Tomorrow Venus will pass between the sun and the earth for the first time since 2004 and for the last time until 2117. Because the rotations of the earth and Venus around the sun are so similar — 365 and 225 days, respectively — it takes a while to catch up with Venus in this way, said Northeastern physics professor Brent Nelson.

The event is similar to a solar eclipse, but because the planet is so far away it looks like a tiny dot traveling across the sun’s surface.

To the naked eye, the sun and the moon appear to be the same size, so a solar eclipse completely blocks the sun. The two have the same “angular size,” which is to say that if you draw a triangle between your position on earth and the position of the moon in the sky at two different points in time, the angle of that triangle will be the same as if you drew a similar triangle with the sun. The ancient Greeks used solar eclipses and trigonometry to estimate the distance between the earth and the sun.

But the angular size of Venus is quite a bit smaller than that of the sun — 1/30, to be precise. “That’s right at the limit of what the human eye can really resolve,” said Nelson. “If it wasn’t so bright you might actually sense that it has size to it. When you see it in relief against the sun as a dark spot, it becomes quite apparent how big it is.”

The first time anyone observed the transit of Venus, back in 1639, they used the event to revise the ancient Greek calculation. Apparently they determined that the sun was much farther away than they had previously thought.

“There was a lot of unease about the sense that the solar system was so vast,” said Nelson. “This ever growing distance between us and the other heavenly bodies was starting to create a sense of dismay. How can we be this tiny relative to all the rest of these things?”

One of the most difficult and important things in astronomy and astrophysics, Nelson said, is asking the questions “how far away is that thing?” As you move father and farther away, you need to employ different tools to answer that same question.

One of those tools is observing the chemical composition of an object using its gaseous spectrum and then measuring its frequency. If that frequency is lower than what it would be on earth, you can assume it is moving away from us.

In the 1920s, physicists realized that all of the very distant bodies — those outside of our solar system — were moving away from us (with the exception, apparently of the nearest galaxy, Andromeda). They discovered that the universe was expanding. In Einstein’s day, the source of that expansion was called the cosmological constant.

I asked Nelson if that notion ever overwhelmed him. “I work on string theory,” he said. “So I’m required to imagine ten dimensions at a time — which is impossible of course. I’m used to the fact that nature need not be easily intelligible.” Ultimately, he said, we can figure it out mathematically, but what that really means in a tangible sense is much harder to understand. The expansion of the universe is one of these weird ideas that is hard for the human brain to imagine. It implies that it’s expanding into something. But that’s not quite right, he said. It only means the distance between any two points in space-time is expanding and that’s it.

In 1999 physicists revealed once again that the universe is incredibly difficult to comprehend. They found that the cosmological constant, which describes the energy of the empty space (the space created by the expanding universe) was not zero. “No one wanted to believe it,” said Nelson. “We wanted to believe that the cosmological constant was zero because we had precisely zero good ideas about it.”

Even weirder than the fact that it’s not zero was the fact that it’s not huge, either. “It’s right in this sweet spot,” said Nelson, where we’re accelerating but not so fast as to blow the universe to pieces in a fraction of a second, in which case “there never would have been chemistry or life or planets or anything at all.”

It begs the question — what would the cosmological constant have to be to create a catastrophe? It turns out that if the cosmological constant were only two or three times bigger, I wouldn’t be writing about it, Einstein would never have thought of it, and the dinosaurs would never have gone extinct because they would never have existed in the first place. If the cosmological constant were different, there would be no universe.

“Well, that’s a lucky coincidence,” said Nelson. Some start going toward intelligent design as a solution, but others are trying to explain it mathematically. One explanation is string theory, which can be fundamentally defined as a quantum theory of gravity. “It’s perfectly designed to address circumstances where both enormously massive bodies and the very very small can coexist,” he said.

That’s what the universe is expected to have looked like in the year of the big bang, before the universe began to expand, when it was still totally compacted. “So there may be things in the universe now that bear the imprint of string theory.”

The first observation of the transit of Venus revealed that the universe was much larger than previously believed. It was, in some ways, the birth of modern cosmology, Nelson said.

Tomorrow afternoon, beginning at 5pm and lasting until sunset, we’ll be able to watch a sort of astronomical home video of that history. Think of how much changes in our understanding of the universe between each transit. How different will things look next time? Maybe string theory will be a totally unexciting idea that everyone takes for granted. Maybe we’ll be able to imagine ten dimensions by then, just the way we can imagine a spherical earth much better than our forebears who believed it was flat.

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Now, an important word of caution, though: DO NOT LOOK DIRECTLY AT THE SUN tomorrow or ever. If you want to watch the transit of Venus, please take precautions like wearing solar viewing goggles or setting up a pinhole projector and watch its shadow. Or, watch it online in real time with SLOOH SpaceCamera.