Every now and again, I like to take a deep breath and regain some perspective.

When I was in graduate school, we were presented with a little cautionary tale about perspective. There was this graduate student, you see, a brilliant young man, who decided to tackle some of the Big Questions in cosmology for his PhD thesis.

For us graduate students, the lesson was clear: always do rough estimates — “reality checks.” They’re quick, they’re easy, and they will save your academic ass. I’ve found that they save your practical ass as well.

Suppose you need to know the number of grains of sand on all the beaches of the world. Hard question, right?

But here’s an easier question: how many grains of sand would there be if the whole surface of the world were covered with sand, say, three meters deep? The earth is around 4000 miles in radius, which is about 6500 km. Surface area of a sphere is 4*pi*radius², so we get about 530 million square kilometers, times 0.003 kilometers deep, for a grand total of 1.6 million cubic kilometers of sand. A grain of sand is at least a tenth of a millimeter across, let’s say, or one ten-millionth of a kilometer — ten million, or 10⁷, grains lined up end-to-end in one kilometer, or 10²¹ grains packed in one cubic kilometer. We had 1.6 x 10⁶ cubic kilometers of sand on the surface of the earth, so we have a grand total of no more than 1.6 x 10²⁷ grains of sand on all the beaches of the earth.

Of course, the answer is much less than that. The earth isn’t covered by a ten-foot-deep layer of sand, and a tenth of a millimeter is pretty fine sand. But this simple calculation took me only a minute or two, and now I’ve got a “reality check” for my more elaborate calculation, which might take weeks or even years to finish. If I get an answer bigger than this reality check at any point, I know I’ve made a huge mistake. And if someone else gives some answer that’s bigger than this, I know they’re spinning bullshit.

Counting grains of sand is not very useful. But the same principle applies when someone tries to sell you something that will “save you money.” Or when someone tries to tell you that a sitting president has spent more taxpayer dollars in four years than all the past presidents combined. Simple “reality checks” are a quick way to call bullshit on those who are trying to take advantage of you.

Reality checks of this sort give you perspective: broad outlines about how the world really works.

Here are a few points of perspective that have been passing through my mind lately, as we move into the mind-bending prevarication and fear-mongering of a presidential election season in the United States.

In 250,000 years, there will be no more humans on the earth. If we don’t drive ourselves into extinction first, “we” won’t be human any more — either way, no more humans. Why do I say this? Because 250,000 years ago, there were no humans on the earth. Even 100,000 years ago, Homo Neanderthalensis was the dominant erect primate, and Homo Sapiens Sapiens was just getting started on his path to world domination. In another 100,000 years, we’ll doubtless be on our way out as a species, and in 250,000 we’ll have joined the dinosaur and the loon.

In 5000 years, our entire civilization will be forgotten, or studied only as “ancient history” from scraps and fragments. Why do I say this? Because our entire history of the world is only around 3000 years old. What we think of as “civilization” is itself no more than 10,000 years old. Asphalt vanishes within a decade or two. Concrete crumbles and steel rusts within a few centuries. Even stone, like that used in Medieval cathedrals or the pyramids of Egypt or the megaliths of Paleolithic Europe, erodes, etches, and breaks over the course of a few thousand years.

In another 300 years, there will be no United States of America. Why do I say this? Because 300 years ago there was no United States of America. Any survey of maps through history shows how fluid and ephemeral national boundaries are over the course of even fifty years. Nations and empires never endure unchanged for a thousand years; only a few have lasted more than a handful of centuries; many lasted a single generation or less. The US is beginning to show fracture lines, and has clearly peaked as a unified world power. I personally doubt it will celebrate a fourth centennial, in 2176, much less a fifth. It might not even reach 2076 intact.

In 100 years, we will no longer burn oil or petrofuels of any kind. Why do I say this? Because oil was first commercialized in the mid-1800’s — it’s only about 150 years old as a commodity. Yet we have just reached/passed peak global production, and are now on the downslope, which is always steeper than the upslope: from here, prices will rise on the whole. At some point well short of the next century mark, oil will become too expensive to burn.

In 50 years, us middle-aged curmudgeons will be dead, or drawing our last breaths of pure oxygen from a tube. The hot, randy, enthusiastic teens of today will be retiring with gray hair and wrinkled skin and pot bellies. The infants still in diapers will have just finished their mid-life crises.

I find these points of perspective useful (and calming) when deluged with all the panicked rhetoric about how the Roberts Court’s decision on Obromneycare has obliterated the boundary between federal and state governments. OMG! The world has come to an end!!!

Meh. Not so much.

In 5000 years, everything we’ve created will be completely forgotten: art, music, literature, laws, bridges, cities, monuments. In only 300 years, the Roberts Court will matter only to historians; whatever distant descendants I have will be living under a different form of government, probably something rebuilt after several bloody interstate wars and at least one descent into despotism — perhaps a Hunger Games sort of world, perhaps more like Ecotopia. In a mere 50 years, I’ll be beyond caring — my grandchildren will be thinking about stiff knees and retirement, and their grandchildren will be crawling around on the floor.

The only thing of lasting value that we pass on to future generations is the good earth: having lived upon it lightly enough that it can continue to serve as the mother of all life. We do this so that our descendants can live their own lives, and gradually forget about us. Just as we have gradually forgotten all those who came before us. Our greatest virtue is our ephemerality. Our invisibility. Our unimportance in the grand scheme of things.

We can be passionate about our politics, our religion, our art and music. But it’s useful to keep a little perspective on the matter.