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Astronomy Reference Libraries

Page 15 of about 169 Articles
Venus
2004-10-19 04:45:40

The Planet Venus is the second planet from the sun. It is often called the evening star or morning star and is brighter than any object in the sky except the sun and the moon. Because its orbit lies between the sun and the orbit of the earth, Venus passes through phases like those of the moon, varying from a large bright crescent when the planet is near inferior conjunction (nearest the...

Mercury
2004-10-19 04:45:40

The Planet Mercury -- in astronomy, nearest planet to the sun, at a mean distance of 36 million mi (58 million km); its period of revolution is 88 days. Mercury passes through phases similar to those of the moon as it completes each revolution about the sun, although the visible disk varies in size with respect to its distance from the earth. Because its greatest elongation is 28, it is seen...

Mars
2004-10-19 04:45:40

The Planet Mars -- in astronomy, 4th planet from the sun, with an orbit next in order beyond that of the earth. Physical Characteristics Mars has a striking red appearance, and in its most favorable position for viewing, when it is opposite the sun, it is twice as bright as Sirius, the brightest star. Mars has a diameter of 4,200 mi (6,800 km), just over half the diameter of the...

Earth
2004-10-19 04:45:40

Earth -- in geology and astronomy, fifth largest planet of the solar system and the only planet definitely known to support life. Gravitational forces have molded the earth, like all celestial bodies, into a spherical shape. However, the earth is not an exact sphere, being slightly flattened at the poles and bulging at the equator. The equatorial diameter is c.7,926 mi (12,760 km) and the...

Mars Moon -- Deimos
2004-10-19 04:45:40

Mars' Moon Deimos -- outermost of two small moons orbiting the planet Mars. Deimos orbits Mars at a distance of about 23,500 km (about 14,100 mi), completing an orbit once every 1.26 Earth days. The moon's orbit is almost circular and is only slightly tilted relative to the Martian equator. Deimos is irregular in shape, measuring about 15 km (about 9 mi) along its longest side and about 11...

Mars Moon -- Phobos
2004-10-19 04:45:40

Mars' Moon Phobos -- in astronomy, innermost moon, or natural satellite, of Mars. Phobos orbits Mars at a distance of only 9,378 km (5,627 mi), closer to its planet than any other moon in the solar system. In fact, it is so close that the force of Mars's gravity is stronger than the force keeping the moon in its orbit, so the radius of Phobos's orbit is decreasing at the rate of about 1.8 m...

Earths Moon
2004-10-19 04:45:40

The Moon -- natural satellite of a planet, in particular, the single natural satellite of the earth. The Earth-Moon System The moon is the earth's nearest neighbor in space. In addition to its proximity, the moon is also exceptional in that it is quite massive compared to the earth itself, the ratio of their masses being far larger than the similar ratios of other natural satellites...

Whirlpool Galaxy M51
2004-10-19 04:45:40

Whirlpool Galaxy -- Discovered 1773 by Charles Messier. The famous Whirlpool galaxy M51 was one of Charles Messier's original discoveries: He discovered it on October 13, 1773, when observing a comet, and described it as a "very faint nebula, without stars" which is difficult to see. Its companion, NGC 5195, was discovered in 1781 by his friend, Pierre Mchain, so that it is mentioned in...

Virgo Cluster of Galaxies
2004-10-19 04:45:40

Virgo Cluster of Galaxies -- This giant agglomeration of galaxies is the nearest big cluster of galaxies, the largest proven structure in our intergalactic neighborhood, and the most remote cosmic objects with a physical connection to our own small group of galaxies, the Local Group, including our Milky Way galaxy. This structure is another discovery by Charles Messier, who noted behind his...

Virgo A Galaxy M87
2004-10-19 04:45:40

Virgo A Galaxy -- Discovered 1781 by Charles Messier. The giant elliptical galaxy M87, also called Virgo A, is one of the most remarkable objects in the sky. It is perhaps the dominant galaxy in the closest big cluster to us, the famous Virgo Cluster of galaxies (sometimes also called "Coma-Virgo cluster" which is more acurate, as it extends into constellation Coma), and lies at the...