For the star in Figure 1, the
parallax angle - P
is half the distance moved by the star between photos. Therefore

P = 0.5 / 2 = 0.25 seconds of arc. (1 second of arc (1") = 1 / 3600)
degrees

Figure 1

From Figure 2, the distance between the Sun and the star is :

d = r / tan P

If P is 1 second of arc:

d = 150 000 000 / tan 1" = 30 million million km

This distance is called one parsec and is a basic unit for measuring
astronomical distances.

Distance in parsecs = 1 / P in seconds of arc

Figure 2

For the star in Figure 1:

d = 1 / P = 1 / 0.25 = 4

Therefore the star is four parsecs away.

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