Long distance observations

A typical example of long-distance observation. The Tatra Mountains as seen from the Magdalenka Hill near Rzeszów, in southeast Poland, at a distance of about 170 km (110 mi).

Long-distance observation is any visual observation, for sightseeing or photography, that targets the Earth's landscape and natural surface features (e.g. mountains, depressions, rock formations, vegetation), as well as manmade structures firmly associated with the Earth's surface (e.g. buildings, bridges, roads) that are located farther than the usual naked-eye distance from an observer. These objects may be natural or artificial.^[1]

Natural

View towards Low Tatras located about 140-180km away from the observation place (Vihorlat Mountains in Slovakia) (Credit: Milan Bališin)^[2]

Artificial

Important is that an observer must also be firmly integrated with the Earth's surface or one of the objects listed above.

The term "long-distance photography" usually excludes astrophotography, as well as photography of distant objects not ground-based, such as the following:^[3]

Main aspects of long distance observations[]

Topographic[]

Object size and feature[]

The objects, whose appearance is different from others are recognizable and detectable easier. It refers to these mountains, where some rock protrusions stand on the top. The same situation applies to the mountains more prominent than adjacent ones. Unlike mountains, industrial and infrastructure objects are usually much thinner, what makes them hard to notice and photography because of their angular width.

Object location[]

The location of the observed object plays an important role, making it visible or not even from a small distance. The best visible are freestanding mountains or mountain ranges isolated from the mountain chain regardless of their relative altitude. Likewise separated mountains, industrial telecoms, and infrastructure objects are also visible from range because they are usually higher than the surrounding area. The telecommunications transmitters are often inherent elements of the mountains, making them easy distinguishable from others.

Topography along the line of sight[]

Sometimes the prominent object can be hidden by another one standing somewhere in the middle between him and the observer. It happens usually inside the massive, often parallel mountain range, where a lot of peaks having a similar altitude block some distant mountain chains visible in the theoretical sense. An opposite situation takes place, when the remote massive chains are separated by a vast plain, lowland, or large water body. The circumstances such that are the most favourable for seeing and capturing objects from the biggest possible distance, what the best example is the current world's record established between the Pyrenees and Alps in Europe.^[4] Both mountain ranges separated by lowland from each other must be high enough to be visible at long range like this. There are only a few places on Earth, where the similar or bigger result can be achieved.^[5]

Astronomical[]

The most important astronomical factors determining the conditions of long-distance observations are:

Diurnal position of the Sun[]

This is the most obvious astronomical factor, as the main source of light shapes the light scattering conditions on haze and visual object appearance.

When an object is located at a similar azimuth to the Sun, then its observation conditions are the worst. Because of the forward light scattering the haze concentrated nearby, the solar azimuth has a whitish appearance blocking the light reflected from an observed object's surface.

The effect of progressively shifting Sun angle on the appearance of a vista as seen from Canyonlands National Park. In each image air quality is the same. 1, 2 – represents a moment after sunrise; 3, 4 – a vista around noon, when the angular distance to the Sun is the biggest, hence visibility is the best (Malm, 2016).

On the other hand, the Sun travels across the sky changing its position against the observed object. It also reflects changes the contrast of this object.

The solar azimuth always goes along with its angle above the horizon. When the Sun shines higher, less amount of light is scattered by the atmosphere towards the observer. Besides, the vista reflects more light, which results in more image-forming information (reflected photons from the vista) reaching the human eye. Otherworldly, the contrast detail and scene are enhanced.^[6] When the distant features are located at similar azimuth to the solar one they are shaded by themselves, revealing much fewer features for an observer. On the contrary of solar azimuth, objects present at the opposite side of the Sun are much better illuminated. It's the best visible during the golden hour when the view towards the antisolar direction is the best. The role here plays also the Atmospheric extinction, which reduces the direct sunlight as Sun is lower above the horizon. As a result of such less light is scattered on the haze particles and molecules giving a view more discernable. As sunlight diminishes around sunrise and sunset the all objects seem to be visible better in all directions except the solar azimuth. Vanishing scattering of direct sunlight causes significant changes in the scene contrast and the visibility of the objects at once. The sunset marks the moment when the forward scattering disappears and reappears at sunrise again. This moment is called the sunrise or sunset transition ^[7] and it's derivative from the light transition phenomenon.^[8][9]

