Cosmic Measurement

May 28, 2024 -- Written By Kevin Jie

The Astronomical Unit

Planetary Level

On Earth, we primarily use two systems to measure physical sizes and distances. These two systems are the Metric System and Imperial System. Although both provide accurate measurements and allow us to grasp how large or small objects and distances are, we begin to lose this context the larger the measurement is. This can be observed on a planetary level, where sizes and distances are so great that numbers become nearly incomprehensible but are still understandable to a point. If one were to say, “Neptune is around 5 billion kilometers away from the sun,” the numbers don’t really mean anything, as our brain can’t mentally comprehend what a billion of anything looks like. It’s this reason that astronomers have developed a separate measurement system to process larger numbers. The smallest of these cosmic measurements is the astronomical unit (au). One astronomical unit is the distance between Earth and the sun, or roughly 93 million miles (150 million kilometers). The astronomical unit is primarily used to compare the distances of planets within the solar system, but it’s rarely used to show size. However, when looking at cosmic measurements that are beyond the astronomical unit, we begin to see their role in measuring sizes more.

Now that we’ve established the values of both the astronomical unit and light-year, we should note what they are used for. The astronomical unit, as stated before, is primarily used to measure distances between planets in the solar system. Though, the use of the astronomical unit is not restricted to the solar system only. The astronomical unit can be used to measure distances between planets in other star systems too. The light-year, on the other hand, is used to measure massive structures that exceed stellar sizes, such as nebulae and galaxies. 

Graphic Made By Astron Analytics

A graphic detailing the measurement of an astronomical unit compared to the kilometer and mile with an example.

Upon viewing structures that are bigger and farther apart than the celestial bodies in the solar system, we again lose context of how great these measurements actually are. This is the switch from a planetary level to a galactic level, where sizes and distances become so great, once again, that astronomers now need another level of measurement to use. On a galactic level, most things are measured using light-years (ly), the distance that light can travel in a year, or around 5.88 trillion miles (9.46 trillion kilometers). Structures on the galactic scale range anywhere from just a few thousands of astronomical units to hundreds of millions. Although we can process what millions look like, the astronomical unit becomes far less efficient when trying to compare large numbers, where saying the measurement in astronomical units becomes a mouthful. Given that one light-year is equivalent to around 63,241 au, numbers are now condensed down from thousands to ones.

The scale doesn’t stop at light-years, though. As we proceed into the galactic and cosmic scale, we now look at the parsec (pc). The calculation of a parsec is a long and complex process that should be covered separately, but there isn’t anything that is necessary to learn about it except for knowing that one parsec comes out to around 3.26 ly. Alongside the parsec is the megaparsec and gigaparsec. These are used to measure the structure of the cosmic web and galactic neighborhoods. One megaparsec (mpc) is equal to one million parsecs while one gigaparsec (gpc) is equal to one billion parsecs. Possible extensions to the cosmic measurement system beyond the gigaparsec have been proposed, but none of them have been officially adopted or used, as it’s considered that measuring things beyond even tens of gigaparsecs is unnecessary. This is because  the observable universe itself  is only 28.5 gpc in wide (~94,000,000,000 ly). Aside from measuring the universe, there really isn’t any other practical use of the gigaparsec in tens.

The numbers may be understandable, but we still lose context. The truth is, it doesn’t matter how small we make the numbers; humans will never truly grasp the absolute scale of cosmic range.

Astron Original Document --

Space: Cosmic Measurement

To put into context the sheer size difference between the two measurements, let’s put them into a situation that we can imagine. If you were to walk the distance of an astronomical unit at an average walking pace of around 3 mph, it would take you around 3,539 years. However, if you were to walk a light-year at the same pace, it would take you 224,000,000 years. Even if you were traveling at the fastest speed a man made object has ever reached, at around 109.6 miles a second, it would still take you 1,700 years.