What is Sky: Definition and 270 Discussions

The sky is everything that lies above the surface of the Earth, including the atmosphere and outer space.
In the field of astronomy, the sky is also called the celestial sphere. This is an abstract sphere, concentric to the Earth, on which the Sun, Moon, planets, and stars appear to be drifting. The celestial sphere is conventionally divided into designated areas called constellations.
Usually, the term sky informally refers to a perspective from the Earth's surface; however, the meaning and usage can vary. An observer on the surface of the Earth can see a small part of the sky, which resembles a dome (sometimes called the sky bowl) appearing flatter during the day than at night. In some cases, such as in discussing the weather, the sky refers to only the lower, denser layers of the atmosphere.
The daytime sky appears blue because air molecules scatter shorter wavelengths of sunlight more than longer ones (redder light). The night sky appears to be a mostly dark surface or region spangled with stars. The Sun and sometimes the Moon are visible in the daytime sky unless obscured by clouds. At night, the Moon, planets, and stars are similarly visible in the sky.
Some of the natural phenomena seen in the sky are clouds, rainbows, and aurorae. Lightning and precipitation are also visible in the sky. Certain birds and insects, as well as human inventions like aircraft and kites, can fly in the sky. Due to human activities, smog during the day and light pollution during the night are often seen above large cities.

View More On Wikipedia.org
  1. P

    I Density fluctuations and the color of the sky

    From posts like this I get that the color of the sky is explained by Rayleigh scattering, but needs density fluctuations. However as atoms are not uniform and are localized, the density is $$\rho(\mathbf r)=\sum_i \delta(\mathbf r-\mathbf r_i)$$ where ##i## sums over all the atom positions...
  2. Z

    Stargazing Has a Deep Field Picture of the Sky Been Captured with a Radio Telescope?

    Has anyone ever taken a "deep field" picture of the sky, like Hubble, but with a radio telescope?
  3. L

    B How bright is the Sun compared to the sky?

    What is the brightness of the sun compared to the sky that it lights up? I have a solar filter that only sees the sun. I just wonder if it is needed to be so dark that I can't see the sky anymore. I have plenty of camera options left to shorten exposure even with a filter a fraction as dense...
  4. S

    B Night Sky of Alpha Centauri: Sol as a Bright Star

    I finally got around to watching the movie "Avatar", which takes place in the Alpha Centauri system. I was thinking that a great little scene between the 2 main characters (i.e., the paraplegic Earthling and the Centauri woman) would have been looking up at the night sky and seeing Sol as a...
  5. R

    I Estimating Hot & Cold Spots in CMB Sky Maps

    Is it possible to estimate the number of hot and cold spots of average diameter of 1° in a full-sky map of the CMB?
  6. S

    Light's properties (Why isn't the sky violet?)

    Why isn't the sky violet? Since violet has a shorter wavelength even than blue, why doesn't it dominate then?
  7. Salmone

    I Rayleigh scattering -- What is the true reason for the color of the sky?

    I have a little question about Rayleigh scattering: I know that Rayleigh cross section is proportional to the inverse of the fourth power of the wavelenght of the incident light and that it is so even the intensity of the scattered beam of light, now: 1. What is the color of the sky due to...
  8. Melbourne Guy

    Anyone watched George Clooney's "The Midnight Sky"?

    I'd seen George talking about The Midnight Sky on Graham Norton's show, and it seemed a bit meh, but at a loose end, I started watching it yesterday. Gotta say, it is the first time I have upped the play speed on Netflix to 1.25 and even then, it dragged. Being about thirty-five minutes in and...
  9. Kwantumnaut

    I A busy and interesting night sky in Mackinaw City

    Hello there, first let me say I hope this is the right section for this topic, and if not please feel free to move it! So a couple days ago I took a trip up to Mackinaw City, Michigan, and saw a few interesting and intriguing things (luckily!) in the sky that night, and was just hoping maybe...
  10. P

    B How to calculate torque on a body falling freely through the sky?

