We are unable to directly measure distances to objects in our night sky. However, we can obtain the distance to relatively nearby stars by using their parallax angles because we measure them with respect to the baseline of the Earth's orbit of 1 AU (see section 11.1 in your text- book). Because even these stars are very far away (up to about 500 parsecs), the parallax angles for these stars are very small. They are measured in units of arc seconds, where 1 arc second is 1/3600 of one degree. To give you a sense of how small this angle is, the thin edge of a credit card, when viewed from one football field away, covers an angle of about 1 arc second. Part 2: Finding Stellar Distance Using Parallax parallaxdistance1.png Consider the star field drawing shown in Figure 1. This represents a tiny patch of our night sky. In this drawing the angle separating the two bright stars Star A and Star B is just 1/2 of an arc second. In Figure 2 below there are pictures of this star field taken at different times during the year. One star in the field exhibits parallax as it moves back and forth across the star field with respect to the other, more distant stars.

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