I was curious about why astronomy still has its own set of units. Wikipedia mentions that:

So I went through various distances and did some work:

1. The distance between a planet and a moon can be expressed in megameters (Mm).
2. The distance between a star and a planet can be expressed in gigameters (Gm).
3. The distance between stars can be represented in petameters (Pm).
4. The size of a galaxy can be represented in exameters (Em).
5. The distance between galaxies can be represented in zettameters (Zm).
6. The size of the observable universe can be represented in yottameters (Ym).

And converting between these units is absurdly trivial.

For example, the current distances between the Sun and:

1. Mercury: 63 Gm
2. Venus: 108 Gm
3. Earth: 150 Gm
4. Mars: 249 Gm
5. Jupiter: 788 Gm
6. Saturn: 1,502 Gm
7. Uranus: 2,857 Gm
8. Neptune: 4,478 Gm

Also, Alpha Centauri is 41 Pm from us, and the diameter of our galaxy is almost exactly 1,000 Em. The distance between us and Andromeda is 18 Zm. The observable universe is approximately 660 Ym in diameter.

This day in age, why are separate astronomical units needed?

I way overthink things sometimes. I've got a better question:

Why isn't astronomy done entirely with scientific notation?

Because it’s not necessary. The point of scientific notation is to allow easy conversions between different scales.

People often bring up the awkward number of feet in a mile, or the relationship between slugs and pounds. But really, when have you ever needed to know how many feet are in a mile? We don’t pace out miles 12” at a time.

Similarly, the number of meters between our planet and another doesn’t really have any meaning. We try to develop our units so they have physical meaning to us. The most logical distance unit in science is probably the radius of a hydrogen atom. But we don’t use that, because it has no meaning to most people.

The point of scientific notation is to allow easy math between values of arbitrary scale.

Here's a quick example: I happen to know that the diameter of the observable known universe is approximately 660 Ym, which is 6.60x10²6 m. Now light basically travels at 3.00x108 m/s. Immediately, I can tell you that light will take about 2.20x10¹8 s to travel from one end of what we can see to the other.

Not too long ago, I was arranging to have carpet installed in my office. I gave the contractor the room's dimensions in inches, but he wanted them in feet+inches. The entire system is awkward and irrational. The only part of USC that makes any sense whatsoever is that volumes are represented by larger units in powers of two.

A meter has meaning to literally everyone on this planet except us. As far as meaning in astronomy goes, whatever the exponent is would give meaning in a hurry. Is it a 6 or a 7? That's probably denoting the distance between a planet and a moon. If it's 9-12, that would be the distance between a star and a planet. And so forth.

And light years (and smaller time units) have worked nicely for a long time.

They are very easy to convert to meters if you like to handle multi digit exponents.

I do have a calulator that goes up to E499.

fpni.  I really don't understand people's obsession with the metric system and insisting it's the one true unit set.  Sure it's nice to do math in multiples of ten.  It's also nice to be able to intuitively quantify things without having to DO math in your head, which is why almost every discipline has a unit normalized to something related to that field that a normal person can pretty easily comprehend.

The Frogs started the whole thing.

The meter was equal to one ten-millionth of the distance from the equator to the pole
measured on a meridian through Paris, France.

Since the earth is pretty far from perfectly symmetrical and uniform at meter type scales.

Because it's fucking cool and has made a number of household projects personally easier.

1. A single set of prefixes is used to convert base units (meters, liters, grams) into scaled units (millimeters, milliliters, milligrams).
2. Scaling between prefixes is as easy as moving the decimal point.
3. Doing math with real world measurements is stupid easy.

I have never had anyone actually defend the English system.

By convention.

Same reason we describe long terrestrial distances in miles instead of mega inches.

Ha! reminds me of talking to a guy in France that worked for the IGN (national geographical institution) about what he was currently working on.
He was translating US maps for NATO use so that the French armed forces could use them.

I was a bit perplexed, and asked what needed translating, and why they would need him (computer engineer/programmer) to do that?
He said, well, for a start, the prime meridian has to go through Paris, not Greenwich (London).
So I said, well, that's pretty easy, you just basically twist the lines of longitude a few degrees to the right ...

He sighed, and said: Yes, it would be that easy if the pole was in the same place.

The French can be a bit odd at times.

This must be why 90% of the world's humans have left it behind like a disease.

BS

MGRS, metric and we all use the same maps.

The US military has used MGRS and the metric system for years.

Astronomers use parsec, for same reason that maps and artillery use milliradian.

They need to compare and do arithmetics on distances, as angles through their optics.

https://units.fandom.com/wiki/Light-year

https://en.wikipedia.org/wiki/Milliradian

5280 isn't an awkward number.  Makes perfect sense.

80 chains to the mile.  10 square chains to the acre.  640 acres to the square mile.

Quarter mile 1320 feet, or 20 chains.

If your professional life revolves around acres, it makes perfect sense.

Moon Metric Meme by FredMan, on Flickr

Eh...

There are 1,000 meters to a kilometer, meaning that there are 1,000,000 square meters in a square kilometer... Of course there are also 10 hectometers to a kilometer and therefore 100 square hectometers in a square kilometer. Of course a square hectometer is simply called a hectare. So in conclusion there are 100 hectares in a square kilometer.

I'm not understanding how miles, chains and feet go together intuitively.

Is your assertion that Liberia and Myanmar have been to the moon?

Attached File

Spend 4 years at Forestry School and you'll figure it out

Who has landed men on the moon and returned them to earth?

The USA and NO ONE ELSE.

The others have done a few controlled landings, but most ended up crashing into the moon.
Good thing no one was on board.

Is your assertion that we made it to the moon and back because we didn't use SI?