Astronomy Answers
Mysterious Lights in the Sky

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What was that mysterious light that you saw in the sky? There are many natural and man-made things that produce light and that can be seen often, so it is usually not necessary to think of UFOs and space aliens when you see a strange light in the sky.

The table directly below shows various possible sources of nightly lights, with a few of their characteristics. The "Source Type" indicates whether the source of the light is natural or artificial. The column "Direct?" shows whether the light reaches you directly from the source, or indirectly, after one or more reflections or scatterings. The "Duration" indicates how long the phenomenon is usually visible, and the "Variation" how fast the brightness of the light typically changes. The column "Sound?" reports whether sound can be heard with the phenomenon.

Phenomenon Source Type Direct? Duration Variation Sound?
City Lights artificial indirect night none no
Tower Navigation Light artificial direct night seconds no
Star natural direct hours none no
Planet natural direct hours none no
Moon natural indirect hours none no
Searchlight artificial indirect hours none no
Satellite natural indirect minutes seconds no
Airplane Navigation Light artificial direct minutes seconds faint
Airplane Landing Light artificial direct seconds irregular maybe
Falling Star natural direct moment irregular rarely
Satellite Flash natural indirect moment
Airplane Flash natural indirect moment

If you see a strange light in the sky, then remember for how long it was visible and how quickly the brightness changed. Then check the above table for possible causes of the phenomenon. Below, the phenomena are explained one by one in greater detail. Compare those explanations against what you saw.

city lights.

In most cities quite a bit of artificial light shines up, without a strong preference for the exact direction (unlike searchlight beams). That light often illuminates clouds hanging above the city, or is scattered in all directions by dust particles in the air. As seen from great distances, a bubble of light then appears to hang over the city.

Professional greenhouses are another source of illumination of the sky. In many professional greenhouses strong lamps are lit at light, and part of the light escapes up into the sky, where it can illuminate a layer of clouds or can form a bubble of light through scattering. Often the light from such a greenhouse is concentrated somewhat more strongly upward than the light from a city, so the effects look a bit more like those of searchlights. Some areas with professional greenhouses leak more light into the sky than a sizable city.

a tower navigation light.

Some buildings are so tall that they are a hazard to air traffic. Such buildings must carry navigation lights at their top that blink at night and so warn airplanes.

a star.

A star looks like a point of light in the sky. Because of always present temperature changes in the air a star appears to twinkle: to move slightly to and fro, irregularly, very many times a second, in arbitrary directions. All stars together form patterns (including the constellations) that seem to move together along the sky, and return to the same places in the sky after about a day.

The brighest star in the sky (Sirius) has a magnitude of −1.5. From a very dark place (which cannot really be found anymore in the Netherlands) a person with good eyesight can see stars to a magnitude of about 6. From an urban area, one has to be pleased if one can see stars of magnitude 2.

a planet.

A planet at first sight looks the same as a star. The brightest planets (Jupiter, Venus) stick out because they can become brighter than the brightest star (Venus even up to 13 times brighter), and because they appear to twinkle less than stars do. Venus can get so bright that it is visble in the sky long before the sky has become sufficiently dark for stars to show. Because Venus is so far away, it appears to move with you, just like the Moon and the Sun do.

Planets move along the sky just like stars, but after many days it becomes apparent that planets also move slowly relative to the stars.

the Moon.

The Moon moves along the sky just like the stars, but also moves relative to the stars, just like the planets do. Because the Moon is so far away, it appears to move with you.

a searchlight.

If a searchlight shines up, then something dark in the sky can reflect part of the light and so become visible. Searchlights can be mounted in fixed places and always point in the same direction (for example, straight up from the top of an attention-grabbing building), but can also be installed only temporarily or have the beam move around (for example, to draw attention to a disco or circus).

