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. One method of discovering exoplanets is detecting fluctuations in a star's brightness over time. Such fluctuations could be caused by a planet's orbit around the star, partially blocking the light that reaches an observer on Earth. In the comic, the exoplanet astronomer is trying to observe the sun through the ground at night, observing that it has decreased in brightness compared to daytime (which it has by 100%). She then rightfully concludes that the star is orbited by at least one planet (the Earth), which is of course true.

The title text refers to another method of discovering exoplanets by detecting light reflected off of them from nearby stars. Observing the light that reflects off of the Earth is in fact how was see everything around us, so one hardly needs to be an exoplanet astronomer to discover the Earth in this way.


[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)