The specific situation occurs at twilight when the Sun is below the horizon. This is the moment when the light scattering takes hold in the atmosphere. In the shaded part of the atmosphere, the secondary scattering takes place. As twilight progresses most of the atmospheric aerosols have an extinction coefficient decreasing in magnitude with increasing wavelength.^[10]

Presence of moonlight[]

The moonlight plays an analogous role in the sunlight. However, this light is about 500k times fainter, than sunlight.^[11] As a result, the long-exposure photography is required to achieve a decent result of the observation. The full moon conditions are pretty much the same as considered for the daylight. This is only one significant natural light source beyond the Sun, which can seriously impact the scene's visibility. All other celestial bodies shine too weak for improving a distant scene visibility at night unless we consider an excellent dark-sky site combined with advanced long-exposure photography techniques. Besides, the moonlight doesn't appear all the time, as our natural satellite moves around the Earth. The illumination conditions shaped by the presence of the Moon change daily and repeat every lunar month, so its influence on the conditions of the long-distance observations at night is not always visible.^[12] Specifically unfavorable conditions occur when Moon shines lower above the horizon at twilight on the other side of the sky, where the Sunset or is about to rise. The forward scattering makes distant objects in an antisolar direction (inside the Earth's shadow) more difficult to spot. A combination of shaded Earth's atmosphere with relatively strong moonlight flattens the contrast between the sky and distant features. In practice the just-noticeable difference falls closer reducing the visual range towards this direction.

Seasonal variations of the sunrise and sunset azimuth[]

Because of the annual variations of Earth's axial tilt the range of sunrise/sunset azimuth changes accordingly. Basically, its changes occur daily with exception of around-solstice periods when are barely noticeable. The quickest change of these azimuths falls roughly at the Equinox.

Sunset above the High Tatra Mts saw from Łęki Strzyżowskie in Poland. See also the green rim at the top of the solar limb (bottom image).

These seasonal changes of solar azimuth come along with shifts of the twilight glow azimuth either. By rough knowledge of the solar azimuth on the given day, we are usually able to capture a distant mountain emerging on its disk. It's beneficial especially during the hazy day when the captured object is not visible.^[12] It happens only rarely when the Sun is completely blocked by haze. This situation is mostly identified with misty conditions or smog. On a clear day, the solar disk visible at the horizon is much brighter than the surrounding sky, if the observed object is too small (i.e. phone transmitter) some filters or short exposures with narrow aperture can be essential. The yearly changes in the twilight azimuth determine the contrast enhancement between the certain part of our horizon and the sky is still illuminated by the Sun. Considering the northern hemisphere after sunset, the wintertime will be supportive for objects visible at the south-western and western horizon, whereas around the summer solstice the north-western horizon will be the best or even the northern at the latitudes, where nautical white nights occur.

Changes the range of azimuth at moonrise and moonset[]

Analogically to the Sun, also Moon can rise or set beyond some distant objects. The major difference is in the brightness,^[13] which plays important role in terms of the thick Earth's atmosphere at the horizon line. When the atmosphere is not clear enough, moonlight can't break through it making Moon invisible yet before the set. Other important feature of the Moon is its long-term movement across the sky. Every 18,9 years, due to the lunar precession it comes into the major lunar standstill period, which is analoguous to the solar solstice. Because the Orbit of the Moon has 5,15° inclination on average it translates into more various azimuths of the rise and set. During major lunar standstill the range of these azimuths is about 10,3° wider than solar ones as it reaches declination of ± 28,6°.^[14]

Jupiter about to set above the Tatra Mountains, whereas the lunar twilight actually has begun. An observer can see a high-level cloud deck still illuminated by Moon, which improves the visibility of these mountains located about 130km ahead (credits: Michał Skiba).

