Astronomy, History of

• Astronomy, History of, history of the science that deals with all the celestial bodies in the universe. Astronomy includes the study of planets and their satellites, comets and meteors, stars and interstellar matter, star systems known as galaxies, and clusters of galaxies. The field of astronomy has developed from simple observations about the movement of the Sun and Moon into sophisticated theories about the nature of the universe.

Astronomical Constants

Constant

Value

Astronomical unit (au)	149,597,870 km
Speed of light in a vacuum (c)	299,792.458 km/sec
Solar parallax	8.794148 arc seconds
Mass of the sun	1.9891 × 1030 kg
Mass of the earth	5.9742 × 1024 kg
Mass of the moon	7.3483 × 1022 kg
Light-year (ly)	9.4605 × 1012 km = 0.30660 pc
Parsec (pc)	30.857 × 1012 km = 3.26161 ly
Obliquity of the eliptic (2000)	23 ° 26 ' 21.448 "
General precession (2000)	50.290966 arc seconds/year
Constant of nutation (2000)	9.2025 arc seconds
Constant of aberration (2000)	20.49552 arc seconds

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Callisto (astronomy)

• Callisto (astronomy), large satellite of the planet Jupiter. Callisto is the eighth known moon in distance from the planet. The moon orbits Jupiter at an average distance of 1,883,000 km (1,170,000 mi). Callisto completes an orbit and rotates in the same amount of time, once every 16.69 Earth days. Its nearly circular orbit parallels Jupiter’s equator.

• Callisto is spherical and is the third-largest moon in the solar system. The moon has a radius of 2,403 km (1,493 mi), making it nearly the same size as the planet Mercury. Since Callisto consists mostly of low-density water ice, however, the moon is only one-third as massive as rocky, metallic Mercury. Callisto’s interior is probably not differentiated into a rocky core surrounded by lighter icy material, like that of the other three large moons of Jupiter—Io, Europa, and Ganymede. Instead, scientists believe that the entire moon is a mixture of rock and ice, with the percentage of rock in the mixture increasing toward the moon’s center.

• Callisto is the most heavily cratered world known. This cratering suggests that the surface has not changed in billions of years and that there is little internal activity affecting it. If Callisto’s internal activity was strong enough, it would melt or disturb the ice near the moon’s surface, and many of the craters would be erased. Instead, Callisto seems to be the most geologically quiet of the four large moons of Jupiter. This is probably because Callisto is the outermost of the large moons. No large outer satellite of Jupiter subjects it to the gravitational tug-of-war with Jupiter that can produce internal heating.
  

• The only sign of change on the surface is the slumping of older crater walls that occurs as the ice that forms them flows downward over extremely long periods of time. Despite Callisto’s apparent stillness, however, in 1998 the Galileo spacecraft revealed signs that the moon may have a liquid or slushy ocean beneath its surface. Callisto may also have a tenuous atmosphere composed mostly of carbon dioxide.

• The largest crater on Callisto is Valhalla, a 300-km (190-mi) basin surrounded by a system of concentric rings 1,500 km (930 mi) wide. In addition, Callisto has 12 known crater chains. Astronomers believe that these chains were caused by comets or asteroids that broke up when they passed too near Jupiter and crashed into Callisto. The longest chain, Gipul Catena, is about 640 km (about 400 mi) long.

• Callisto was discovered independently by Italian astronomer Galileo and German astronomer Simon Marius. Callisto and the three other large moons of Jupiter—all of which were found in 1610—are collectively known as the Galilean moons. Marius named Callisto and the other Galilean moons for mythical lovers of the Greek god Zeus, whom the Romans renamed Jupiter. Callisto was a nymph who was changed into a bear by Zeus’s jealous wife Hera.
  

• In Greek mythology, Callisto is the basis of the constellation Ursa Major. Craters on Callisto are generally named for heroes and heroines of northern mythologies. The United States Voyager space probes and the United States Galileo orbiter have provided a wealth of information about Callisto. When the Voyager probes passed Jupiter in 1979 they provided some detailed photographs. Galileo made its first flyby of Callisto in 1996 and examined the moon several times before its mission ended in 2003.

