faster” Hmm, not so much. Due to conservation of angular momentum, the more mass at the equator and thus further from the centre of the Earth, the slower the Earth will rotate. Think of the classic rotating ice skater. When she pulls her arms in she speeds up. But when she holds her hands out and so moves mass away from the centre of rotation, she slows down.
Yes, I guess this point is slightly more complicated. What’s happening with the ice (and this is again confusing because of the above reports from the satellites) is that as it melts, apparently the Earth’s crust may spring back redistributing more mass in the poles also. So I think the more important effect is that we would have mass also along the axis of rotation, hence overall the moment of inertia would be increased. So overall, the last hundred years looks like a combination of factors that have worked to counteract the slowing trend that progresses over the age of the Earth.
Sorry, Becky, but I have another correction to add: it turns out we currently have an almost precisely average amount of ice at our poles, at least since our satellites have been keeping track.(http://arctic.atmos.uiuc.edu/cryosphere/iphone/images/iphone.anomaly.global.png).
Yes, the Arctic is currently running below average (though not at record lows), but the Antarctic has more than made up for that the last few years with record highs in sea ice extent and thickness. Bottom line: sea ice changes are not contributing measurably to changes in day length, at least not at the moment.
Smokey is talking about a recent Forbes article that was based on a misunderstanding of basic statistics. Here is the rebuttal:
https://www.atmos.illinois.edu/~wlchapma/Forbes.article.response.pdf
To sum it up:
“The long-term record of global sea ice (illustrated below) shows a long-term decline of global sea ice of about 5.5%. One is free to argue whether this decline in global sea ice is important, or whether it is a result of human impacts on the climate; however, it is misleading to claim that polar sea ice has not decreased over the historic record.”
Actually, I had no idea Forbes had done such an article; for the sake of hearing the argument being rebutted, I’d love to see a link to that as well. As for the rebuttal itself, one must remember that the red line in the graph is mathematically calculated, albeit using the actual measurements. The result is that both the amounts AND the trend may be said to be “factual,” but they may easily lead observers to contradictory conclusions. Said another way, while the long-term trend indicated by the record IS negative (any attempt to say otherwise would be incorrect/false), the current amount of sea ice is, simultaneously, at or above the long-term record average.
Let me be as clear as I am able: “The long-term trend is currently negative:” TRUE, to the tune of 5.5% over the length of the record in question.
“Current sea ice levels are at/above the historical average:” ALSO TRUE, according to the very same observational record. (*)
This is the reason I brought up the subject. While Becky’s initial assertion — that less polar ice = slower rotation — may have followed logically, I felt it necessary to point out that the actual amount of polar ice is currently not much different from “normal,” leaving her conclusion in doubt due to empirical observation. (**) In other words, “‘less ice = slower spin’, but what if ‘=/= less ice?'”
I appreciate that she has since added the caveat that the spring-back of mass in the Earth’s crust may actually work in the opposite direction. However, this adds a new variable: since most polar ice actually in contact with the earth’s crust is in Antarctica — a place where sea ice amounts have consistently increased over the available record, resulting in a very much POSITIVE trend (http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/seaice.anomaly.antarctic.png) — one might find that the resulting deformation of the crust as a result of those gains may actually work independently of (or even contrary to) the global polar ice trends OR amounts.
To sum up: equating the length of the day to polar ice levels is not nearly cut & dried, and to say that “less polar ice = slower/faster spin” — while perhaps a true statement — ignores the actual distribution of ice/total mass CURRENTLY at our poles (both separately and collectively) versus the equator, as well as the location of said ice. This alone was the reason I brought it up.
As it happens, the geologic record indicates that the earth has seen both MUCH more and less polar ice than we currently have, meaning even a current loss of 55.0% of the ice caps over the last 40 years might (I did say “might”) be statistically insignificant to the problem in question. After all, we’re effectively talking about the effects on the coefficient of drag experienced by a swimming gray whale due to an increase/decrease in skin bacteria — possibly calculable, but certainly academic! ^_^
(*)Statistics: where one set of numbers can suggest two or more mutually exclusive “truths” simultaneously. No wonder Mark Twain loved the subject so very much. (:-P)
(**)Caveat: As regards the earth’s rotation, I am more than willing to consider that the polar ice TREND may be more important than the actual AMOUNT, due to delays in the transfer of mass through the system. I am sure such delays are at least theoretically possible, I simply do not know of them myself; I’m sure someone out there does, or could figure them out if pressed.
Agreeing with JIHarmer. More mass around the equator…love handles…does cause the rotational rate to slow…making the day longer.
Why is the high tide also on the side of the earth that is facing away from the moon?
The far side of the earth is actually further from the moon than anywhere else. Since it is farther from the moon’s influence it also produces a high tide. That is why we have two high tides in a 24 hour period and not just one.
An excellent question, AlbertB.
Purleegates is correct: while the water on the near side is pulled toward the moon gravitationally, the water on the far side is being slung away from the Earth/Moon barycenter by the orbital motion of the system. In other words, centripetal/centrifugal force is what (mainly) produces the far side bulge.
Imagine a big sister holding hands with her much younger sibling, swinging her about in a circle. Now imagine that Big Sis has a long pony tail: does it point towards Little Sis, or does it swing out behind to some degree?
The following site does a fair job of explaining the process in both words & diagrams: http://oceanlink.island.net/oinfo/tides/tides.html. It also goes into the many other factors which influence the tides, including the Sun’s gravitational pull, the Moon’s changing orbital characteristics (range, orbital inclination), and so on.
Thank you all for the explanation and the links. We need the concept of “barycenter” in addition to the concept of the attraction of the moon to fully explain the double tides. In school lessons they did not, so we were left wondering about this phenomena.
This following site does a good job of explaining it too.
https://youtu.be/j3mhkYbznBk?t=1512