In practice, the moonrise or moonset can happen above objects located far south or north against the extremal azimuth range observed for the sunrise and sunset. Another thing, which plays a minor role in the facilitation of long-distance observations during the night time is the , which can be observed mostly on high-level clouds located ahead of the distant object.^[15] Additionally, the Earth's atmosphere behaves likewise under solar twilight conditions, being the brightest roughly above Moon plunged under the horizon. Its impact on the nocturnal distant objects visibility wasn't confirmed yet.

The appearance of the Owl Creek Mountains before and during the 2017 total solar eclipse. Visibility was significantly improved when the whole line of sight fell within in the lunar shadow.^[16]

Rare phenomena[]

There is a group of celestial events, which can ease watching of the remote objects, but occur rarely or even extremely rarely. They are restricted with timing or space:

.

Noctilucent clouds - the event confined to the early summer period only, which because of its brightness can produce serious contrast difference between the distant object plunged in the darkness. It might happen just only under clear air mass conditions, which reduces the atmospheric extinction. A combination of noctilucent clouds appearance combined with unusually clear weather stresses the rarity of observation such as this.

Meteorological[]

Haze concentration[]

The level of haze in the atmosphere, which primarily visible aftermath is the color of the sky. As research shows, the blueness of the sky can vary significantly depending on the density of aerosols. In the clearest conditions, which apply mainly for the "free atmosphere" above the inversion or planetary boundary layer the sky has the deep blue color, unlike inside hazy layer, where it acts like pale blue or even bluish-white.^[19] The dirty coloration of the sky arises out of the attenuation of light traveling through the part of the atmosphere full with various aerosols and applies also to the clouds (especially their shaded bases), which watched from the distance look fainter than these observed above our heads and it's described by Beer-Lambert law.^[20] Important is also the shape and orientation of the particles present in the terrestrial atmosphere and their lifetime counted from coagulation to sedimentation,^[21] which varies between minimum 1 hour in the planetary boundary layer to about 1 week in the "free troposphere".^[22] It at some point defines the pace of visibility changes throughout the day following i.e. variations of humidity level. The relative humidity determines strongly the shape of aerosol particles, which eventually impacts their scattering properties. For instance, in a humid environment, the light scattering is more effective, because the aerosol particles have regular shapes.
In arid conditions, the shape of aerosols is set by wind, which keeps them suspended for a long time.^[23] The level of relative humidity determines the ability of suspended aerosols to absorb the water droplets in the atmosphere, which leads to their coagulation and condensation. In turn, rapid changes in the level of scattered light are observed deteriorating significantly the visual range.^[24] This visual range deterioration is expressed by significant loss of contrast in the distant feature due to the effect of light scattering through the haze particles. After twilight, the haze layer observed clearly especially within the Planetary boundary layer leads to enhanced extinction of the light scattered by the atmosphere illuminated by the Sun. As a consequence the contrast between the distant object silhouette and the twilight sky beyond is reduced.

Optical[]

Scattering of light[]

The visibility impairment, caused by dense haze or water droplet (atmo.arizona.edu).

Scattering of light plays important role in the visibility of distant features. Everything depends on three major factors, which are the presence of the major source of light, the degree of atmosphere clarity between the observer and distant feature, and the local pattern of the light scattering which depends on the light reflection from some objects or clouds. Regardless of the degree of aerosol pollution in the atmosphere, we always list two major types of light scattering:
- Forward scattering - typical for angular distance from the major source of light smaller than 90°,
- - occurring at an angular distance higher than 90° from the major source of light.
In daylight conditions, the distant objects located at the antisolar direction are better visible, because the backward scattering doesn't reduce the visibility as strong as forward scattering.

Visibility conditions under an overcast sky between an observer and distant feature are the best because of the lowest level of light scattering by aerosols and air molecules.