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The 20 Brightest Stars as Seen from Earth

The 20 Brightest Stars as Seen from Earth

A star's brightness is referred to as its magnitude. "Apparent magnitude" is brightness as seen from Earth. The 20 stars with the highest aparent magnitudes are listed here. "Absolute magnitude" is intrinsic brightness as measured at a standard distance of 32.6 light-years or 10 parsecs from the star. The star with the highest absolute magnitude known in the universe, the Pistol Star, does not appear in this list because it is so far from Earth.

Scientific name

Common name

Distance (light-years)

Alpha Canis Majoris	Sirius	9
Alpha Carinae	Canopus	98
Alpha Centauri	Rigil Kent	4
Alpha Boötis	Arcturus	36
Alpha Lyrae	Vega	26
Alpha Aurigae	Capella	42
Beta Orionis	Rigel	910
Alpha Canis Minoris	Procyon	11
Alpha Eridani	Achernar	85
Alpha Orionis	Betelgeuse	510
Beta Centauri	Hadar	460
Alpha Aquilae	Altair	17
Alpha Tauri	Aldebaran	65
Alpha Crucis	Acrux	360
Alpha Scorpii	Antares	330
Alpha Virginis	Spica	260
Beta Geminorum	Pollux	36
Alpha Piscis Austrini	Fomalhaut	22
Alpha Cygni	Deneb	1,830
Beta Crucis	Mimosa	420
Alpha Leonis	Regulus	85

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Alpha Centauri

• Alpha Centauri, closest star system to Earth and third brightest star in the sky. Alpha Centauri is located in the constellation Centaurus and is sometimes called Rigil Kentaurus, which literally means “foot of the centaur.” Alpha Centauri is actually a triple star system that appears as a single point of light because its two largest and brightest members, Alpha Centauri A and Alpha Centauri B, are too close together for the naked eye to tell them apart and its third member, Alpha Centauri C, is too small and dim to be seen at all.

• The Alpha Centauri system is only visible from the Southern Hemisphere and the southernmost portion of the Northern Hemisphere. In Texas, Louisiana, and Florida, for example, it appears very low in the southern sky and is most easily visible in May.

• Alpha Centauri A and Alpha Centauri B lie about 4.35 light-years from Earth. A light-year is the distance light travels in a year, equal to about 9,460 billion km (5,880 billion mi). The two stars circle their common center of gravity, a point in space between them, about once every 80 years. The average distance between A and B is about 3.6 billion km (2.2 billion mi), which is a bit more than the average distance between the Sun and the planet Uranus.

• Alpha Centauri C orbits A and B at a tremendous distance—about 1,500 billion km (930 billion mi)—so far out that C takes millions of years to circle the two larger stars. Alpha Centauri C is also called Proxima Centauri; in the current portion of its orbit it is theclosest star to the solar system, at a distance of 4.2 light-years.

• Alpha Centauri A is a yellow star, slightly larger and brighter than the Sun, of spectral type G2 and apparent magnitude +0.01. Spectral type indicates a star’s surface temperature and the predominant color of the light it gives off. Apparent magnitude is a measure of how bright stars appear in the sky—a small, nearby star may appear just as bright as a much larger star that is farther away. The lower the apparent magnitude, the brighter the star appears. Alpha Centauri B is a yellow-orange star somewhat smaller and cooler than the Sun of spectral type K1 and apparent magnitude +1.34. Proxima Centauri is a red dwarf star of spectral type M5, much smaller and cooler than the Sun.

Spectral Types of Stars

Astronomers categorize stars according to the the characteristics of the light that the stars emit, which are related to the stars’ temperatures. O stars are the hottest stars, and M stars are the coolest. Our Sun is a G star.

