How do you measure the parallax of a star?

In order to measure parallax, you need two points that are known to be apart in distance.

The result is more accurate the larger the base (distance).

The largest possible base comes from two opposing points on Earth's orbit; in that case, the base is roughly 300 million kilometers across.

Note: To see how your finger moves in relation to the background, place it at arm's length and look with one eye first, then the other. You will notice that your base is approximately 7 cm!

When a star is observed to have changed apparent position as the Earth orbits the Sun, this shift is known as parallax and can be used to calculate the star's distance using simple geometry and the known distance between the Earth and the Sun. The shift is measured by taking pictures of the star at different times of the year when the Earth is on opposite sides of its orbit and calculating the angle of shift using trigonometry.