    if a body is pivoted at a point it is quite easy to calculate the torque as we know the axis of rotation. but, if a body is under free fall and we apply a force of F Newtons at a distance of x from its centre of mass , then how do we determine the axis of rotation?
  11. berkeman

    I Dark Sky Parks -- coming to a park near you

    This sounds interesting and promising... https://www.cnn.com/travel/article/dark-sky-waterton-glacier-international-peace-park-trnd/index.html
  12. Doctor U

    Exploring the Possibilities of Sky Color

    As far as I know, the sky is blue because of the natural vibration of the molecules of the air enter in a ressonance with the ultraviolet spectrum, so when the sun light hits the air molecules the scattering occours more on the ultraviolet part, but blue part of the visible spectrum is more...
  13. BillTre

    Bronze Age or Iron Age? Age of Nebra Sky Disk is disputed

    The age of the Nebra (from Germany) Sky Disk (claimed to be the oldest known representation of the heavens) is in dispute. It was found in the black market, so provenance is unclear. NY Times article here.
  14. P

    I What is the correct way to calculate the area of the sky in square degrees?

    Most of the calculations that I have seen that measure the area of the Sky involve doing this: 2*pi*r = 360. => r = 57.295 degrees. And then 4*pi*(57.295)^2 = 41251.83 square degrees. Now the units check out fine, but here are the places where I am having trouble understanding this derivation...
  15. I

    B Why certain stars seem to move very little in the sky

    Why is it that some celestial bodies when observed from the Earth appear to move very little in the the sky throughout the seasons, even though the Earth in the meantime has traveled millions of miles in its orbit around the sun? From my location in Montreal, Canada, there are only very few...
  16. P

    Rayleigh Scattering Conceptual Question about the Color of the Sky

    We know ##\lambda_{blue} < \lambda_{red}## so citing the formula above (in the relevant equations) it's apparent that blue light is scattered more than red light. But presumably this would hold for ##\theta = 0## as well but when we look at the sun head on we don't see blue, we see red/orange...
  17. skyshrimp

    Is the Stationary Light Near the Moon Venus?

    I'm in Surrey, London, UK. I've noticed this stationary light in the sky near the moon when there are no stars visible. What is it?
  18. E

    Night sky cooling, cooling by transmission or as 'heat sink'

    Howdy all. Regarding the phenomenon of radiative night sky cooling, where an object (such as a blackbody) on the surface of the planet cools via radiating energy into space: can the cooling be ascribed to the transmittance of the radiation through the atmosphere (as such a clearer night with...
  19. Jiman

    English Word Differences: Exploring the Nuances of Similar Terms

    How did you find PF?: Via a website link I want to know difference of the words in English
  20. W

    Abaqus FEA - how to choose temperature for the sky on a clear night

    Hi, I want to model the phenomena that a windshield gets covered in ice during a night with clear sky even tough the air holds a temperature of a few degrees above zero Celsius (at which water freezes for sea level pressure). Clearly the windshield gets a temperature below zero Celsius, and...
  21. Meruem

    B What is the black that we see in the night sky?

    Summary: Can black means many thing? The black we see in night sky has to mean many things First of all, from point of views of human eyes then the photons can't reach our eyes so it appears black. Secondly, the objects appear smaller as we go farther from that object. So, at some point object...
  22. M

    Stargazing Mysterious Phenomenon Seen in the Night Sky

    There is supposed to be a meteor shower tonight, so I was looking up at the sky, and saw something that at first I thought could be one, but it moved too slowly, so I watched as it danced in a zig zag across the sky, as high as a star, appearing as a star, yet floating slowly enough through the...
  23. davenn

    Stargazing Short videos on the Messier catalog of deep sky objects

    Some of you may be familiar with the 60 Symbols YouTube channel, that have many great physics videos They also have a sister channel that is dedicated to discussing space related subjects, called Deep Sky Videos. Here I present their collection of videos on the Messier catalog Enjoy this...
  24. C

    What percent of the sky can an astronomer see at one time?