Spectral Class

Effective Temperature Star Color

Principal Characteristics

O	25,000 K Blue star	This stage is characterized by lines of helium, oxygen, and nitrogen in the spectrum of the photosphere. O stars are extremely hot, very bright stars that emit large amounts of ultraviolet radiation.
B	11,000 K - 25,000 K White-blue star	In this group the lines of helium in the spectrum reach a maximum intensity and then fade. The intensity of the hydrogen lines regularly increases in all the subdivisions of stage B. Type B stars are typically represented by the star Epsilon Orionis.
A	7500 K - 11,000 K White star	This group comprises the so-called hydrogen stars. The spectra of these stars is dominated by absorption lines of hydrogen. Sirius, the Dog star, is a typical type A star.
F	6000 K - 7500 K Yellow-white star	This group is composed of stars characterized by an elevated intensity of the H and K lines of calcium and of lines characteristic of hydrogen. A notable star in this category is Delta Aquilae.
G	5000 K - 6000 K Yellow, solar star	This group is composed of stars with prominent H and K calcium lines and less prominent hydrogen lines. The spectra of numerous metals, in particular iron, are also present. The Sun belongs to this group, and therefore G stars are frequently called solar stars.
K	3500 K - 5000 K Orange-yellow star	This group comprises stars having strong calcium lines and lines indicating the presence of other metals in their spectra.The violet light of class K stars is less intense than the stars' red light. This group is typically represented by Arcturus.
M	3500 K Red star	This group is composed of stars whose spectra are dominated by bands resulting from the presence of metallic-oxide molecules, notably those of titanium oxide. The violet end of the spectra is less intense than that of K stars. The star Orionis is typical of this group.

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Aerospace Medicine

• Aerospace Medicine, branch of preventive medicine that is concerned with the physiological and psychological stresses on the human body in flight. The study of effects within the earth’s atmosphere is also called aviation medicine; beyond this atmosphere the study of effects is also called space medicine. Aerospace medicine was recognized as a subspecialty by the American Medical Association in 1953.

AVIATION MEDICINE

• Specialists in aviation medicine study the reactions of humans to the stresses of air travel. They are concerned with the proper screening of candidates for flight training, the maintenance of maximum efficiency among aircrews, and with clinically oriented research into the effects of flight on the body. They also cooperate actively with aeronautical engineers in the development of safe aircraft.

• Aviation medicine is rooted in the early 18th-century physiological studies of balloonists, some of whom were physicians. In 1784, a year after the first balloon flight by the French physicist Jean Pilâtre de Rozier, a Boston physician, John Jeffries, made the first study of upper-air composition from a balloon.

• The first comprehensive studies of health effects in air flight were made by the French physician Paul Bert, who published his research on the effects of altered air pressure and composition on humans in 1878 under the title La pression barometrique. In 1894 the Viennese physiologist Herman Von Schrötter designed an oxygen mask with which the meteorologist Artur Berson set an altitude record of 9150 m (30,000 ft).

• With the advent of the airplane, the first standards for military pilots were established in 1912. Significant work in this area was directed by the physician Theodore Lyster, an American pioneer in aviation medicine. Lyster set up the Aviation Medicine Research Board in 1917, which opened a research laboratory at Hazelhurst Field in Mineola, New York, in January 1918. The School of Flight Surgeons opened the following year, and in 1929 the Aero Medical Association was founded under the direction of Louis H. Bauer. In 1934 facilities were built at Wright Air Field in Dayton, Ohio, to study the effect of high-performance flight on humans.

• Technical advances included the first pressurized suit, designed and worn by the American aviator Wiley Post in 1934, and the first antigravity suit, designed by W. R. Franks in Britain in 1942. In an effort to help design better restraint systems for military jet aircraft, the U.S. flight surgeon John Stapp conducted a series of tests on a rocket-powered sled, culminating on December 10, 1954, when Colonel Stapp underwent deceleration from a velocity of 286 m (937 ft)/sec in 1.4 sec.

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