got to wondering about this, you hear the numbers from time to time, but wanted to review the methods:
size of our galaxy: 100-150K ly
distance to andromeda galaxy: 2.5M ly
size of the observable universe: 90B ly
size of the universe: 23T ly
this article gives a pretty nice not-too-technical description of how those numbers are assayed:
http://www.bbc.com/earth/story/20160610-it-took-centuries-but-we-now-know-the-size-of-the-universe
Very interesting article, thanks.
shareLight years usually.
Parsec ... larger unit, but less meaningful and less used.
parsec |ˈpärˌsek| (abbr.: pc)
noun
a unit of distance used in astronomy, equal to about 3.26 light years (3.086 × 1013 kilometers). One parsec corresponds to the distance at which the mean radius of the earth's orbit subtends an angle of one second of arc.
Astronomical Unit is used on a smaller scale ...
astronomical unit |ˈˌæstrəˈnɑməkəl ˈjunət| (abbr.: AU)
nounAstronomy
a unit of measurement equal to 149.6 million kilometers, the mean distance from the center of the earth to the center of the sun.
yeah. definitely see the utility of the au (for near-scale intra-solar system measurements, comparisons), sounds like its useful to astronomers, which makes sense, of course: lightyears, for their workflow, would require a conversion :
If, for example, we observe a star in January, and then look at it again in July, the Earth will have gone halfway around its orbit. We’re looking at the star from two locations around 200 million miles (~300 million km) apart. If the star is reasonably close, then – from one side of Earth’s orbit to the other – it will appear to move ever so slightly.
Add some trigonometry, and the parallax angle, combined with the size of Earth’s orbit, lets astronomers calculate the distance to the star.
These angles are miniscule. They’re too small for degrees to be a practical unit of measurement. That’s why parallax angles are typically measured in arcseconds – a unit of measurement equivalent to the width of an average human hair seen from 65 feet (20 meters) away – not degrees. There are 3,600 arcseconds in one degree.
And here’s how we arrive at parsecs as a unit of distance: one parsec is the distance to an object whose parallax angle is one arcsecond.
The term parsec is just over 100 years old. It first appeared in a 1913 paper by English astronomer Sir Frank Watson Dyson, and the term stuck. If you see a star with 1/2 arcsecond of parallax, it is two parsecs away. At 1/3 arcsecond, it is three parsecs away. And so on.
Basically, astronomers liked it because it made the math easier!
https://earthsky.org/space/what-is-a-parsec