Larger Unitsįor even greater astronomical distances, multiples of the parsec are used, including the kiloparsec (kpc), the megaparsec (Mpc), and the gigaparsec (Gpc), which are, , and parsecs respectively. In practical terms, if we observe a nearby star from two points in Earth’s orbit six months apart, and that star appears to move by one arcsecond compared to more distant stars, then that star is said to be one parsec away. The term is a portmanteau of “parallax” and “arcsecond”, reflecting its definition. The term parsec comes from the method of parallax measurements used to determine the distances of nearby stars. These relationships can be represented by the equations: In terms of light-years, a commonly used unit in astronomy, one parsec is approximately equal to 3.26 light-years. Conversion to Other UnitsĪ parsec is approximately equal to 3.09 trillion kilometers, or 1.92 trillion miles. An astronomical unit is the average distance between the Earth and the Sun, approximately 149.6 million kilometers. there are about 8.32 “light-minutes” in one Astronomical Unit.The parsec (pc) is defined as the distance at which one astronomical unit (AU) subtends an angle of one arcsecond. One Astronomical Unit (1 AU) is the mean distance from the centre of the Earth to the centre of the Sun. The nearest large galaxy – M32, the Andromeda galaxy – is around 752 kpc from Earth, and the entire observable universe is around 28 billion parsecs (28 gigaparsecs or 28 Gpc) in diameter. Our galaxy, the Milky Way, is estimated to be around 27,000 parsecs (i.e. (Note that the Earth would move two arcseconds over the course of the year, at this distance, as the radius and not the diameter of the Earth’s orbit is used to define the parsec.) The radius of Earth’s orbit around the Sun (1AU, see below) would appear to subtend an angle of one arcsecond if you were a distance of one parsec away. Note that all stars are further than one parsec from Earth, and Proxima Centauri – the nearest star to Earth – is around 1.3 parsecs away.Īn alternative way to picture this, is to imagine you are looking back at Earth from deep space. In other words, if there were a star located exactly one parsec from Earth, it would appear to shift in position around a fixed point against the distant background stars by two 3,600ths of a degree (two arcseconds), and back again, over the course of a year, due to the motion of the Earth around the Sun. The word parsec is a contraction of the words “parallax of one arcsecond”, since a parsec is defined as the distance from Earth that a star would need to be in order to exhibit an annual parallax of 1 arcsecond. The ParsecĪ parsec is a unit of distance used by astronomers, which is equivalent to around 3.26163344 light-years. A parsec a unit of distance equal to about 19 trillion miles (more than 30 trillion km) is more closely related to how astronomers go about the business of figuring out the size of the. The observable universe is about 91 billion light-years in diameter (534,943,482,710,000,000,000,000 miles), at the present time, although the size of the entire universe is unknown. Proxima Centauri our nearest star (other than the Sun), is approximately 4.243 light-years away. 5.87849981 x 10 12 miles, or roughly 6 trillion miles. Since light travels at a constant speed, in a vacuum, of around 3 x 10 8 metres per second (or, more precisely, around 299,792,458 metres per second), a light-year is around 9.4605284 x 10 12 kilometers – i.e. One light-year is the distance that light travels in a year, through the vacuum of space.
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