    Homework Statement The astronomer fell into the well. The well is circular, depth H = 15m, radius r = 1m. The astronomer has eyes at h = 1.75m. Observation is from the well axis, refraction is neglected. How many % of sky can an astronomer see at one time? How many % of sky can an astronomer...
  25. C

    Observing galaxies: area of sky would I need to survey

    Homework Statement Given that there are 10-2 Ellipticals per Mpc3 and my garden telescope can reach to 14 mag. How large an area of sky would I need to survey to find 100 Elliptical galaxies ? (assume the typical absolute magnitude for an Elliptical galaxy is -21 mag).Homework Equations...
  26. X

    I How to find the position of a celestial object in the sky?

    Hi all, I am an electrical engineering student currently involved in a project aimed to create a simple GoTo azimuth mount. I don't have a lot of background in astronomy, so I am a bit confused on how to find the position of a star in the sky. After some research on my own, I figured out that...
  27. fando1234

    B A silly question about why the sky is blue

    I know that 'the sky is blue because higher frequency blue wavelengths are scattered more than red'. What has always confused me is that when I imagine this, I imagine the blue wavelengths bouncing around between atoms in the atomosphete. Whilst red light interacts less, so travels through...
  28. Baluncore

    B SOFIA IR Observatory in the Southern Sky

    NASA747 has been flying out of Christchurch, New Zealand since 5th of June 2018. Another 20 flights are planned in this block. https://www.nasa.gov/feature/sofia-to-study-southern-skies-in-new-zealand The “Stratospheric Observatory for Infrared Astronomy”, SOFIA, flies an interesting tracking...
  29. kolleamm

    Stargazing A good telescope with tracking for deep sky objects

    I've had a refractor telescope and I've managed to get some okay photos of them moon and jupiter. The only thing it lacked however was a tracking system. I would really like to buy a low maintence telescope that has a straightforward tracking system I can use to take pictures of things such as...
  30. quasarLie

    B Extracting Spectra from Sloan Digital Sky Survey Data Releases 13 and 14

    Hello, What are the quasar observation criteria by GAIA and SDSS(what are the condition used by SDSS to define an observed object as a quasar), why some brilliant objects are observed by SDSS and not by GAIA? thanks
  31. B

    Does iFLY Truly Mimic the Experience of Skydiving?

    Who has tried IFLY where you fly in the air inside the tube? Is the experience really like sky diving? Is it that fun? next month I may try it if it's ok. Any technical reference how it all works (the turbines and suction power, etc.)?
  32. S

    I How bright will Musk's Tesla car be in the sky?

    I wonder if amateur astronomers will be able to see it.
  33. quasarLie

    Inclination angle between the orbital plane and the plane of the sky

    Homework Statement I have the inclinination i which is the angle between the orbital plane and the plane of the sky, i [0,pi/2] if it is in the direct sense ans [pi/2, pi] in the indirect, so here i have an angle i=40 in the direct sense how can i calculate the i in the indirect sense Homework...
  34. davenn

    Stargazing Southern Sky Image: View the Southern Cross, Coal Sack & Omega Centauri

    An image specially for those in the northern hemisphere that won't see these things ... upper right - Southern Cross (on it's side) on the lower side of it, the Coal Sack Nebula. Lower centre, the two pointers, Alpha and Beta Centauri. and finally in the upper left, the huge globular cluster -...
  35. R

    Stargazing Mysterious Evening Sky in Ireland - Solve the Mystery!

    It's early evening here (Ireland). The Sun is definitely set, and there is a bright half Moon. However there are no stars or planets visible at all, anywhere in the sky. There is no cloud, at least nothing substantial, the Moon is perfectly bright, no sign of cloud in that part. There are low...
  36. jedishrfu

    B What are the mystery lights in the sky and how can you identify them?

