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Opposition Surge
The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer. The term is most widely used in astronomy, where generally it refers to the sudden noticeable increase in the brightness of a celestial body such as a planet, moon, or comet as its phase angle of observation approaches zero. It is so named because the reflected light from the Moon and Mars appear significantly brighter than predicted by simple Lambertian reflectance when at astronomical opposition. Two physical mechanisms have been proposed for this observational phenomenon: shadow hiding and coherent backscatter. Overview The phase angle is defined as the angle between the observer, the observed object and the source of light. In the case of the Solar System, the light source is the Sun, and the observer is generally on Earth. At zero phase angle, the S ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Armstrong Heiligenschein
Armstrong may refer to: Places * Armstrong Creek (other), various places * Armstrong River (other), various rivers Antarctica * Armstrong Reef, Biscoe Islands Argentina * Armstrong, Santa Fe Australia * Armstrong, Victoria Canada * Armstrong, British Columbia * Armstrong, Ontario * Armstrong, Thunder Bay District, Ontario * Armstrong, Ontario (Indian settlement) United States * Armstrong, California * Armstrong, Delaware * Armstrong, Florida * Armstrong, Georgia * Armstrong, Illinois * Armstrong, Indiana * Armstrong, Iowa * Armstrong, Minnesota * Armstrong, Missouri * Armstrong, Oklahoma * Armstrong, Texas * Armstrong, Wisconsin * Armstrong County, Pennsylvania * Armstrong County, Texas * Armstrong Lake (Blue Earth County, Minnesota), a lake in Minnesota * Armstrong Township, Vanderburgh County, Indiana * Armstrong Township, Pennsylvania (other), more than one, including ** Armstrong Township, Indiana County, Pennsylvania ** Armstrong ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Illuminance
In photometry (optics), photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception.International Electrotechnical Commission (IEC): ''International Electrotechnical Vocabulary.'ref. 845-21-060, illuminance/ref> Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance. In SI derived units, SI units illuminance is measured in lux (lx), or equivalently in lumen (unit), lumens per square metre (Lumen (unit), lm·meter, m−2). Luminous exitance is measured in lm·m−2 only, not lux. International Electrotechnical Commission (IEC): ''International Electrotechnical Vocabulary.'ref. 845-21-081, luminous exitance/ref> In the CGS system, the unit of illuminance is the phot, which is equal to . The foot-candle is ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Tom Gehrels
Anton M.J. "Tom" Gehrels (February 21, 1925 – July 11, 2011) was a Dutch–American astronomer, Professor of Planetary Sciences, and Astronomer at the University of Arizona, Tucson. Biography Youth and education Gehrels was born at Haarlemmermeer, the Netherlands on February 21, 1925. He was born in bible-belt Netherlands, and was forced to attend church regularly, an act he despised. When he was older he rejoiced when he found out his childhood church had been destroyed. During World War II he was, as a teenager, active in the Dutch Resistance. After he escaped to England, he was sent back by parachute as an organizer for Special Operations Executive SOE committing sabotage against the German forces. After the war, he attended the University of Leiden where he graduated with a degree in physics and astronomy in 1951. He continued his education at the University of Chicago where he obtained his doctorate in astronomy and astrophysics in 1956 under Professor Gerard P. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Glory (optical Phenomenon)
A glory is an optical phenomenon, resembling an iconic saint's halo (religious iconography), halo around the shadow of the observer's head, caused by sunlight or (more rarely) moonlight interacting with the tiny water drop (liquid), droplets that comprise mist or clouds. The glory consists of one or more concentric, successively dimmer rings, each of which is red on the outside and bluish towards the centre. Due to its appearance, the phenomenon is sometimes mistaken for a Rainbow#Full-circle rainbow, circular rainbow, but the latter has a much larger diameter and is caused by different physical processes. Glories arise due to interference (wave propagation), wave interference of light total internal reflection, internally refracted within small droplets. Appearance and observation Depending on circumstances (such as the uniformity of droplet size in the clouds), one or more of the glory's rings can be visible. The rings are rarely complete, being interrupted by the shadow of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Heiligenschein
(; ) is an optical phenomenon in which a bright spot appears around the shadow of the viewer's head in the presence of dew. In photogrammetry and remote sensing, it is more commonly known as the hotspot. It is also occasionally known as Cellini's halo after the Italian artist and writer Benvenuto Cellini (15001571), who described the phenomenon in his memoirs in 1562. Nearly spherical dew droplets act as lens (optics), lenses to focus the light onto the surface behind them. When this light light scattering, scatters or reflects off that surface, the same lens re-focuses that light into the direction from which it came. This configuration is similar to a cat's eye retroreflector. However a cat's eye retroreflector needs a refractive index of around 2, while water has a much smaller refractive index of approximately 1.33. This means that the water droplets focus the light about 20% to 50% of the diameter beyond the rear surface of the droplet. When dew droplets are suspended on tr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Antisolar Point
