So why would anyone want to use parsecs instead? The light-year even has a usefulness that goes beyond simple measurement, because it tells us that when we observe an object X light-years away, we’re seeing as it was X years in the past. It’s simply the distance that light travels in a year, and it’s been in use since at least 1838. By contrast, the light-year is much easier to understand. Parsec vs light yearĪs logical as the definition of a parsec is, it’s still likely to come across as unnecessarily complicated to most people. In fact, real stellar parallaxes are smaller than that, meaning that their distances are always greater than a parsec. So that’s where the parsec comes from: it’s the hypothetical distance at which a star would show a parallax of exactly one second. Measuring that angle and then halving it (because we have two equal and opposite offsets relative to the Sun) gives us the star’s parallax. The star will appear to have moved through a small angular distance relative to the background objects. Then six months later, when the Earth is on the other side of the sun, they take another photograph of the same bit of sky, according to NASA. They take a photograph of a patch of sky containing the star they’re interested in and other, more distant objects such as galaxies. The same principle enables astronomers to measure the distance to nearby stars. If you could measure that angular difference, then knowing the distance between your eyes enables you to calculate the distance to the pencil. That’s the parallax effect, and it happens because the angular direction to the pencil is slightly different when seen by your left and right eyes. Parallax can be demonstrated by looking at a pencil with one eye or the other.
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