Scientists trying to learn how our planet’s atmosphere might appear to other worlds have finally sorted a clever experiment. When we measure the compositions of alien atmospheres, we do it by looking for their star’s light to pass through their atmospheres, and then we measure their atmosphere as the difference between starlight and starlight plus atmosphere. Compositions are measured with spectrographs that allow us to see the complex dips and wiggles in the random of light from a system. This isn’t pretty science. This is science by graph.

In January 2019, astronomers pointed the Large Binocular Telescope at the moon during a lunar eclipse and looked at the blood red moon that was illuminated strictly by sunlight that had been bent through the Earth’s atmosphere. As the eclipse progressed, astronomers were able to make out more and more elements and molecules in our atmosphere in the reflected sunlight. It is clear that distant observers would be able to see that we have an atmosphere rich in molecular oxygen and water vapor, as well as elements like sodium, calcium, and potassium. And I have to admit, I didn’t know all those elements were in our atmosphere because they aren’t anywhere near the top of the atmosphere’s ingredient list. This is an experiment we can do under different conditions to build a good understanding of just how eclipses can reveal our world’s hidden characteristics. Our world after all, is just one of 1000s in our galaxy, and if we can figure out how we look, it will help us understand how we see others.

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Total lunar eclipse: observing the Earth as a transiting planet (aip.de)