# Learn How the Night Sky Works Plain-language explainers on why the sky goes dark, how stars and the moon move, and what causes aurora, eclipses, meteor showers, and light pollution. ## Why is the sky dark at night? The sky is dark at night because you are standing on the side of Earth facing away from the sun. Sunlight cannot reach you directly, so the atmosphere above you has nothing bright to scatter, and space beyond it is mostly empty and cold. It is the same reason a room goes dark when you turn off the lamp: no light source, no light reaching your eyes. NASA's Space Place has a kid-friendly walkthrough of this basic geometry, and [timeanddate.com](https://www.timeanddate.com/astronomy/night/) explains why the transition from day to night is gradual rather than instant, because of twilight. Sources: [NASA Space Place, "Why Do We Have Day and Night?"](https://spaceplace.nasa.gov/day-night/en/), [timeanddate.com, "Night"](https://www.timeanddate.com/astronomy/night/). ## Earth's rotation and the apparent motion of stars Earth spins once on its axis roughly every 24 hours, and that spin, not any motion of the stars themselves, is what makes the whole sky appear to wheel slowly overhead each night. Stars near the North Star (Polaris) trace small circles around it, while stars near the horizon rise in the east and set in the west, just like the sun. The [Royal Astronomical Society of Canada](https://www.rasc.ca/rotation) has a clear explainer on how this rotation produces the apparent motion, and long-exposure "star trail" photographs make the circular paths easy to see. Sources: [RASC, "Earth's Rotation"](https://www.rasc.ca/rotation), [NASA Space Place, "What Is a Constellation?"](https://spaceplace.nasa.gov/starry-night/en/). ## What causes the aurora (northern and southern lights) Aurora happens when charged particles from the sun, carried on the solar wind, collide with gases in Earth's upper atmosphere and make them glow, the same basic principle as a neon sign. Oxygen tends to glow green or red, nitrogen tends to glow blue or purple, and the display is strongest near the magnetic poles, which is why Canada and the northern US states see it more often than the rest of the country. [NOAA's Space Weather Prediction Center](https://www.swpc.noaa.gov/phenomena/aurora) tracks this activity in real time and explains the physics; a strong solar storm can push visibility much farther south for a night or two. Sources: [NOAA SWPC, "Aurora"](https://www.swpc.noaa.gov/phenomena/aurora), [NOAA SWPC, "What Is the Aurora?"](https://www.swpc.noaa.gov/communities/what-aurora). ## What causes eclipses A solar eclipse happens when the moon passes directly between the sun and Earth, blocking some or all of the sun's disk from view for observers in its shadow's path. A lunar eclipse happens when Earth passes between the sun and the moon, and Earth's shadow falls on the moon, often turning it a dim red-orange because of sunlight scattered through Earth's atmosphere. Both events only happen a few times a year because the moon's orbit is tilted relative to Earth's, so the sun, Earth and moon rarely line up exactly. [NASA's eclipse website](https://eclipse.gsfc.nasa.gov/SEmono/reference/eclipse.html) is the definitive reference for how and why eclipses occur, with visualizations from [NASA's Scientific Visualization Studio](https://svs.gsfc.nasa.gov/vis/a000000/a005100/a005187/). Sources: [NASA Eclipse Website, "Eclipses and the Sun-Earth-Moon System"](https://eclipse.gsfc.nasa.gov/SEmono/reference/eclipse.html), [NASA SVS](https://svs.gsfc.nasa.gov/vis/a000000/a005100/a005187/). ## What causes meteor showers Meteor showers happen when Earth's orbit carries it through a trail of dust and debris left behind by a comet or asteroid. Those particles, most no bigger than a grain of sand, burn up in the atmosphere at tens of thousands of miles per hour, creating the streaks of light called meteors. Because Earth crosses the same debris trails at the same points in its orbit every year, showers like the Perseids and Geminids return on predictable dates. The [International Meteor Organization](https://imo.net/resources/what-is-a-meteor-shower/) and the [American Meteor Society](https://amsmeteors.org/meteor-showers/meteor-faq/) both maintain the reference explanations and yearly shower calendars that astronomers rely on. Sources: [IMO, "What Is a Meteor Shower?"](https://imo.net/resources/what-is-a-meteor-shower/), [AMS, "Meteor FAQs"](https://amsmeteors.org/meteor-showers/meteor-faq/). ## Moon phases The moon does not produce its own light; it reflects sunlight, and the phase you see depends on the angle between the sun, Earth and moon as the moon orbits Earth roughly every 29.5 days. A full moon happens when the moon is opposite the sun in the sky, so its whole near side is lit; a new moon happens when it sits between Earth and the sun, so the near side is dark to us. [NASA's Scientific Visualization Studio](https://svs.gsfc.nasa.gov/4955) has a widely used animation of a full lunar cycle, and the [US Naval Observatory](https://aa.usno.navy.mil/faq/moon_phases) publishes the reference definitions and timing for each phase. Sources: [NASA SVS, "Moon Phase and Libration"](https://svs.gsfc.nasa.gov/4955), [USNO, "Phases of the Moon"](https://aa.usno.navy.mil/faq/moon_phases). ## Light pollution and skyglow Light pollution is excess or misdirected artificial light that brightens the night sky, wastes energy, and hides stars that would otherwise be visible. Skyglow, the diffuse orange-gray brightening visible over cities from miles away, happens when artificial light is scattered by the atmosphere, particularly by clouds, dust and humidity. [DarkSky International](https://darksky.org/resources/what-is-light-pollution/) tracks the different forms light pollution takes, from glare to clutter to skyglow, and [lightpollutionmap.info](https://www.lightpollutionmap.info/) lets you check how much artificial skyglow affects any location using satellite brightness data. Sources: [DarkSky International, "What Is Light Pollution?"](https://darksky.org/resources/what-is-light-pollution/), [lightpollutionmap.info](https://www.lightpollutionmap.info/). ## The Bortle scale The Bortle scale is a nine-level scale, from Class 1 (an excellent, truly dark rural sky where the Milky Way casts visible shadows) to Class 9 (an inner-city sky where only the moon, planets and a handful of the brightest stars are visible. It was created by amateur astronomer John Bortle in 2001 to give observers a common language for describing how much light pollution affects a given sky. [DarkSky International](https://darksky.org/resources/what-is-bortle-scale/) publishes the standard summary of each class, and the [original scale as printed in Sky & Telescope](https://www.icq.eps.harvard.edu/ICQBortle.html) remains the reference text amateur astronomers cite. Sources: [DarkSky International, "The Bortle Scale"](https://darksky.org/resources/what-is-bortle-scale/), [Sky & Telescope / ICQ, "The Bortle Dark-Sky Scale"](https://www.icq.eps.harvard.edu/ICQBortle.html). Ready to put this to use? Find a certified dark sky place near you on [/places](/places), check the upcoming calendar on [/events](/events), or start simple with [Stargazing for Kids and Beginners](/stargazing-for-beginners). For the data sources professional and amateur astronomers use day to day, see [/research](/research). --- Canonical: https://darkskiesdirectory.com/learn