Long distance observations

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. 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.

Presence of moonlight The moonlight plays a role analogous to the sunlight, but it is about 500,000 times fainter. 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. 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.

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