Now, let's determine the actual distance to each of the stars. To do this, we need to look closely the triangle we established earlier:

• One side of the triangle is the distance between the Earth and Sun, which we know has a length of one astronomical unit (1 a.u.).
• Another is the distance from the Sun to the star, d.
• The angle p is called the parallax angle.
• The angle Earth-Sun-Star is a right angle (90 degrees).

Your textbook explains that the parallax angle p (measured in arcsec) and the distance to the star d (measured in units called parsecs) are related by a simple equation:

You have measured the parallax angles p for Stars A, B, C on the previous page. Compute the distance to each star using the above formula, and enter the answers on your Homework Answer Sheet.

The distances output from this equation are in units called parsecs (abbreviated pc). Parsecs are the usual units used by professional astronomers, but most students are more familiar with light years (ly). You can convert any distance measured in pc to units of ly by multiplying by 3.26 ly/pc (for example, a star that is 10 pc away is 32.6 ly away). Do this conversion for Stars A, B, and C on your Homework Answer Sheet.

Before ending this assignment, stop a moment to reflect: did you get the right answers to Questions 2 and 3? Do you feel you understand the concept of parallax now? If not, work your way through this exercise again.