Quite opposite situation occurs at twilight when twilight wedge^[25] becomes visible. At this moment the distant objects located in opposition to the solar azimuth are less visible due to vanishing contrast between the sky and ground, which loses its luminance quickly. The effect of light scattering depends on the size of the particles, whereas the weakest is typical for near-Rayleigh conditions and the strongest for dese haze particles suspended in the atmosphere. The substantial presence of aerosols in the Earth's atmosphere, especially within the Planetary boundary layer degrades the scene significantly. This phenomenon tends to produce a distinctive gray hue, which affects atmospheric transparency.^[26] Light from the atmosphere and light reflected from an object is absorbed and scattered by aerosol particles leading to significant deterioration of visibility.
This regularity applies to the clear day when the sky is free of clouds. It happens very often, that cloudiness occurs. Clouds block the direct sunlight decreasing the light scattering at once. Thus the visual range is extended. The presence of clouds results in nonuniform solar illumination across the line of sight and inhomogeneous irradiance of the atmosphere at once. Thick clouds determine a perfect light diffusion, which is next radiated uniformly in all directions.^[27] Considering the viewing line between the observer and watched the distant object, the illuminated aerosols directly by Sun scatter light more efficiently on the contrary to shaded aerosols. For the observers, the best situation occurs when the cloud cover stretches between their observation place and remote objects. However, the sky beyond these objects remains clear and bright. In turn, the contrast between the shaded distant feature and the bright sky beyond is the best, giving the highest chance to see this object. On the other hand, is fairly not possible to detect any details of object texture, as it remains completely shaded.

Landscape (object) features[]

An example of snow-capped mountains visible from about 200km distance under clear atmospheric conditions (credits: Shukaj Vitalij)

Every landscape feature has its own color, texture, form, and brightness.^[6] The easiest feature to recognize from the distance is definitely the form. Mountains act as domes, beacons, buttes, or steep objects (triangles, protrusions, etc). The second element, which can help with recognizing the distant object is its color and texture. In the case of mountains, especially not forested we can see their structure changes annually (summer-winter) by the presence of snow coverage. The color pattern of the landscape comes along with its brightness. Brightness makes the object more or less visible against the background. When the object's surface is brighter the light is reflected more effectively. It makes the reflected beam from this object stronger and more capable to reach the distant observer. The object illuminance influences on the scattering coefficient. Additionally, we can take into account also the anthropogenic features of the object, which come down to the artificial light emission or reflection. Some skyscrapers can perfectly reflect the sunlight, producing glitters visible far away. On the other hand, if an object shines during the night, can be also visible much further than normally would be.

Blueness of a distant horizon[]

The blueness of the horizon visible on the Monument Valley example. The distance from the feature was 1,5 and 15km, but the middle image shows the enhanced blueness of the horizon as seen towards solar direction due to forward scattering.

The blueness of the horizon causes the smallish part of the air-tiny hydrocarbon particles released by vegetation, which next chemically react with ozone molecules.^[28] At the outcome, the blue light is scattered selectively giving remote features a blue appearance. Another reason behind it is the Diffuse sky radiation scattering mostly short wavelengths, which gives the air molecules a bluish appearance unless we are watching something towards the rising or setting Sun. Sometimes, when the aerosol density is high, the blueness appearance might come out from the nitrogen dioxide gas concentration.

Light reflection at angle of incidence[]

The light reflection appearance can slightly push back the visibility threshold of some distant feature. Counterintuitively to the forward light scattering, which significantly deteriorates the vista towards the incident source of light, the light beams, which come to the observer by specular-looking reflection can significantly emphasize the contrast between these two types of surface. One surface, in this case, is the light reflector, which can be a water body or thick haze layer and another one is an object with individual surface features and low albedo.

The role of contrast triangle in long-distance observations: the difference in illumination produces contrast between the water, the sky just above the horizon, and the distant object. By this little contrast enhancement between the distant object and the sea surface, making it slightly better visible for an observer.