The antisolar point is the abstract point on the celestial sphere directly opposite the Sun from an observer's perspective. This means that the antisolar point lies above the horizon when the Sun is below it, and vice versa. On a sunny day, the antisolar point can be easily found; it is located within the shadow of the observer's head. Like the zenith and nadir, the antisolar point is not fixed in three-dimensional space, but is defined relative to the observer. Each observer has an antisolar point that moves as the observer changes position. The antisolar point forms the geometric center of several optical phenomena, including subhorizon haloes, rainbows, glories, the Brocken spectre, and heiligenschein. Occasionally, around sunset or sunrise, anticrepuscular rays appear to converge toward the antisolar point near the horizon. However, this is an optical illusion caused by perspective; in reality, the "rays" (i.e. bands of shadow) run near-parallel to each other. Also ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cassini–Huygens
''Cassini–Huygens'' ( ), commonly called ''Cassini'', was a space research, space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, including its Rings of Saturn, rings and Moons of Saturn, natural satellites. The Large Strategic Science Missions, Flagship-class robotic spacecraft comprised both NASA's ''Cassini'' space probe and ESA's Huygens (spacecraft), ''Huygens'' lander (spacecraft), lander, which landed on Saturn's largest moon, Titan (moon), Titan. ''Cassini'' was the fourth space probe to visit Saturn and the first to enter its orbit, where it stayed from 2004 to 2017. The two craft took their names from the astronomers Giovanni Cassini and Christiaan Huygens. Launched aboard a Titan IV, Titan IVB/Centaur on October 15, 1997, ''Cassini'' was active in space for nearly 20 years, spending its final 13 years orbiting Saturn and studying the planet and its system a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radar
Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track aircraft, ships, spacecraft, guided missiles, motor vehicles, map weather formations, and terrain. The term ''RADAR'' was coined in 1940 by the United States Navy as an acronym for "radio detection and ranging". The term ''radar'' has since entered English and other languages as an anacronym, a common noun, losing all capitalization. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwave domain, a transmitting antenna, a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the objects. Radio waves (pulsed or continuous) from the transmitter reflect off the objects and return to the receiver, giving ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hugo Von Seeliger
Hugo von Seeliger (23 September 1849 – 2 December 1924), also known as Hugo Hans Ritter von Seeliger, was a German astronomer, often considered the most important astronomer of his day. Biography He was born in Biala, completed high school in Teschen in 1867, and studied at the Universities of Heidelberg and Leipzig. He earned a doctorate in astronomy in 1872 from the latter, studying under Carl Christian Bruhns. He was on the staff of the University of Bonn Observatory until 1877, as an assistant to Friedrich Wilhelm Argelander. In 1874, he directed the German expedition to the Auckland Islands to observe the transit of Venus. In 1881, he became the Director of the Gotha Observatory, and in 1882 became a professor of Astronomy and Director of the Observatory at the University of Munich, which post he held until his death. His students included Hans Kienle, Ernst Anding, Julius Bauschinger, Paul ten Bruggencate, Gustav Herglotz, Richard Schorr, and especially Karl Schwarz ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rings Of Saturn
Saturn has the most extensive and complex ring system of any planet in the Solar System. The rings consist of particles in orbit around the planet made almost entirely of water ice, with a trace component of Rock (geology), rocky material. Particles range from micrometers to meters in size. There is no consensus as to what mechanism facilitated their formation: while investigations using theoretical models suggested they formed early in the Solar System's existence, newer data from ''Cassini–Huygens, Cassini'' suggests a more recent date of formation. In September 2023, astronomers reported studies suggesting that the rings of Saturn may have resulted from the collision of two moons "a few hundred million years ago". Though light reflected from the rings increases Saturn's apparent brightness, they are not themselves visible from Earth with the naked eye. In 1610, the year after Galileo Galilei's first observations with a telescope, he became the first person to observe Saturn' ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hapke Parameters
The Hapke parameters are a set of parameters for an empirical model that is commonly used to describe the directional reflectance properties of the airless regolith surfaces of bodies in the Solar System. The model has been developed by astronomer Bruce Hapke at the University of Pittsburgh. The main parameters are: #\bar_0 — Single scattering albedo. This is the ratio of scattering efficiency to total light extinction (which includes also absorption), for small-particle scattering of light. That is, K_s/(K_s+K_a), where K_s is the scattering coefficient, and K_a is the absorption coefficientSingle Scattering Albedo scienceworld.wolfram.com. # — The width of the opposition surge ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Phase Angle (astronomy)
In observational astronomy, phase angle is the angle between the light incident onto an observed object and the light reflected from the object. In the context of astronomical observations, this is usually the angle Sun-object-observer. For terrestrial observations, "Sun–object–Earth" is often nearly the same thing as "Sun–object–observer", since the difference depends on the parallax, which in the case of observations of the Moon can be as much as 1°, or two full Moon diameters. With the development of space travel, as well as in hypothetical observations from other points in space, the notion of phase angle became independent of Sun and Earth. The etymology of the term is related to the notion of planetary phases, since the brightness of an object and its appearance as a "phase" is the function of the phase angle. The phase angle varies from 0° to 180°. The value of 0° corresponds to the position where the illuminator, the observer, and the object are co ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
