Long distance observations

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.

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.

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.[5] 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.[6]

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

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.[7] 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.

A visual range comparison on shifting solar position at the same moments against sunrise and sunset. 1 – view at 15m after sunrise; 2 – view at 15m before sunset. Comparing to the Pic. 3 both vistas appear to look more clear due to vanishing sunlight. Moreover, a local sunset transition is to be noticed in image 2, where: A – shows sunlit haze and B shows shaded haze. View from Royal Sun Hotel near Los Gigantes at the Tenerife. (Deckchair.com)

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 [8] and it's derivative from the light transition phenomenon.[9][10]

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.[11]

A visual range comparison when Sun is 6 deg below the horizon (civil twilight) at the symmetric moments against noon. In this case, the much better view is in the evening, when Gomera and La Palma are located near the solar azimuth. View from Royal Sun Hotel near Los Gigantes at the Tenerife. (Deckchair.com)

It finally extends the visual range significantly, but only towards the solar azimuth on the contrary of daylight conditions. As our sight moves away from the aforementioned solar azimuth, the contrast between the horizon and the sky reduces gradually upon the horizon and Earth's shadow coincidence, where drops abruptly. Finally, on the remained part of the horizon distant objects are not discernible enough. This situation occurs as long as the stratosphere (ozonosphere) is illuminated, which finally lasts upon the end of the nautical twilight.

The moonlight plays an analogous role in the sunlight. However, this light is about 500k times fainter, than sunlight.[12] 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.[13] 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.

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.[14] 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.

Analogically to the Sun, also Moon can rise or set beyond some distant objects. The major difference is in the brightness,[15] 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°.[16]

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 lunar twilight, which can be observed mostly on high-level clouds located ahead of the distant object.[17] 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 a whole line of sight fallen in the lunar shadow [18].

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:

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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 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.[22][23]

The [26] 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.[27]

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 [28] 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,[29] 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 seal level rise [30] analysis or the solar eclipse and lunar eclipse simulations.

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.[31] 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,[32] where loading terrain data for the given area was required.[33]

The Peakfinder is a modern panorama simulator founded by Fabio Soldati.[34] 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.[35]

• 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,[36] where a user can create a polygonal type of horizon, derivative from the Cartes du Ciel open-source planetarium program. By the Horizone application [37] a user can easily grab a computed horizon from the Heywhatsthat.com for any place in the world.[38] It can be useful for tracking some planet sets above distant features.