One light-year is the distance that light travels in a year, through the vacuum of space.
Since light travels at a constant speed, in a vacuum, of around 3 x 108 metres per second (or, more precisely, around 299,792,458 metres per second), a light-year is around 9.4605284 x 1012 kilometers – i.e. 5.87849981 x 1012 miles, or roughly 6 trillion miles.
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.
A parsec is a unit of distance used by astronomers, which is equivalent to around 3.26163344 light-years. This is around 1.91735116 × 1013 miles (roughly 19 trillion miles) or 3.08567758 × 1013 kilometres (roughly 31 trillion km).
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.
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.
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.
An alternative way to picture this, is to imagine you are looking back at Earth from deep space. 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 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.)
Our galaxy, the Milky Way, is estimated to be around 27,000 parsecs (i.e. 27 kiloparsecs or kpc) across. 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.
The Astronomical Unit
One Astronomical Unit (1 AU) is the mean distance from the centre of the Earth to the centre of the Sun.
This is around 149,597,871 kilometres, 92,955,807 miles or around 1.58128451 × 10-5 light-years – i.e. there are about 8.32 “light-minutes” in one Astronomical Unit.