visuo   46

Why do stars twinkle?
According to many astronomers and educators, twinkle (stellar scintillation) is caused by atmospheric structure that works like ordinary lenses and prisms. Pockets of variable temperature - and hence index of refraction - randomly shift and focus starlight, perceived by eye as changes in brightness. Pockets also disperse colors like prisms, explaining the flashes of color often seen in bright stars. Stars appear to twinkle more than planets because they are points of light, whereas the twinkling points on planetary disks are averaged to a uniform appearance. Below, figure 1 is a simulation in glass of the kind of turbulence structure posited in the lens-and-prism theory of stellar scintillation, shown over the Penrose tile floor to demonstrate the random lensing effects.

However appealing and ubiquitous on the internet, this popular explanation is wrong, and my aim is to debunk the myth. This research is mostly about showing that the lens-and-prism theory just doesn't work, but I also have a stellar list of references that explain the actual cause of scintillation, starting with two classic papers by C.G. Little and S. Chandrasekhar.
nibble  org:junk  space  sky  visuo  illusion  explanans  physics  electromag  trivia  cocktail  critique  contrarianism  explanation  waves  simulation  experiment  hmm  magnitude  atmosphere  roots  idk 
december 2017 by nhaliday
light - Why doesn't the moon twinkle? - Astronomy Stack Exchange
As you mention, when light enters our atmosphere, it goes through several parcels of gas with varying density, temperature, pressure, and humidity. These differences make the refractive index of the parcels different, and since they move around (the scientific term for air moving around is "wind"), the light rays take slightly different paths through the atmosphere.

Stars are point sources
…the Moon is not
nibble  q-n-a  overflow  space  physics  trivia  cocktail  navigation  sky  visuo  illusion  measure  random  electromag  signal-noise  flux-stasis  explanation  explanans  magnitude  atmosphere  roots 
december 2017 by nhaliday
galaxy - How do astronomers estimate the total mass of dust in clouds and galaxies? - Astronomy Stack Exchange
Dust absorbs stellar light (primarily in the ultraviolet), and is heated up. Subsequently it cools by emitting infrared, "thermal" radiation. Assuming a dust composition and grain size distribution, the amount of emitted IR light per unit dust mass can be calculated as a function of temperature. Observing the object at several different IR wavelengths, a Planck curve can be fitted to the data points, yielding the dust temperature. The more UV light incident on the dust, the higher the temperature.

The result is somewhat sensitive to the assumptions, and thus the uncertainties are sometimes quite large. The more IR data points obtained, the better. If only one IR point is available, the temperature cannot be calculated. Then there's a degeneracy between incident UV light and the amount of dust, and the mass can only be estimated to within some orders of magnitude (I think).
nibble  q-n-a  overflow  space  measurement  measure  estimate  physics  electromag  visuo  methodology 
december 2017 by nhaliday
How do you measure the mass of a star? (Beginner) - Curious About Astronomy? Ask an Astronomer
Measuring the mass of stars in binary systems is easy. Binary systems are sets of two or more stars in orbit about each other. By measuring the size of the orbit, the stars' orbital speeds, and their orbital periods, we can determine exactly what the masses of the stars are. We can take that knowledge and then apply it to similar stars not in multiple systems.

We also can easily measure the luminosity and temperature of any star. A plot of luminocity versus temperature for a set of stars is called a Hertsprung-Russel (H-R) diagram, and it turns out that most stars lie along a thin band in this diagram known as the main Sequence. Stars arrange themselves by mass on the Main Sequence, with massive stars being hotter and brighter than their small-mass bretheren. If a star falls on the Main Sequence, we therefore immediately know its mass.

