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ROBERT SIEGEL, HOST:

You're listening to ALL THINGS CONSIDERED from NPR News.

Tomorrow brings a rare celestial event. Venus will come between the Earth and the sun. It's called the Venus Transit. From here, it will appear as though a dark dot is moving across the sun. The whole journey lasts six hours and 40 minutes.

Astrophysicist and NPR science blogger Adam Frank says it's worth taking note of the Venus Transit because of what it tells us about our time on Earth.

ADAM FRANK, BYLINE: A few hundred years ago, Venus passing in transit across the face of the sun was a big scientific deal. In 1769, for example, astronomers around the planet coordinated their observations of a transit and used it to accurately measure the distance from the Earth to the sun. Back in the day, that was the equivalent of a mission to Mars. But now, we can nail the Earth-sun distance with a precision of meters. So why should you or anyone else care that our sister planet Venus will appear to march across the sun's disk late Tuesday afternoon?

While transits remain one of astronomers' favorite tools for discovering alien planets orbiting other stars, there's a far deeper and far more intimate reason why the Venus transit matters. It's all about time. The next Venus transit will be in 2117. That's 105 years from now. It's unlikely that anyone hearing the sound of my voice will still exist then. Think about that: The next time the orbits of Venus and Earth align just so to create a transit, the world will be entirely populated by an entirely unborn generation. That essential point about time is really what makes this transit worth a moment of your own.

It's rare for us to get pulled over the narrow time horizons defining our lives. We struggle through the daily cycle of waking and work, looking to the longer cycle of weekends for some rest. On even longer timescales, we make appointments or plan vacations months in advance. We may even think on yearlong cycles, dreaming of that summer home we want to buy. But short of worrying about retirement and not doing very much about it, the time horizon of our lives never stretches further than a few years.

Then along comes a celestial event like the Venus transit, announcing this won't happen again for 105 years. It comes as a kind of shock, and we have no experience for this kind of timescale. It's not like our favorite TV shows disappear for centuries. Oh, by the way, "Game of Thrones" will be back in 2125. These long time horizons are not easy for human beings to digest, but their recognition is an essential piece of our evolution in both science and civilization. And that's the point.

While the astronomy behind a Venus transit might not be news, the celestial mechanics of our own trajectories through life and the universe are an ongoing story. Tomorrow's transit of Venus reminds us of something essential. We are so busy worrying about getting the kids to school before homeroom, getting to work before the shift starts or getting to the gym for that spin class that we completely forget that time spins on many different cycles.

While our heads are down waiting for a Facebook page to update on our cellphones, the solar system continues relentlessly on its steady, stately dance of gravity, matter and motion. The sky is the original clock, and it has always reminded humans that there are times and cycles that dwarf our own. Now that we've become a global culture whose actions affect the planet on timescales of centuries or more, this kind of reminder couldn't be more, well, timely.

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SIEGEL: That's astrophysicist Adam Frank. His most recent book is called "About Time: Cosmology and Culture at the Twilight of the Big Bang."

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SIEGEL: This is ALL THINGS CONSIDERED from NPR News.

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