Observation Usual Astronomical Measure Units

The Light-Year. 1 light-year is 5,880,000,000,000 miles, 5,880 billion miles or 5.9 trillion miles (9,463 billion km). One light-year is the distance the light scours during one year. The speed of light is 186,000 miles per second -300,000 km per second- (671 million miles per hour). This measure unit gives somehow an historical view about the Universe: when a star is said located at 300 light-years, this means that it is seen as it was 300 years ago, i.e. about the year 1700; when a galaxy is at 4 million years, it is seen it as it was 4 million years ago, at the time when first men appeared. With such a scale, the Universe may be considered 145,000 trillion-mile (234,000 billion of billion-km) wide!

The Astronomical Unit (AU). 1 astronomical unit is the mean Sun-Earth distance, 149,597,870 kilometers (92,960,116 miles). One light-year is 63,240 AU. By 2012, the International Astronomical Union voted a new definition of the AU. As it formerly relied upon a changing mathematical equation, depending upon the mass of the Sun, the length of a day, and a fixed number known as the 'Gaussian gravitational constant' it has been changed into a fixed number with the new definition better able to handle Einstein's General Relativity. The new fixed number is the best estimate of the original expression

The Parsec. The parsec is the distance of an object having a parallax of 1 arc second: 3.26 light-year; 1 kiloparsec (kpc) is 1,000 parsecs; 1 megaparsec (Mpc) is 1 million parsecs. 1 light-year is 0.3069 parsec. note: the parallax of a celestial object is its -minute- apparent change of location against the starry background due to that all astronomical observations occurs from the Earth, which makes that a observer, each 6 months, finds himself at a opposite location, on the orbit of the Earth, each side of the Sun