The optical features of the distant object observed differ completely from not only the surface, which tends to reflect the light but also from the horizon background beyond, being affected by forwarding scattering.^[29][30] The important element here is the plane of incidence, which is the angle between a ray incident on the surface and a perpendicular line to the surface at the point of incidence. In a practical sense, the angle of incidence is always equal to the angle of reflection. When the light source is low above the horizon, the light beam can be almost parallel to the surface producing the grazing incidence. Concluding, the observer can see the reflected beam on the surface exactly at the plane of incidence. This plane of incidence determines the Sun glitter appearance, which exact pattern is determined by the precise location of the watcher. It is composed of a multitude of suns stating as the little mirror, as a perfectly smooth surface can contain one glint. These glints are rather elliptical with an aspect ratio depends on the observer's altitude.^[31] Along with the low position of the bright source of light above the horizon comes the light reflectance value, which gradually increases as the source of light is lower above the horizon. Since latitude plays a role in the elevation of the Sun above the horizon, light penetration is always less at higher altitudes.^[32] The light reflection on the water body has a diffuse character, which means, that the angle of incidence does not appear as a straight line. Its border is usually very fuzzy with gentle reduction of single glitters when moving away from the major line. At the end of every line of light reflection, an observer can spot sudden darkening of the horizon called the , which can push the visibility threshold a bit beyond.
The light reflection at the angle of incidence applies also to various other sources of light not only direct but also scattered like by clouds or the sky. Therefore they appear mirror-like on the smooth water. The role of scattered light reflected on the waterbody is significant especially during twilight at solar azimuth, which builds up a big contrast between the illuminated sky and shaded distant landscape feature. The analog phenomenon, but usually short-lived is the . Glint is only the moment when the light is reflected, but it can be seen far away due to the light strength, even from about 200 km distance.^[33]Sun glitter seen from 179km distance.

The visibility of distant features above the haze or cloud deck can be enhanced by contrast difference caused by the light reflected from the haze, from the sky just above horizon beyond and the shaded feature itself.

An analog phenomenon applies to clouds or haze. The common denominator here is just the difference between the medium on which surface the light is reflected or scattered. The describing effect might occur everywhere where the light reflection or scattering near the angle of incidence occurs at a denser medium. This denser medium can be the haze trapped inside the inversion layer, which remains somewhat the plain water surface. Because of the different physical states of the air body including haze, the way of light distribution varies significantly. The haze layer causes a much wider angle of reflection because the solar beam is cloven on small particles. The same situation applies to cloud deck marking the top of inversion layer. The cloud deck marks the area, where the dew point is reached, which ramps up the light reflection considerably. Observers located above the haze or cloud layer can effectively see the 3 major levels of brightness: from the inversion layer (clouds or haze), from the object, and also from the illuminated atmosphere beyond. These circumstances can be altered by snow coverage, which changes significantly the albedo of the distant feature surface unless it's forested. The high albedo of the distant object being just underneath the Sun flattens the contrast, making it less visible for an observer. On the other hand, the mountains located at the solar azimuth in the wintertime, when the Sun is low, usually shade themselves. Therefore the albedo plays a minor role here.

Geometric[]

Essential tools[]

Planning the long-distance observations often requires studying the destination area. The observer obviously can see distant objects on-site, although without decent tools is unable to identify them properly. The traditional tourist map might be not enough for this purpose, especially because of their primary objective. We have obviously a wide choice of maps for hiking tourism, which contains a reach set of names of peaks, passes and valleys ^[34] and detailed representation of the relief, which should result in a good orientation in hard terrain.^[35] A vast majority of these maps is large-scaled, which is impractical for identifying remote objects, as their locations are far outside of the tourist map. For proper recognition of these far-off silhouettes, an observer needs at least a few maps such as this. Moreover, the process of manual object identification is usually time-consuming and impossible on-site without advanced topography knowledge acquired before.

With the growth of the Internet, this method is not used anymore or used occasionally for smaller areas or for mountain guide course purposes. In exchange for it, an observer can do a relevant investigation yet at home, before setting off on the destination site by using at least a few tools available on the market.

