The principle of 'parallax' is used in the determination of distances of very distant stars. The baseline $AB$ is the line joining the Earth's two locations six months apart in its orbit around the Sun. That is,the baseline is about the diameter of the Earth's orbit $\approx 3 \times 10^{11} \; m$. However,even the nearest stars are so distant that with such a long baseline,they show parallax only of the order of $1''$ (second of arc) or so. $A$ parsec is a convenient unit of length on the astronomical scale. It is the distance of an object that will show a parallax of $1''$ (second of arc) from opposite ends of a baseline equal to the distance from the Earth to the Sun. How much is a parsec in terms of metres?

• A
  $1 \; \text{parsec} \approx 1.6 \times 10^{11} \; m$
• B
  $1 \; \text{parsec} \approx 9.3 \times 10^{20} \; m$
• C
  $1 \; \text{parsec} \approx 6.2 \times 10^{12} \; m$
• D
  $1 \; \text{parsec} \approx 3.09 \times 10^{16} \; m$