Space Stories - Parallax: Measuring the Cosmos

The 365 Days of Astronomy

Hosted by Richard Drumm, our Editor.

The word of the day today is parallax! Specifically stellar parallax.

So what’s parallax & what does it mean to astronomers? As it happens, I was part of the UVA Parallax team in the early 1980s. Best part-time job ever! So for once I might know a little bit about what I’m talking about! I’ll do my best to describe it so you can understand the science.

An easy method to help grasp the concept is for you to hold out your thumb at arm’s length and close one eye, then the other, switching back & forth between them while looking at your thumb. The location of your thumb seems to shift with each eye’s view. If your, uh, thumb was 20 feet away from your eye, the shift would be much less. You can measure the baseline, the distance between your eyes and, using a little trigonometry calculate the distance to your thumb. Simple!

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