    NASA article on how to identify mystery lights in the sky using a simple taxonomy https://apod.nasa.gov/apod/ap170924.html
  37. P

    B Can someone please identify what I saw? (lights in the night sky)

    I'm the farthest thing in the world from an expert, but I've seen the usual number of pictures and Discovery Channel programs... and I never saw anything like I saw last night (8-1): I'm in San Diego, California. I went outside Tuesday night a little bit after 11 PM, and was casually looking...
  38. R

    Why does Venus always appear close to the Sun in the sky?

    1. The problem statement, all variables, and given/known data Venus is sometimes described as either the “Morning Star” or the “Evening Star”, since it can only be seen near sunrise and sunset, very close to the Sun in the sky. Why does Venus always appear close to the Sun in the sky for an...
  39. J

    Is the Sun's Movement Different in the Southern Night Sky?

    hi. i am from sydney australia. over the xmas holidays i spent some time in tasmania, and had a very memorable night sleeping on a 1300m high mountain with 360° veiws and crystal clear night. i stayed awake all night hoping to see the southern lights but no luck. however i did see something else...
  40. J

    Stargazing Strange looking star in the sky

    Hi I am a new member, I felt compelled to find a forum to talk about what i have been watching in the night sky. I have been watching the sky most evening now for the last couple of weeks, since one Saturday evening around 12pm I noticed what at first sight looked like a plane siting in the...
  41. weezy

    I How would the sky look if ether drag hypothesis was true?

    I just learned how stellar abberation and the ether theory while studying Robert resnick's book. What I'm thinking is how the sky would look IF ether were to exist and be dragged along the Earth as it revolved the sun. Since Earth's local supply of ether is moving along with it i.e. rest w.r.t...
  42. sophiecentaur

    Stargazing The Pleiades: an underrated glory of the night sky

    I was doing my usual untargeted tour of the sky last night and checking the objects I'm now familiar with when I saw a new twinkle down on the NE horizon. The binoculars made it look interesting so I looked at it with my telescope. What a fantastic fairyland of light. No wonder the Pleiades are...
  43. G.K. Masterson

    Writing: Input Wanted Travel to an Alien Sky: Far in Milky Way?

    How far in the Milky Way would one have to travel to land upon a planet where the night sky would have zero familiar stars? Basically, I'd like to set up a world somewhere so far from our section of the galaxy that the only things that are the same are distant galaxies (which I'm assuming would...
  44. Shadowkat678

    B What Could Explain the Mysterious Red Pulsing Light in the Sky?

    Hey, I'm new here, and I really need an explanation for this. It's been driving me up the walls! Also, it looks like this got posted in a different area then I tried to put it. Tried putting it in astrology thinking it might have something to do with a space phenomenon. I would delete this and...
  45. Phil A

    Stargazing Stars in sky given arbitrarily sized/oriented window

    Hi All, I've been taking a bit of a crash course in astronomy for a raspberry pi project I'm toying with. I basically want to create a window to the night sky without the actual window but a screen to replace it. Barring the practicality (psh, who needs that) of doing this, it's been a lot of...
  46. B

    How advanced are spy drones used in the military and law enforcement?

    In this movie Eye in the Sky about drone deployment in the middle east. This is this small electronic beattle which can fly and has built in camera to spy inside houses. Is this technology real? Where can one get it? Should there be none, what is the smallest drone you saw that has good...
  47. terryds

    Understanding the Effects of Distance on Light Intensity

    If I shot/flash lights up to the sky, will the lights or photons fly up to the sky then to the space then to the top of the universe?? Will it stop?
  48. A

    I Why is the Sky Blue? Understanding Born's Rule & Rayleigh Scattering

    I recently asked a physics professor to cite some observable natural phenomena whose explanation essentially requires Born's rule. He cited the sky's being blue. But I've had some trouble confirming this through online searches. Typically one finds the claim that the blue colour of the sky is...
  49. DiracPool

    Close Calls from Falling Objects Out of High Rises

    Does anyone have any stories of close calls from objects falling out of windows of high rises in the city (or elsewhere)? I was watching the Ken Burnes series on the civil war recently and they said that the women in New Orleans used to dump their chamberpots on the occupying Union soldiers...