On the Earth, the metric system is the norm. Common to was used to determine distances in meters and more, in kilometers. In astronomy, observations are turning towards targets whose distances are much greater. In the traditional jargon, often referred these long numbers "astronomical" and that's saying something because if we had to make the distance of the nearest star to us after the sun, Proxima Centauri, the Odometer value is already so long that an ordinary calculator would not have enough increments to handle. The astronomical dimensions are so large that they combine with the time scale in which the speed of light is the vector.

To handle more easily astronomical dimensions, new angular units and distances appeared :

  • The arcdegrees, arcminutes and arcseconds
  • The astronomical unit
  • The light-year
  • The Parsec
The astronomical unit

This unit created in 1958 is used to measure the distances between the solar system objects. It is found overwhelmingly used for transient objects such as comets and asteroids. It is historically based on the Earth-Sun distance approximately 150 million km (more exactly 149 597 870 700 km - fixed value end of August 2012 in Beijing, China, during the 28th edition of the General Assembly IAU).

The earth has an elliptical orbit around the sun, the earth-sun distance is not constant, it is then necessary to clarify the definition of this value as the average distances on a Gaussian year (365 256 898 3-day orbital period). Compared to the sun, we note some distance as Jupiter at 5.21 AU, the asteroid belt between 2 and 3.5 AU, 30.11 AU for Neptune, the Kuiper belt 30 to 55 AU, Pluto 29-49 AU or even the distance of the Oort cloud at about 50,000 AU.

The light-year
The light-year is without doubt the most famous astronomical unit. It corresponds to the distance theoretically traveled in kilometers or AU during a Gaussian year with the light is about 10,000 billion kilometers (exactly 9 460 895 000 000 kilometers - equivalent to 241.077 63 astronomical units). The galaxies are distant from our own multi-million light-years away to several thousand billion light years.
The Parsec
The parsec (corresponding to 206,265 astronomical units), short for "parallax-second" comes from a trigonometric ratio in the Earth-Sun distance and an angle of one arcsecond relative to a celestial object located far away outside the solar system. This report is the "parallax method" for determining the observer distance from a distant celestial object.
The arcdegrees, arcminutes and arcseconds

In astronomical observation, everything is relative to our view from Earth. overwhelmingly used with a telescope exposed units that they are the degrees, minutes and seconds of arc to quantify any (apparent distances between celestial objects and their area). As many amateur astronomers, the handling of these units soon became common. They are very commonly used on objects belonging to the terrestrial suburbs such as planets, sun/moon eclipses, passages two inferior planets to the sun, comets, asteroids and plutoids.

These units are used in particular to express :

  • The apparent diameters of planetary disks
  • The distances between the two components of a double star
  • The distances between objects in conjunction
  • The apparent scope of an optical instrument

The hand remains a handy measuring tool to estimate the larger dimensions.

For practical reasons, the professional astronomers use the parsec more than light-years to express distances. The parsec is born works and first interstellar measures Friedrich Bessel in 1838.

Briefly, this report means that when a celestial object at exactly 3.2616 years-light viewed from the earth in two opposite points of its orbit, it seems to waver a arcsecond on its virtual position occupied in the sky. Parallaxes have low values. If we apply this example constant compared to a star like Proxima Centauri, one obtains a 0.76 arcseconds flicker for a distance of 4.28 light-years. More distant the object is since the earth and more the angular value decreases ; why the "parallax method" has a limit 100 parsecs around for a virtual wobble of the star in the sky less than 10 arcmilliseconds.

Distances and units




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