Special units of measurement are needed to cope with the Universe's vast distances - light years, for example. A light year is the distance light travels in one year through a vacuum, about 9.46 trillion km.

A parsec is 3.26 light years, as Chris Lintott explains.

An astronomical unit is the mean distance between the Earth and the Sun, about 150 million km.

So the star Eta Carinae is an estimated 7,500 light years, 2,301 parsecs or 474,308,078 astronomical units from the Sun.

**Image:** An X-ray image of the star Eta Carinae (credit: NASA/CXC/SAO)

Professor Marcus Du Sautoy explains to comedian Alan Davies how some mathematicians envision a potential shape for the Universe.

Sir Patrick Moore and Chris Lintott describe some of the beautiful sights in our galaxy, the Milky Way.

Sir Patrick Moore and Chris Lintott explain some of the basic units and methods used to measure distance in the Universe.

Sir Patrick Moore and Chris Lintott explain astronomy terms like declination, right ascension and procession.

Sir Patrick Moore shows a clip from a 1971 Sky at Night programme in which he used a cricket pitch to demonstrate the principle of parallax. He then discusses the measurement of star distances with his guest, Andrew Murray.

Distance measures are used in physical cosmology to give a natural notion of the distance between two objects or events in the universe. They are often used to tie some observable quantity (such as the luminosity of a distant quasar, the redshift of a distant galaxy, or the angular size of the acoustic peaks in the CMB power spectrum) to another quantity that is not directly observable, but is more convenient for calculations (such as the comoving coordinates of the quasar, galaxy, etc.). The distance measures discussed here all reduce to the common notion of Euclidean distance at low redshift.

In accord with our present understanding of cosmology, these measures are calculated within the context of general relativity, where the Friedmann-Lemaître-Robertson-Walker solution is used to describe the universe.

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