Rather than blink their eyes, however, astronomers move the Earth. Likewise, astronomers measure angles to find the distances to stars. If you measure the angle over which your finger appears to move, you can figure out how far your finger is from your face. This apparent shift is called parallax, from a Greek word meaning alternation. So the finger’s location, relative to stuff in the background, looks different. Each eye sees your finger from a slightly different angle. As you alternate eyes, you’ll notice your finger appears to dance back and forth in front of your face. Hold your finger in front of your face, focus on something in the distance, and close first one eye, then the other eye. To find the distance to a nearby star, astronomers use triangulation.
A parsec – a unit of distance equal to about 19 trillion miles (more than 30 trillion km) – is more closely related to how astronomers go about the business of figuring out the size of the universe. But light-years aren’t as useful as parsecs when it comes to measuring those distances. The concept of a light-year – the distance light travels in a single earthly year, or about 6 trillion miles (nearly 10 trillion km) – is a great way to think about distance scales in the universe.
#Parsec definition professional#
If you ever heard professional astronomers talking among themselves, you wouldn’t hear much talk of light-years. But, from one position to the other, the tree appears to move with respect to the mountains.
From each position, the observer sees the same tree. Illustration of the shift in perspective that happens as an observer moves between the 2 positions, in this case, with respect to a tree and distant mountains.
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