Why is time in the galaxy far far away described by reference to Yavin rather than Endor?

Question

In the Star Wars universe, dates are usually given as "[number of years] BBY" or "[number of years] ABY" - that is, X years "Before the Battle of Yavin" or X years "After the Battle of Yavin". The Battle of Yavin, of course, marked the destruction of the first Death Star.

This is puzzling, because the Empire was still going strong after Yavin, whereas the Battle of Endor was catastrophic for Imperial forces - the second Death Star was destroyed, the Emperor and Vader were killed, the Imperial Fleet was scattered, Imperial leadership fell into disarray and in-fighting, the Rebellion emerged as the dominant power in the galaxy, etc.¹

Out-of-Universe, "BBY/ABY" makes sense, because the climactic moment of the first Star Wars movie is the Battle of Yavin, and is familiar to everyone who has seen the film.

In-universe, it makes less sense, because the consequences of the Battle of Yavin were far less pronounced than the consequences of the Battle of Endor.

So why does the galaxy far, far away use "BBY/ABY" rather than "BBE/ABE"?

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¹ The Battle of Jakku was arguably even more significant, since it marked the end of the Empire as a sizable military entity, and ended with the surrender of most Imperial forces and resources. However, the Battle of Jakku only entered canon recently, long after "BBY/ABY" was established, so it is probably too late to switch to "BBJ/ABJ".

Answer 1

I've searched everywhere I can think of but I can't find a source that gives an in-universe reason for the choice of the Battle of Yavin as the epoch rather than the Battle of Endor. That said, an argument can be made that the year of the Battle of Yavin is more significant in-universe than 4 ABY (the year of the Battle of Endor).

The following distinct (though related) events occurred in 0 BBY/0 ABY and were of the highest importance to the Empire, the Rebel Alliance, and the galactic community as a whole:

• Dissolution of the Imperial Senate
• Destruction of Alderaan by the Death Star
• The recruitment of the most famous (and at times, only) Jedi to the Rebel cause: Luke Skywalker
• The destruction of the Death Star at the Battle of Yavin

In contrast, there was only one event of such importance in 4 ABY:

• The Battle of Endor

Granted, there were several outcomes to the Battle of Endor that were extremely remarkable: the destruction of the second Death Star, and the deaths of both the Emperor and Darth Vader. However, the events of 0 ABY would still seem to be more important to most people, in-universe, than these outcomes. For one, it was not well known -- even among the Rebels! -- that the Emperor was a Sith Lord, so his death would be seen by the vast majority of people as more symbolic than truly important. Furthermore, the second Death Star wasn't complete and was essentially confined to the Endor system; in contrast, the first Death Star was fully operational and mobile, and had destroyed the planet of Alderaan -- it was an absolutely hated terror weapon that killed billions of people and could have killed billions more if not for the Rebel victory at the Battle of Yavin. Moreover, although Darth Vader terrorized and killed many people he did not kill billions like the Death Star at Alderaan -- so even his death was less notable than the destruction of the first Death Star, at least in the mind of the average person. Finally, neither battle resulted in the complete destruction of the Empire -- a sizable portion of the Imperial Fleet (including a Super Star Destroyer) remained even after the Battle of Endor; the Empire wasn't fully defeated until the Battle of Jakku, and that battle lacked the climactic destruction of a planet-killing superweapon.

The Battle of Endor appears to be more important than the Battle of Yavin to us, the audience, but only because we are privy to the evil of the Emperor and Darth Vader. To most people in the galaxy far, far away, the destruction the first Death Star and its ability to kill billions of people in mere moments was far more important and notable -- and so the Battle of Yavin makes more sense as the epoch.

Answer 2

It makes sense to me.

You have a period where effectively three (probably more to come) planet-killing bases are constructed and destructed in the space of thirty years. Each one weakens or destroys some level of evilness but leaves some significant force behind to a greater or lesser degree. In such a world -- er, galaxy -- it completely makes sense to use the destruction of the first one as the epoch.

Answer 3

Some relevant information has come in since this post and its answers were last updated:

We now know from "Rogue One" that the war between the Rebels and the Empire started the same year as the Battle of Yavin, specifically with the Battle of Scarif.

Before that, the Rebel Alliance was engaged in an underground resistance/guerrilla type conflict with the Empire, not a full fledged war. There are also hints in the movie that some factions in the Alliance weren't planning on armed conflict at all, and were looking for some sort of negotiated political resolution to their struggle (for example the fact that they refer to Saw Gerrera as an extremist for his willingness to use violence against the empire - if they had been planning for a war, then there would be nothing 'extremist' about his actions).

The events depicted in "Rogue One" and the Battle of Scarif mark the start of the actual civil war that eventually overthrew the Empire, and it makes sense for the New Republic to consider that year as the beginning of its new era.

A parallel can be drawn with U.S. history: The American Revolutionary War lasted from 1775 to 1783, yet Americans don't consider the U.S. to have been "born" in 1783, nor do they consider it to have been born in 1775. They consider the U.S to have been "born" in 1776, the year of the declaration of independence. Based on the background information provided in Rogue One, one can consider the year of the Battle of Yavin to be the year that the Rebels "declared independence" from the Empire.

That still leaves the question of why is it "BBY" and not "BBS"? One could argue that few people besides military historians knew about the events at Scarif, whereas most of the general population knew about the Battle of Yavin and the destruction of the first Death Star, so BBY became the accepted nomenclature.

Answer 4

One practical in universe explanation (although I haven't found any documentation to support it) would be that after picking a time reference such as BBY/ABY it would be problematic to change it with every major event. History would have to be reevaluated every few years as the reference is changed such that some event that was 3 BBY is now 7 BBE or changed from 2 ABY to 2 BBE. If the reference is changed with every major event it would likely cause unnecessary confusion.