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.
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.