In astronomy, which of the following measurements is the longest?

Parsecs measure extremely long distances between astronomical objects outside of the solar system. One parsec is equal to approximately 31 trillion kilometres (19 trillion miles). To put this into perspective, the stars visible to the naked eye are within 500 parsecs of the sun.

One parsec is equivalent to 3.26 light-years. This means it would take a photon of light over three years to travel one parsec. Although parsecs help to record long distances, the distances between the solar system and other galaxies are extremely vast. To cope with these large distances, astronomers use terms such as kiloparsecs (1,000 parsecs or 3,260 light-years), megaparsecs (one million parsecs or 3.26 million light-years) and gigaparsecs (one billion parsecs or 3.26 billion light-years).

The word parsec is a portmanteau of "parallax of one second" and was coined by the British astronomer Herbert Hall Turner in 1913. Parallax is a perceived change in the position of an object seen from two different places. Astronomical measurements of positions are taken at different times of the year to compare the distances from different positions in the Earth's orbit.

There are 1 million kilometres (621,371 miles) in a gigametre, therefore there are 31 million gigametres in a parsec. A lunar distance is a mere 384,400 km (238,855 miles).