Long-distance observation is any visual observation, for sightseeing or photography, that targets any object visible from a great distance, with the possibility of seeing them closely. Such observations exclude the following:
Types of long distance observations Concerning the position of an observer and observed distant object, we can divide long-distance observations by the following types:
The primary criterion is the integration of an observer with the Earth's surface or the object, which is firmly integrated with the ground.
Ground-to-ground The ground-based long-distance observations cover 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. The natural are:
The artificial ones are:
Ground to air The observer situated on the ground is able to observe some distant objects visible within the lower atmosphere, which can be i.e. distant plane passing just above the local horizon.
Aerial This type of long-distance observation refers to a situation, where an observer or photography device isn't integrated with the ground at the moment of observation. In this case, the observation can be made from:
The aerial observations can also include other objects, which are already in the Earth's atmosphere.
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, which makes them hard to notice and photograph 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 the range because they are usually higher than the surrounding area. The telecommunications transmitters are often inherent elements of the mountains, making them easily 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 it 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 a vast plain, lowland, or large water body separates the remote massive chains. The circumstances that are the most favorable for seeing and capturing objects from the biggest possible distance, the best example is the current world's record established in South America . 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 a similar or bigger result can be achieved.
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. On the other hand, the Sun travels across the sky changing its position against the observed object. It also reflects changes in 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 toward 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.Malm W.C., 2016, Visibility: The seeing of near and distant landscape features, Elsevier Inc., New York
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.Horvath H., 1967, Atmospheric visibility, (in:) Atmospheric Environment, vol. 15, i. 10-11, pp. 1785–1796.
Presence of moonlight
The moonlight plays a role analogous to the sunlight, but it is about 500,000 times fainter.Kyba C.C.M., Mohar A., Posh T., 2017, How bright is moonlight? (in:) Astronomy & Geophysics, vol. 58, i. 1, pp. 31–32. As a result, long-exposure photography is required to achieve a decent observation. Full moon conditions are pretty much the same as considered for the daylight. The Moon is the only significant natural light source beyond the Sun, which can seriously impact the scene's visibility. All other celestial light sources are too weak to improve visibility at night, except perhaps at an excellent dark-sky site combined with advanced long-exposure photography techniques. Besides, since the Moon orbits the Earth on a lunar monthly cycle, moonlight is only available intermittently, and varies from one day to the next.Krukar M., (2020), „Oszukać atmosferę”, (in:) Geografia w Szkole 2/2020 | (Polish) Specifically, unfavorable conditions occur when Moon shines lower above the horizon at twilight on the other side of the sky, where the sun is setting 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 the exception of around-solstice periods when are barely noticeable. The quickest change of these azimuths falls roughly at the Equinox. These seasonal changes of solar azimuth come along with shifts of the twilight glow azimuth either. By rough knowledge of the solar azimuth on a given day, we are usually able to capture a distant mountain emerging on its disk. It's beneficial, especially during a hazy day when the captured object is not visible. 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 a certain part of our horizon and the sky that 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
Analogously to the Sun, the Moon can rise or set beyond some distant objects on Earth. The major difference is in the brightness,Calder, W. A. & Shapley, H., 1937, A photoelectric comparison of the brightness of the Sun, Moon, Capella, Vega, and Deneb, (in:) Annals of the Astronomical Observatory of Harvard College; v. 105, no. 22, Cambridge, Mass. : The Observatory, 1937., pp. 445–452. which plays an important role in terms of the thick atmosphere of the Earth at the horizon line. When the atmosphere is not clear enough, moonlight cannot break through it, making the Moon invisible even before astronomical sunset. Furthermore, every 18.9 years, due to the lunar precession, a lunar standstill period occurs. This is to some extent analogous to the solar solstice. Because the Moon's orbit has 5.15° inclination on average, it translates into more various azimuths of the rise and set. During a major lunar standstill the range of these azimuths is about 10.3° wider than solar ones as it reaches a declination of ± 28.6°. 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 feature that slightly affects long-distance observations at night is the lunar twilight, which can be observed mostly on high-level clouds located ahead of the distant object.
Rare phenomena This is a group of celestial events which can ease watching of the remote objects, but occur rarely or even extremely rarely. They are restricted in timing or space: Total solar eclipses – allows extension of the visual range extension, making some far-off mountains visible even in hazy conditions.Vollmer M., Shaw J.A., 2018, Extended visual range during solar eclipses, (in:) Applied Optics, vol. 57, no. 12, p. 3250–3259. However, this extension is restricted to the umbral edges only. In practice, this means that an observer cannot see objects at a longer distance than the diameter of the lunar shadow. The solar eclipse configuration is also important: when it occurs near sunrise or sunset or even below the horizon, the lunar shadow is extremely elongated from one side of the sky to the other. The section of the sky that is in shadow automatically reduces the contrast with the far-off horizon, making it hard to distinguish.
Meteors – these have a very short duration, but sometimes produce brighter light than moonlight. As they are so rare, there are no confirmed distant observations. Rocket contrails – also very rare. If an observer is far enough from the launch site, this type of contrail can be seen just above the horizon far earlier before the astronomical dawn, and locally improve the contrast of a dark distant object. Planetary ephemerides – very rarely, a planet such as Jupiter or Venus will rise or set above some prominent distant object or construction. This type of observation is extremely difficult because it needs to be photographed with a combination of large zoom with long exposure. Noctilucent clouds – this only occurs in early summer. Because of its brightness, it can produce a large contrast difference between the distant objects plunged into the darkness. It might only occur under clear air mass conditions, which reduces atmospheric extinction. The rarity of such events is due to the required combination of noctilucent clouds and unusually clear weather.
Meteorological
Haze concentration