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An important relationship between an object's astronomical (geometric) albedo, absolute magnitude and diameter is given by: |
where is the astronomical albedo, is the diameter in kilometers, and is the absolute magnitude. |
Radar Albedo |
In planetary radar astronomy, a microwave (or radar) pulse is transmitted toward a planetary target (e.g. Moon, asteroid, etc.) and the echo from the target is measured. In most instances, the transmitted pulse is circularly polarized and the received pulse is measured in the same sense of polarization as the transmitt... |
Those components of the received echo that return from first-surface reflections (as from a smooth or mirror-like surface) are dominated by the OC component as there is a reversal in polarization upon reflection. If the surface is rough at the wavelength scale or there is significant penetration into the regolith, ther... |
For most objects in the solar system, the OC echo dominates and the most commonly reported radar albedo parameter is the (normalized) OC radar albedo (often shortened to radar albedo): |
where the denominator is the effective cross-sectional area of the target object with mean radius, . A smooth metallic sphere would have . |
Radar Albedos of Solar System Objects |
The values reported for the Moon, Mercury, Mars, Venus, and Comet P/2005 JQ5 are derived from the total (OC+SC) radar albedo reported in those references. |
Relationship to Surface Bulk Density |
In the event that most of the echo is from first surface reflections ( or so), the OC radar albedo is a first-order approximation of the Fresnel reflection coefficient (aka reflectivity) and can be used to estimate the bulk density of a planetary surface to a depth of a meter or so (a few wavelengths of the radar wavel... |
. |
See also |
Cool roof |
Daisyworld |
Emissivity |
Exitance |
Global dimming |
Irradiance |
Kirchhoff's law of thermal radiation |
Opposition surge |
Polar see-saw |
Radar astronomy |
Solar radiation management |
References |
External links |
Albedo Project |
Albedo – Encyclopedia of Earth |
NASA MODIS BRDF/albedo product site |
Ocean surface albedo look-up-table |
Surface albedo derived from Meteosat observations |
A discussion of Lunar albedos |
reflectivity of metals (chart) |
Land surface effects on climate |
Climate change feedbacks |
Climate forcing |
Climatology |
Electromagnetic radiation |
Radiometry |
Scattering, absorption and radiative transfer (optics) |
Radiation |
1760s neologisms A, or a, is the first letter and the first vowel of the modern English alphabet and the ISO basic Latin alphabet. Its name in English is a (pronounced ), plural aes. It is similar in shape to the Ancient Greek letter alpha, from which it derives. The uppercase version consists of the two slanting sides... |
In the English grammar, "a", and its variant "an", are indefinite articles. |
History |
The earliest certain ancestor of "A" is aleph (also written 'aleph), the first letter of the Phoenician alphabet, which consisted entirely of consonants (for that reason, it is also called an abjad to distinguish it from a true alphabet). In turn, the ancestor of aleph may have been a pictogram of an ox head in proto-S... |
When the ancient Greeks adopted the alphabet, they had no use for a letter to represent the glottal stop—the consonant sound that the letter denoted in Phoenician and other Semitic languages, and that was the first phoneme of the Phoenician pronunciation of the letter—so they used their version of the sign to represent... |
The Etruscans brought the Greek alphabet to their civilization in the Italian Peninsula and left the letter unchanged. The Romans later adopted the Etruscan alphabet to write the Latin language, and the resulting letter was preserved in the Latin alphabet that would come to be used to write many languages, including En... |
Typographic variants |
During Roman times, there were many variant forms of the letter "A". First was the monumental or lapidary style, which was used when inscribing on stone or other "permanent" media. There was also a cursive style used for everyday or utilitarian writing, which was done on more perishable surfaces. Due to the "perishable... |
At the end of the Roman Empire (5th century AD), several variants of the cursive minuscule developed through Western Europe. Among these were the semicursive minuscule of Italy, the Merovingian script in France, the Visigothic script in Spain, and the Insular or Anglo-Irish semi-uncial or Anglo-Saxon majuscule of Great... |
15th-century Italy saw the formation of the two main variants that are known today. These variants, the Italic and Roman forms, were derived from the Caroline Script version. The Italic form, also called script a, is used in most current handwriting; it consists of a circle and vertical stroke on the right ("ɑ"). This ... |
Italic type is commonly used to mark emphasis or more generally to distinguish one part of a text from the rest (set in Roman type). There are some other cases aside from italic type where script a ("ɑ"), also called Latin alpha, is used in contrast with Latin "a" (such as in the International Phonetic Alphabet). |
Use in writing systems |
English |
In modern English orthography, the letter represents at least seven different vowel sounds: |
the near-open front unrounded vowel as in pad; |
the open back unrounded vowel as in father, which is closer to its original Latin and Greek sound; |
the diphthong as in ace and major (usually when is followed by one, or occasionally two, consonants and then another vowel letter) – this results from Middle English lengthening followed by the Great Vowel Shift; |
the modified form of the above sound that occurs before , as in square and Mary; |
the rounded vowel of water; |
the shorter rounded vowel (not present in General American) in was and what; |
a schwa, in many unstressed syllables, as in about, comma, solar. |
The double sequence does not occur in native English words, but is found in some words derived from foreign languages such as Aaron and aardvark. However, occurs in many common digraphs, all with their own sound or sounds, particularly , , , , and . |
is the third-most-commonly used letter in English (after and ) and French, the second most common in Spanish, and the most common in Portuguese. About 8.167% of letters used in English texts tend to be ; the number is around 7.636% in French, 11.525% in Spanish, and 14.634% for Portuguese. |
Other languages |
In most languages that use the Latin alphabet, denotes an open unrounded vowel, such as , , or . An exception is Saanich, in which (and the glyph Á) stands for a close-mid front unrounded vowel . |
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