1371: Brightness

Explain xkcd: It's 'cause you're dumb.
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Recently, some exoplanet astronomers have managed to use careful analysis of reflected light to discover Earth during the day.
Title text: Recently, some exoplanet astronomers have managed to use careful analysis of reflected light to discover Earth during the day.


Exoplanets are planets outside of our solar system, and exoplanet astronomers are astronomers who attempt to discover and study such planets.

Megan is using a common exoplanet discovery technique to discover a planet around a nearby star. When a planet passes between an observing astronomer and a star, the planet will block some tiny part of the light coming from that star, causing it to appear dimmer for some amount of time. The Kepler telescope used this technique to find evidence for exoplanets.

But here Megan is standing on the surface of the Earth at night, looking at the ground, and therefore presumably looking in the direction of the sun. By observing that it is completely occluded at night, she correctly concludes that the Sun is orbited by at least one planet: the Earth.

The title text alludes to using more complicated techniques to observe light reflected by small planets like the Earth, for example by detecting polarized light reflected from the planet's atmosphere. In some sense, observing the light that reflects off of the Earth during the day is in fact how we see everything around us.

The title text may also be a reference to 1231: Habitable Zone where an astronomer is messed up by someone who puts a mirror in front of the telescope.

More details at: Methods of detecting exoplanets


[Megan is standing on a black night-time background, staring at the ground.]
Megan: Based on this decrease in the star's brightness, I believe it is orbited by at least one planet.
Exoplanet astronomers at night

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Funny. But of course, while this technique, when applied to the sun, correctly infers the earth, it would also infer a planet around pretty much any star except Polaris; presumably incorrectly in at least some cases. 13:39, 21 May 2014 (UTC)

I like that - good point... though, there should be a small sliver of Earth where Polaris will be visible during the "day" and will sink slightly below the horizon for the "night", so I would think you could even toss that star into the group, right? It's not EXACTLY above the north pole (it's off by almost 1 degree, I believe) Brettpeirce (talk) 15:08, 21 May 2014 (UTC)

Polaris is not visible at all in the southern hemisphere. Someone who lives exactly on the equator would in theory see it rise and set, but it's tough to observe something that's one degree above the horizon. Jim E (talk) 15:45, 21 May 2014 (UTC)

Assuming he's talking about exoplanet astronomers on Earth, the title-text would require a double reflection. Something on the day-side of the Earth would have to reflect sunlight to space, and something in space would have to reflect this reflected light back into a telescope on the day-side of the Earth. What could this be? The Moon? During a solar eclipse, or even otherwise? (light reflected off the "dark" PART of the moon (washed out by the light reflected by the illuminated part) 14:27, 23 May 2014 (UTC)

Nope. A "double reflection" does only happen at a mirror based telescope (most common today). But a refractor telescope just uses lenses to look straight into the space. A mirror at that path would just show yourself like at your bathroom, or at larger distances your house, or the Earth... --Dgbrt (talk) 22:23, 23 May 2014 (UTC)

The brilliance of a star orbited by an exoplanet appears dimmer every time the exoplanet rotates around the star, i.e. once a year in exoplanet time. But in Megan's situation, the sun is occluded by the Earth every 24 hours. Therefore she may conclude that the Earth rotates around the sun in 24 hours... Seudo (talk) 13:02, 24 May 2014 (UTC)

Please feel free to disagree - I'd love a critique - but the first time I read the title text and every subsequent time I read it, I interpret it as the human eye performing "careful analysis" of the "reflected light" from every surface around us... that is, seeing with the naked eye - no mirrors or refraction, no other heavenly bodies involved Brettpeirce (talk) 13:10, 28 May 2014 (UTC)

I'm pretty sure Randall refers to 1231: Habitable Zone because the Polarimetry Method was only successful at this exoplanet: HD 189733 b. But this observations were not done at daylight. Analysing a reflection of the Earth requires daylight, otherwise the reflected object would be black. So "recently" just refers to a former comic here but not that unique and maybe questionable findings by some astronomers. Randall would criticise this single work and media hype on a "blue" planet. An example how he is confused on press releases it this comic: 1189: Voyager 1--Dgbrt (talk) 21:13, 28 May 2014 (UTC)