In a First, Curiosity Watched Mercury Transit the Sun from Mars

For the first time, we've seen Mercury transit the Sun from Mars. NASA's Curiosity rover sent back pictures of the planet closest to the sun faintly darkening the sun's surface as it appears to pass across the sun's face.

There's something almost charmingly retrofuture about this milestone. The transits of the inner planets were important to early astronomers dating all the way back to Kepler.

"This is a nod to the relevance of planetary transits to the history of astronomy on Earth," Mark Lemmon, who works on the Mastcam crew, said in a press release. "Observations of Venus transits were used to measure the size of the solar system, and Mercury transits were used to measure the size of the sun."

As it literally takes the planets aligning to happen, transits on Earth are a rare thing. Next transit of Mercury that we'll see on Earth won't happen until May 9, 2016. If that seems far off, consider that we won't see Venus pass between us and the Sun until December 2117.

Mercury transits happen more frequently on Mars; the next one happens on April 15 next year. So what Mercury-transits from Mars lack in panache, they make up for in regularity. Which is good, because that was…well it lacked some panache, didn't it?

Of course, even one planet closer, here on Earth, transits of Mercury aren't exactly sky-splitting affairs, so you'll have to forgive the Curiosity camera that's another 54.6 million kilometers away, for making the whole thing seem sort of insignificant. In the video NASA released, Mercury is dwarfed by a pair of Earth-sized sunspots.

But on the plus side the sun has some healthy sunspots—signs that the solar peak, while weak, is still happening.

And as long as we're being optimistic, witnessing transits on Earth have been benchmarks for astronomical progress. The last transit of Venus in 2012 was the first to be witnessed from space. This Mercurial transit was the first photographed from another planet.

In these tense, post-shuttle days, space fans have had plenty of opportunities to feel discouraged, so it's important to stay optimistic and take victories where you can, and hold out hope that eventually someone will be underwhelmed by seeing Mercury transit from Mars in person.