A short video showing the geometry of the Sun, Earth, Moon, and Hubble during the observations of a lunar eclipse.

During a lunar eclipse, if you were standing on the Moon you’d see the Earth blocking the Sun.

The sunlight passing through Earth’s atmosphere hits the Moon and then is reflected back to us, so a telescope back here can be used to look for it?

Since we’re looking in the UV we need to be in space (because the Earth’s air absorbs it), so why not use Hubble Space Telescope.

They point the observatory at the Moon during the 21 January 2019 lunar eclipse, and (spoiler alert) in the end they were able to detect ozone in Earth’s atmosphere this way.

We know the way the Earth’s air changes with height (like the pressure, density, and temperature) as well as the composition (for example, carbon dioxide and water vapor are lower in the atmosphere, while the ozone layer is much higher up), so the way sunlight passes through Earth’s air can be modeled.

They created a model that includes things like O2 (the oxygen we breathe) and even the way the Earth’s air scatters light, called Rayleigh scattering (which makes sunsets red, and is also why lunar eclipses turn the Moon red).

They also made the models such that they could include ozone absorption as well as what it would look like if the Earth had no ozone; that way they can see if their observations matched.

Hubble observations of the Moon during a lunar eclipse (left, different observations shown in different colors) versus models of Earth’s atmosphere.

These are exactly the places where ozone absorbs light, and the model clearly shows that if you leave out ozone the spectrum would look much different.

In other words, they saw the Earth’s ozone absorbing sunlight reflected by the Moon.

Artwork of Hubble observing the Moon during a total lunar eclipse.