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Please sign up for an account and contribute to the Explain XKCD wiki!  We need explanations for comics, characters, themes, memes and everything in between.  If it is referenced in an [http://www.xkcd.com XKCD] web comic, it should be in here.
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Revision as of 21:16, 24 March 2013

Welcome to the explain xkcd wiki!

We have collaboratively explained Expression error: Unrecognised punctuation character ",". xkcd comics, and only Expression error: Unrecognised punctuation character ",". (Expression error: Unrecognised punctuation character ",".%) remain. Add yours while there's a chance!

Latest comic

Go to this comic explanation

Spectroscopy
Although right now I'm more excited about ESPRESSO's radial velocity measurements, so I'm listening to This Kiss, her song about measuring "centrifugal motion" on "a rooftop under the sky".
Title text: Although right now I'm more excited about ESPRESSO's radial velocity measurements, so I'm listening to This Kiss, her song about measuring "centrifugal motion" on "a rooftop under the sky".

Explanation

This comic mixes the method of using spectroscopy to detect oxygen on exoplanets (planets outside our Solar system) with the lyrics for the Faith Hill song "Breathe" (listen to "Breathe" on YouTube).

From the lyrics:

I watch the sunlight
dance across your face
I can feel you breathe

In the comic the word feel has been changed to see. The two first panels are one line in the song. The last line is from the chorus and is repeated five times during the song, although not right after the first two lines.

In the first and second panel the singer examines the spectra of a remote planet by watching the sunlight during the transit of the planet as this sunlight dances across the planet's face. Finally we determine that breathable oxygen exists. Since we cannot (as Faith can) feel the planet we have to see it. And by doing this I can see you breathe.

Measuring the light output of stars (spectra) we are able to determine a number of details of the star, including rotation, relative radial velocity, chemical composition, temperature, and to some degree, distance and size. When a planet, as pictured, moves between the star and the observer, then by looking at the spectrum received, the viewer is able to determine the contents of the planet's atmosphere from the specific wavelengths of light that are absorbed in this. If it turns out that the atmosphere absorbs the lines corresponding to molecular oxygen (O2) this is a clear indication that the planet has large quantities of breathable oxygen (but not necessarily life). Since for most (though not all) forms of life as we know it to exist and breathe, there must be oxygen in large amounts in the atmosphere, it is clear why Randall would be interested in exoplanets with oxygen.

This comic came out four days after this article about NASA's New NExSS Initiative. NASA will search for signs of life on other planets, for instance by using "the light passing through the atmospheres of these exoplanets". And they "will study chemicals that have been detected on other worlds, such as oxygen and methane, to see if they were produced by biology".

The title text refers to determining radial velocity in the ESPRESSO program (Echelle SPectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations). By noting that the radial velocity of the star changes slightly as the planet that orbits it moves around the star (centrifugal acceleration), the ESPRESSO program should be able to detect the masses of planets as they are moving towards the Earth in their orbit around their distant stars. The ESPRESSO program is so precise that it should be able to detect planets as small as Earth and the other of the Solar systems inner planets.

Randall is now even more excited about ESPRESSO than he is about the oxygen levels, because it is now possible to detect these "very" small planets. So he is no longer listening to "Breathe", but to another Faith Hill song: "This Kiss" (listen to "This Kiss" on YouTube).

From the lyrics:

It's centrifugal motion
On the rooftop under the sky

The first line is part of the chorus and it is repeated four times. But not in connection with the second line, which is changed a bit, so the is changed to an a. Also the "on" is not part of the quoted line in the title text.

The song is not about measuring but, of course, about the (a) kiss. Since the ESPRESSO is part of the Very Large Telescope, it is located on the Cerro Paranal mountain in the Atacama desert in Chile at an elevation of 2,635 meters (8,645 ft.) above sea level. So it could be said that it is measuring on a rooftop under the sky. Although it is radial velocity it measures not centrifugal motion, the object it does measure will all be experiencing this fictitious force (also see 123: Centrifugal Force), as the planets are in orbit around a star.

Randall has previously made several references to exoplanets in his comics, most notable are the two comics with the same name: 786: Exoplanets and 1071: Exoplanets. The latter comic came out when there were exactly 786 exoplanets found. Today more than 1900 have been discovered (1915 as of Wikipedia on the release day of this comic), much more than twice that amount. And now they can find even smaller planets, and detect the atmosphere since the first exoplanet comic came out in 2010.

Transcript

[A dark panel with a bright star in the center. To the left a planet (drawn as a new moon) approaches the star. Text is written above in white with two musical notes, one on each side of the text.]
I watch the sunlight
[Same image but now the planet transits the star. Small lines around the planet indicate the atmosphere, as seen from the light from the star passing through it. Text is again written above in white with two different musical notes, one on each side of the text.]
Dance across your face
[A white frame with a black line. It Is the spectrum of the planets atmosphere. Two distinct absorbtion peaks are visible. The first one is labeled with an arrow. Text is again written above, now in black, with two, again, different musical notes, one on each side of the text.]
I can see you breathe
Label: O2
[Below the panels is the following caption:]
Faith Hill on exoplanet spectroscopy


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Lots of people contribute to make this wiki a success. Many of the recent contributors, listed above, have just joined. You can do it too! Create your account here.

You can read a brief introduction about this wiki at explain xkcd. Feel free to sign up for an account and contribute to the wiki! We need explanations for comics, characters, themes, memes and everything in between. If it is referenced in an xkcd web comic, it should be here.

  • List of all comics contains a complete table of all xkcd comics so far and the corresponding explanations. The red links (like this) are missing explanations. Feel free to help out by creating them! Here's how.

Rules

Don't be a jerk. There are a lot of comics that don't have set in stone explanations; feel free to put multiple interpretations in the wiki page for each comic.

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Please only submit material directly related to —and helping everyone better understand— xkcd... and of course only submit material that can legally be posted (and freely edited.) Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.

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