In addition to these methods, we also have an excellent understanding of how stars work. Our models of stellar structure are excellent predictors of the properties and evolution of stars. As it turns out, the mass of a star determines its life history from day 1, for all times thereafter, not only when the star is on the Main Sequence. So actually, the position of a star on the H-R diagram is a good indicator of its mass, regardless of whether it's on the Main Sequence or not.
nibble  q-n-a  org:junk  org:edu  popsci  space  physics  electromag  measurement  mechanics  gravity  cycles  oscillation  temperature  visuo  plots  correlation  metrics  explanation  measure  methodology 
december 2017 by nhaliday
The weirdest people in the world?
Abstract: Behavioral scientists routinely publish broad claims about human psychology and behavior in the world’s top journals based on samples drawn entirely from Western, Educated, Industrialized, Rich, and Democratic (WEIRD) societies. Researchers – often implicitly – assume that either there is little variation across human populations, or that these “standard subjects” are as representative of the species as any other population. Are these assumptions justified? Here, our review of the comparative database from across the behavioral sciences suggests both that there is substantial variability in experimental results across populations and that WEIRD subjects are particularly unusual compared with the rest of the species – frequent outliers. The domains reviewed include visual perception, fairness, cooperation, spatial reasoning, categorization and inferential induction, moral reasoning, reasoning styles, self-concepts and related motivations, and the heritability of IQ. The findings suggest that members of WEIRD societies, including young children, are among the least representative populations one could find for generalizing about humans. Many of these findings involve domains that are associated with fundamental aspects of psychology, motivation, and behavior – hence, there are no obvious a priori grounds for claiming that a particular behavioral phenomenon is universal based on sampling from a single subpopulation. Overall, these empirical patterns suggests that we need to be less cavalier in addressing questions of human nature on the basis of data drawn from this particularly thin, and rather unusual, slice of humanity. We close by proposing ways to structurally re-organize the behavioral sciences to best tackle these challenges.
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november 2017 by nhaliday
How & Why Solar Eclipses Happen | Solar Eclipse Across America - August 21, 2017
Cosmic Coincidence
The Sun’s diameter is about 400 times that of the Moon. The Sun is also (on average) about 400 times farther away. As a result, the two bodies appear almost exactly the same angular size in the sky — about ½°, roughly half the width of your pinky finger seen at arm's length. This truly remarkable coincidence is what gives us total solar eclipses. If the Moon were slightly smaller or orbited a little farther away from Earth, it would never completely cover the solar disk. If the Moon were a little larger or orbited a bit closer to Earth, it would block much of the solar corona during totality, and eclipses wouldn’t be nearly as spectacular.
nibble  org:junk  org:edu  space  physics  mechanics  spatial  visuo  data  scale  measure  volo-avolo  earth  multi  news  org:mag  org:sci  popsci  sky  cycles  pro-rata  navigation  degrees-of-freedom 
august 2017 by nhaliday
How to estimate distance using your finger | Outdoor Herbivore Blog
1. Hold your right arm out directly in front of you, elbow straight, thumb upright.
2. Align your thumb with one eye closed so that it covers (or aligns) the distant object. Point marked X in the drawing.
3. Do not move your head, arm or thumb, but switch eyes, so that your open eye is now closed and the other eye is open. Observe closely where the object now appears with the other open eye. Your thumb should appear to have moved to some other point: no longer in front of the object. This new point is marked as Y in the drawing.
4. Estimate this displacement XY, by equating it to the estimated size of something you are familiar with (height of tree, building width, length of a car, power line poles, distance between nearby objects). In this case, the distant barn is estimated to be 100′ wide. It appears 5 barn widths could fit this displacement, or 500 feet. Now multiply that figure by 10 (the ratio of the length of your arm to the distance between your eyes), and you get the distance between you and the thicket of blueberry bushes — 5000 feet away(about 1 mile).

- Basically uses parallax (similar triangles) with each eye.
- When they say to compare apparent shift to known distance, won't that scale with the unknown distance? The example uses width of an object at the point whose distance is being estimated.

per here:
Select a distant object that the width can be accurately determined. For example, use a large rock outcropping. Estimate the width of the rock. Use 200 feet wide as an example here.
outdoors  human-bean  embodied  embodied-pack  visuo  spatial  measurement  lifehack  howto  navigation  prepping  survival  objektbuch  multi  measure  estimate 
august 2017 by nhaliday
On the measuring and mis-measuring of Chinese growth | VOX, CEPR’s Policy Portal
Unofficial indicators of Chinese GDP often suggest that Beijing’s growth figures are exaggerated. This column uses nighttime light as a proxy to estimate Chinese GDP growth. Since 2012, the authors’ estimate is never appreciably lower, and is in many years higher, than the GDP growth rate reported in the official statistics. While not ruling out the risk of future turmoil, the analysis presents few immediate indications that Chinese growth is being systematically overestimated.
org:ngo  econotariat  study  summary  economics  growth-econ  econometrics  econ-metrics  measurement  broad-econ  china  asia  sinosphere  the-world-is-just-atoms  energy-resources  trends  correlation  wonkish  realness  article  multi  news  org:foreign  n-factor  corruption  crooked  wealth  visuo  electromag  sky  space 
july 2017 by nhaliday
The Easiest Way to Memorize the Algorithms of Rubik's Cube: 7 Steps
Learn to solve the Rubik’s cube in no time by taking advantage of your visual memory. You’ve probably been playing with Rubik’s cube and wondering how to solve it. Eventually, you may have succeeded by following an online tutorial. While there are several methods to solve the 3x3 cube, these techniques usually consist of a number of algorithms that look something like this: T R Ti Ri Ti Fi T F.

Some people can memorize such sequences without difficulty. But what about the rest of us who are better at remembering peoples faces rather than their names? The good news is that the algorithms can be converted into easy to memorize graphics so that you don’t need to spend days learning the sequences by heart!
diy  puzzles  visuo  spatial  explanation  howto  math.GR  rec-math  wordlessness 
july 2017 by nhaliday

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