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# Distances in the universe

If they ask you for a unit of distance, surely you will think in kilometers to say the distance that separates two cities, or in centimeters to tell how long the table in front of you is. But these units are too small to refer to distances in space.

The units of length used in the study of the universe are:

## Light-year

light year  is the distance that light travels in one year.

It is a unit of length, not time, and is equivalent to the path of a photon of light traveling at a speed of 300,000 km/s during one year. Equivalent to approximately 9.46 × 1012  km (9 460 730 472 580.8 km).

1 year = 365.25 days.

1 day = 1 day x (24 hours / day) x (60 minutes / hour) x (60 seconds / minute) = 86400 seconds

1 year = 365.25 days x (86400 seconds / day) = 31557600 seconds

d = v * t

d =  299 792 458  m/s x 31 557 600 s = 9 460 730 472 580 800 m = 9 460 730 472,580 800 km

Light travels about 9.5 trillion kilometers in a year.

## Astronomical Unit

An astronomical unit (AU) is equivalent to the average distance between the Earth and the Sun , about 150,000,000 km.

## Parsec

The parsec is a unit of length whose name derives from the  English  parallax of one arc second.

parsec is the distance at which an object would be found from which the distance between the Earth and the Sun (1 AU) would have a parallax of 1 arc second.

A parsec is equivalent to:

1 parsec = 206 265 ua = 3.2616 light years = 3.0857  × 1016  m Kes47 ( ? ) [CC BY 3.0], via Wikimedia Commons