Viewfinderpanoramas.org[]

The ^[36] is the oldest known platform dealing with the long-distance lines of sight worldwide, created by Jonathan de Ferranti in 2006. The major feature of this website is a downloadable base of various summit panoramas worldwide.^[37]

Heywhatsthat.com[]

Another old tool dedicated to planning the distant views capture. Founded by Michael Kosowsky in 2007. It offers worldwide flexibility with panorama generation and its further download as the visibility cloak. The visibility cloak feature shows roughly the area, from where a given mountain can be visible. In turn, a user can make relatively quickly a complex Viewshed analyses for a random place in the world.

The example of multi summit visibility cloak rendered from Heywhatsthatcom with further processing and final display in Google Earth. Each color represents the viewshed of the distant mountain possible to see from this location.

Obviously, it's based on the STRM data, which includes pure relief only. The main attitude of this tool is a possibility to transfer the generated data both to Google Earth^[38] as well as the Stellarium v0.20 or higher. The KML panoramas produced by this website can be also used in terms of the multi-summit techniques,^[39] giving a possibility to analyze a few visibility cloaks from one place.

The Heywhatsthat website gives also us a chance to analyze our viewshed by applying the terrestrial refraction values. This website is not dedicated only to the far line of sight analysis. It's also a perfect tool for the sea level rise ^[40] analysis or the solar eclipse and lunar eclipse simulations.

Urlich Deuschle panorama generator[]

This tool appears to be the best on the market because it allows rendering a real view estimation from the given place. The mechanism is analogous to the heywhatsthat.com, as it uses the STRM data. Instead of a visibility cloak, we are getting a view towards the distant area determined by the range of azimuth and enlargement.^[41] Moreover, the tool identifies the distances to all the visible features instantly pointing the maximum distance from our angle of view defined in the azimuth range. This panorama generator perfectly supersedes its predecessor, the Kashmir 3D software,^[42] where loading terrain data for the given area was required.^[43] Apart from the standard use, the Urlich Deuschle panorama generator enables users to tweak the viewing simulations by adding the refraction coefficient or imitating the real-outdoor panoramic views by changing the rendering color scale.^[44]

Peakfinder[]

The Peakfinder is a modern panorama simulator founded by Fabio Soldati.^[45] Its mechanism is very similar to the Urlich Deuschle generator, although we have a restricted zoom level.

The polygonal horizon line in the Stellarium 0.20.2 open-source planetarium rendered from the horizon computed by Heywhatsthat.com.

On the other hand, the mountain peak base is much better developed because it's based on the OpenStreetMap database. The major features of this portal are solar and lunar ephemeris, which are very helpful in planning to see the distant landscape features a front of the solar or lunar disk.^[46]

Others[]

• Stellarium 0.20 and higher - this Astro software has an option of modelling your own horizon for observation purposes. The customizing landscapes option is broader from version 0.20 onwards,^[47] where a user can create a polygonal type of horizon, derivative from the Cartes du Ciel open-source planetarium program. By the Horizone application ^[48] a user can easily grab a computed horizon from the Heywhatsthat.com for any place in the world.^[49] It can be useful for tracking some planet sets above distant features.

Human perception[]

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Weather predictions[]

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Equipment[]

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Longest distance observations by country[]

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World records[]

Currently, World records of the most distant landscape photography can be divided by:

Other lines of sight:

The longest line of sight in the British Isles is from Snowdon to Merrick - 232 km. It has been photographed by Kris Williams in 2015.^[52]

The longest line of sight that has been photographed within the USA is Denali from Mount Sanford at 370 km distance.^[53]

Other long distance photographs include:

• Puig D'en Galileu in Serra de Tramuntana to Pic de Saloria in Pyrenees - 324  km - Marcos Molina.^[54]
• Wellington, New Zealand to Mount Taranaki - 330  km - Naked eye line of sight.^[55]

References[]

External links[]

• Longest line of sight in the UK - discussion
• The furthest you can see on a clear day - discussion