Talk:Megaparsec

How can anything be more than 4384 megaparsecs away. If anything is 4384 megaparsecs or more that's more than 13.7 billion light years away and thus light hasn't had enough time since the beggining of time to get here.

can someone give the historical definition of a parsec?
hello,

can someone please enlightn me on who creted this unit of mesure and how it's size was fixed?

thanks, cyrille

How come no one has answered you...I think they or someone had to come up with a unit to measure great distances in space, however they forgot about the seed of light in relationship with light reaching the earth. By this measure we or the earth can be seeing new sources of light, or life in some form every day, if the right humans look hard enough. Any how I'm stuck here on earth with nothing to do but work and pay bills. What a drag... I only wish we could travel the seed of light, but again it would not be fast enough to travel great distances. We need to travel the speed of light times one million to really see the outer limits. 186 thousand plus per second times one-million. Let me know what that figures out to be. I would appreciate it. Then again what would you call the answer and how would you slow down once you got to that speed, wherever you were going, to smell the flowers & explore the unknown.... Really wild... Let me know your answer. I will work on it also. Thanks, joe

Additional; Parsec From Wikipedia, the free encyclopedia Jump to: navigation, search For other uses, see Parsec (disambiguation). The parsec (symbol pc) is a unit of length used in astronomy. It stands for "parallax of one arc second".

It is based on the method of trigonometric parallax, one of the most ancient and standard methods of determining stellar distances. The angle subtended at a star by the mean radius of the Earth's orbit, around the Sun, is called the parallax. The parsec is defined to be the distance from the Earth of a star that has a parallax of 1 arcsecond. Alternatively, the parsec is the distance at which two objects, separated by 1 astronomical unit, appear to be separated by an angle of 1 arcsecond. It is, therefore, approximately:

AU = 206,264.8062 AU = 3.08567758×1016 m = 30,856,775,800,000 kilometers = 19,173,511,600,000 miles = 3.26156378 ly (light years). See 1 E16 m for a list of comparable lengths and scientific notation for an explanation of the notation.

Since the parallax method is the fundamental calibration step for distance determination in astrophysics, its unit of choice, the parsec, is the most used unit of distance in scholarly astronomical publications. Newspapers and popular science magazines however prefer a more intuitive unit, the light year.

The first direct measurements of an object at interstellar distances (of the star 61 Cygni, by Friedrich Wilhelm Bessel in 1838) were done using trigonometry using the width of the Earth's orbit as a baseline. The parsec follows naturally from this method, since the distance (in parsecs) is simply the reciprocal of the parallax angle (in arcseconds).

Though it had probably been used before, the term parsec was first mentioned in an astronomical publication in 1913, when Frank Watson Dyson expressed his concern for the need of a name for that unit of distance. He himself proposed the name astron, while Carl Charlier had suggested siriometer. However, Herbert Hall Turner's suggestion, parsec, was eventually adopted.

There is no star whose parallax is 1 arcsecond. The greater the parallax of the star the closer it is to the Earth, and the smaller its distance in parsecs. Therefore the closest star to the Earth will have the largest measured parallax. This belongs to the star Proxima Centauri, with a parallax of 0.772 arcseconds, and thus lying approximately 1.295 parsecs, or 4.225 light-years, away from us.

The measurement of distances of celestial bodies from the Earth in parsecs is a key aspect of astrometry, the science of making positional measurements of celestial bodies.

Because of the extremely small scale of parallactic shifts, ground-based parallax methods provide reliable measurements of stellar distances of no more than 325 light-years, or about 100 parsecs, corresponding to parallaxes of no less than 1/100 of 1 arcsecond, or 10 mas (1 mas or milliarcsecond = 1/1000 arcsecond).

Between 1989 and 1993 the Hipparcos satellite, launched by the European Space Agency (ESA) in 1989, measured parallaxes for about 100,000 stars, with a precision of about 0.97 mas, and obtained accurate measurements for stellar distances of around 1000 pc.

NASA's FAME satellite was due to be launched in 2004, to measure parallaxes for about 40 million stars with sufficient precision to measure stellar distances of up to 2000 pc. However, the mission's funding was withdrawn by NASA in January 2002.

The ESA's GAIA satellite, due to be launched in mid-2012, will be of sufficiently high astrometric precision to measure stellar distances to within 10% accuracy as far as the galactic centre about 8 kpc away in the Sagittarius constellation.

Contents [hide] 1 Distances in parsecs 2 How to calculate the value of a parsec 3 Popular usage 4 See also

[edit] Distances in parsecs One kiloparsec, abbreviated kpc, is one thousand parsecs. One megaparsec, abbreviated Mpc, is one million parsecs.

8.6kpc From Earth to the center of the Milky Way 17Mpc From Earth to M100 [edit] How to calculate the value of a parsec

In the diagram above (not to scale), S represents the Sun, and E the Earth at one point in its orbit. D is an object at a distance of one parsec from the Sun. By definition, the angle D is one arcsecond and the distance ES is one astronomical unit. By trigonometry, the distance SD is

One astronomical unit is equal to approximately 1.49598×108 km, so

The attoparsec (atto being the prefix indicating 10-18) is a unit humorously used by some hackers. It is approximately 3.1 centimetres - just a little over an inch.

[edit] Popular usage Parsecs are a common measure of distance in Star Wars, and are occasionally used in Star Trek.

[edit] See also Conversion of units Megaparsec The Millennium Falcon, "the ship that made the Kessel Run in less than twelve parsecs." Small-angle formula Gigaparsec Retrieved from "http://en.wikipedia.org/wiki/Parsec"

Mpc/h
What is the meaning of Mpc/h ? I find this as a unit scale on some simulation images of galactic clustering. For example, check http://www.aip.de/People/AKhalatyan/COSMOLOGY/MareNostrum/

I later found that h is the Hubble Constant (H), as a fraction of 100 km/sec/Mpc. Since H is about 70 km/sec/Mpc, and 70 = 0.7 * 100, we have h = 0.7.

216.221.56.20 03:37, 18 March 2006 (UTC)Pierre Charland, pierrecharland@hotmail.com


 * Much of the uncertainty in distances is due to the uncertainty in h. By expressing distances as a ratio to h, the uncertainty of the ratio can be much less, and whatever the current value of h is may be plugged in later to find the right distance. -- Xerxes 00:19, 22 March 2006 (UTC)