If the air above the searchlight is clean and empty, then the light beam is invisible. If the weather is a little hazy, then tiny dust particles or drops of water in the air will scatter part of the light, and then you can see where the beam of light is. If the light hits part of the bottom of a cloud, then you'll see a spot of light there. If the rest of the cloud is not illuminated, then the spot seems to hang in the air. If the cloud moves, then the spot can suddenly move up or down or change its shape. If the direction of the searchlight is changed, then the spot of light seems to move to and fro as well (often in complicated combinations of circles).

If the cloud moves completely beyond the beam of light, or if the searchlight is redirected to a direction where there are no clouds, then the spot of light suddenly disappears. If a different cloud enters the beam of light, or the beam is pointed at a different cloud, then the spot of light can suddenly reappear.

If a small thing such as a bird flies through the beam of light, then that thing will be illuminated for a few moments, and then it will be visible for a few moments.

an artificial satellite.

Most artificial satellites have curved parts that reflect sunlight back to Earth, from where they appear as small points of light that move across the sky in a few minutes in a straight line. Because most satellites rotate around their axis in a few seconds and aren't perfect spheres, the brightness of satellites often goes up and down in a regular period of a few seconds.

If a satellite enters the shadow of the Earth, then it suddenly no longer reflects sunlight to Earth, so it seems to disappear, as seen from Earth. If a satellite leaves the shadow of the Earth, then it becomes visible again just as suddenly. If the satellite has an orbit around the Earth that is sufficiently high, then it can be seen even many hours after sunset or before sunrise.

an airplane navigation light.

Most airplanes have navigation lights at various extremities that blink at night. If such an airplane passes overhead at great height at night, then it appears as a regularly blinking light (or small clump of lights) that moves in a straight line across the sky. This sounds quite a bit like the description of what an artificial satellite looks like. You can tell a satellite and an airplane apart because (1) airplane navigation lights often have a strong (red) color, and satellites often do not, (2) a satellite looks like just a point of light even through binoculars, while for even a small airplane at 10 km distance details (such as navigation lights on both wings) can easily be seen through binoculars, (3) a satellite never produces sound, but an airplane does. The sound of a high airplane can only be heard from a quiet place.

an airplane landing light.

When airplanes get ready to land, then they turn on powerful landing lights that shine forward and down a bit. Because airplanes usually have to make some curves to line up correctly for the landing, the landing lights can accidentally shine towards you, and then they look much brighter for a while than when they don't shine directly at you. Of course, airplanes tend to use their landing lights only near an airport where they are about to land (or from where they just took off).

a falling star.

A falling star or shooting star or meteor draws a straight trail along the sky of something like 10 degrees long in the blink of an eye. That trail can sometimes continue to glow for a few seconds. Some falling stars give a bright flash near the end. Most falling stars are much fainter than the brightest stars, but some can be much brighter than the brightest stars.

Some people think they have heared sounds that had something to do with a falling star, but such sounds are very rare, if they do in fact exist at all.

From a reasonably dark place you can see (if there are no clouds) a few falling stars every night. At some times of the year there are "meteor showers" during which you can see a few falling stars per hour from a reasonably dark place.

a satellite flash.

Some artificial satellites also have (besides curved parts) large flat shiny parts (for example, solar panels). When such a flat part has just the right orientation relative to both the Sun and the observer, then such a satellite reflects much more sunlight to the observer than the curved parts alone can do. Because the satellites usually rotate around their axis, the required orientation will be rare and last only for a few moments. Such a satellite can therefore show a sudden flash in the sky that lasts only a moment but can get much brighter than any star.

On Earth, there is a similar situation when sunlight is reflected to you from the window of a far-away building (when the Sun is low in the sky). If you wait for a little while, or take a few steps to the side, then the flash disappears again.

The Iridium communication satellites are well-known for producing such flashes.

an airplane flash.

Some parts of an airplane are reasonably flat and can therefore reflect sunlight to the observer, if the airplane has just the right orientation relative to the Sun and to the observer, just like for a satellite flash. In practice, such an airplane flash through reflection only occurs around sunrise or sunset.


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Last updated: 2016−02−07