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FACE ON MARS This photograph of the Cydonia Mense region of Mars was taken by NASA's Mars Global Surveyor in 1998. It is a coincidental alignment of rocks and other geologic formations that happens to look like a human face from this angle.

FACULAE Falculae are bright patches on the sun that are associated with sun spots or any region which is brighter than the surrounding area on a planet or a moon.

FALLING STAR A falling star is not a star at all; it is a meteor (made of rock and/or iron) which is burning up in the Earth's atmosphere.

FAMILY OF CONSTELLATIONS There are many families of constellations, constellations that are either close to one another or have some other relationship. Some constellation families include the Zodiac (the 12 constellations representing the signs of the Zodiac), the Ursa Major Family (10 constellations, including Ursa Major, Ursa Minor Canes Venatici, Bootes, Coma Berenice, Corona Borealis, etc.), the Perseus Family (9 constellations, including Cassiopeia, Cepheus, Andromeda, Perseus, etc.), the Hercules Family (19 constellations, including Hercules, Sagitta, Aquila, etc.), the Orion Family (5 constellations, including Orion, Canis Major, Canis Minor, Monoceros, and Lepus), the Heavenly Waters (9 constellations, including Delphinus, Columba, Equuleus, Vela, Puppis, etc.), the Bayer Group (11 constellations, including Hydrus, Dorado, Volans, Apus,etc.), the La Caille Family (13 constellations, including Norma, Fornax, Circinus, etc.).

FINDERSCOPE or FINDER A finderscope (or finder) is a low-power telescope attached to (and aligned with) a higher-power telescope. The finder is used to locate objects in the sky easily; the higher-power telescope is then used for closer examination.

FIREBALL A fireball is any meteor that is brighter than Venus (magnitude -4). A sonic boom often follows a fireball and they can cast a shadow. They are caused by millimeter-sized (or bigger) meteoroids disintegrating in the atmosphere.

FIREWORKS Fireworks, the forerunners of rockets, were invented by the Chinese. The ancient Chinese invented gunpowder (made of saltpeter, sulfur, and charcoal dust) roughly 2,000 years ago, and used it for making fireworks, which were used to celebrate important occasions. Chinese fireworks were probably brought to Europe in the 1400's by European Crusaders.

First Animal and First Dog in Space Laika (meaning "barker" in Russian) was the first dog in space. She was part Siberian husky and was launched in Sputnik 2 on November 3, 1957. Laika survived in Earth orbit for four days. She died in space when the batteries to the cabin over-heated. Sputnik 2 fell back to Earth on April 14, 1958, burning up during re-entry.

FIRST LIGHT First light is a term that refers to the earliest light emitted when the first galaxies were born. The first light occured long after the Big Bang happened. The Big Bang occured roughly 15 billion years ago, but the Universe was dark until the first light came millions of years after the Big Bang, after the smooth distribution of matter following the Big Bang coalesced into clumps, eventually forming galaxies.

FIRST MONKEY IN SPACE The first monkey in sub-orbital space was a squirrel monkey called Gordo. Gordo was launched into space on December 13, 1958, in the nose cone of the US Army rocket Jupiter AM-13. Gordo did well in the flight, but drowned in the Atlantic Ocean when the flotation device on the nose cone failed and it sank. The next two monkeys in space, Able and Baker, were launched on May 28, 1959 (Jupiter AM-18). They were successfully retrieved after a 300 mile sub-orbital flight.

FISSION Nuclear fission is a reaction in which an atom's nucleus is broken apart, releasing a tremendous amount of energy. Nuclear fission is used in many power plants on Earth (and is used in atomic weapons). Nuclear fission is not an important reaction in the Sun (for nuclear fission, the nuclear reaction that occurs in the Sun, click here).

FIZEAU, HIPPOLYTE Armand Hippolyte Louis Fizeau Fizeau (1819-1896) was a French physicist who was the first person to measure the speed of light on the Earth's surface. He measured the speed of light in 1849 using a device that consisted of a light, a toothed wheel and a distant mirror. He calculated light's speed by adjusting the speed of the wheel (the distance between wheel and mirror was 5 mi/8 km) so that the time it took the wheel to move the width of one tooth was equal to the time it took the light to travel from the wheel to the mirror and back again. He also measured the speed of light in other media, and found that light travels faster in air than in water. Fizeau also realized that the motion of a star affects its spectrum. He also did early work in daguerreotype photography.

FLAMSTEED NUMBER In each constellation, every star is given a Flamsteed number accordng to the system devided by John Flamsteed in early eighteenth century. In this scheme, the westernmost star in each constellation is given the number "1", the next star to the east is given the number 2, then 3, etc. The north-south position of a star in ignored.

FOCAL LENGTH Focal length is the distance from the lens (or mirror) in a telescope to the point at which the object being observed is focused.

FOCUS A focus (pleural foci) is a central point at which rays of light, heat or any other type of radiation meet after being reflected or refracted.

FOCUSER The focuser is a knob on a telescope that brings the image into focus. It works by moving the eyepiece in and out a slight amount.

FOLD MOUNTAINS Fold mountains are a type of mountain range that is formed when two continental plates collide (or one continental plate colliding with an oceanic plate). The colliding crust is compressed and pushed upwards (uplifted), forming mountains. For example, the Himalayas were slowly formed when the Indian plate collided with the Asia-European plate millions of years ago.

FOSSA Fossa are long, narrow, shallow depressions on the surface of a planet.

FOUCAULT PENDULUM A Foucault pendulum is a simple pendulum (a weight on a long string attached to a support) that tracks the rotation of the Earth. As the pendulum swings, the Earth rotates under the pendulum, so the pendulum seems to rotate. It was first demonstrated by Jean Bernard Léon Foucault, in 1851 at the Paris World's Fair.

FOUCAULT, JEAN Jean Bernard Léon Foucault (1819-1868) was a French physicist who was the first to demonstrate how a pendulum could track the rotation of the Earth (the Foucault pendulum) in 1851. He also invented the gyroscope (1852), showed that light travels more slowly in water than in air (1850), and improved the mirrors of reflecting telescopes (1858).

FRAUNHOFER, JOSEPH Joseph Fraunhofer (1787-1826) was a German physicist who first studied the Sun's spectra. In the early 1800s, he observed that the continuous spectrum was marred by over 700 dark lines (now called Fraunhofer lines). Fraunhofer's work with the spectra and also with diffraction gratings was seminal in the science of spectroscopy. At the time, it was unknown what caused these lines (they are caused by elements in the Sun's atmosphere absorbing light at their characteristic wavelengths).

FREE FALL Free fall is a state that occurs while in orbit around a planet. In free fall, things appear to weightless. During free fall, the orbiting object falls toward the planet the exact same amount as the planet's surface curves beneath the object; the object seems to be continually falling toward the planet but never reaches it.

FULL MOON A full moon appears as an entire circle in the sky; it is illuminated by the Sun and from the Earth, we see its entire daytime side. A full moon occurs once each lunar month, when the Moon is on the opposite side of the Earth from the Sun.

FUSION Nuclear fusion is an atomic reaction in which many nuclei (the centers of atoms) combine together to make a larger one (which is a different element). The result of this process is the release of a lot of energy (the resultant nucleus is smaller in mass than the sum of the ones that made it; the difference in mass is converted into energy by the equation E=mc2). Suns are powered by nuclear fusion, mostly converting hydrogen into helium. In this process, 4 hydrogen nuclei combine to form a single helium nucleus; about 0.3% of the original mass is converted into energy