1. Measuring the Distance to Stars

2. The Parallax Method To the naked eye stars are so far away that they do not appear to change positions even though we are rotating (8,000 miles wide) and revolving (186, 000,000 miles). However with the invention of photography, we had new tool to record stars and look for tiny shifts in their position

3. The Parallax Method The first time we were able to see a tiny shift in the position of a star was in 1916 during the First World War. The shifts are less than an arcminute and were only possible because of the improvements of photography and telescopes.

5. The Parallax Method

6. We use the orbit of the Earth to make our measurements. By taking pictures 6 months apart we have moved about 186,000,000 miles. This change of position makes the nearby stars appear to shift compared to the more distant stars. By recording the angle of the shift we can calculate the distance to the star.

7. The Parallax Method For very small angles we can assume that the sine of the angle is very close to the value of the angle itself (using radians as the measure). Therefore: Sin θ = opp/hyp and »Hyp = opp/sin θ or Hyp = opp/ θ

8. The Parallax Method However, to simplify things astronomer have developed an easier formula based on the parsec. 1 parsec = 3.26 light year, where –1 light year = 5.88 trillion miles The formula is d = 1/p where d is distance in parsecs and p is the parallax measured in arc seconds.

9. The Parallax Method