Randomness is not the issue
The most common complaint against Eclipse is that it is too random. The critics will site examples where they would cast Wrath fifteen times in a row without an Eclipse proc to show that the proc rate on Lunar Eclipse is to low. This conclusion is wrong for two reasons.
First, random numbers are random. I completely understand how frustrating a run of bad luck is, but it is no different then any other class. A mage can cast Arcane Blast 15 times in a row without a critical strike as well. In the long run, these periods of bad luck will be balanced by periods of good luck, but people tend to remember the bad luck better then the good luck. For example, I can remember only one time where someone set me an email or a tell about their good luck (Grats Murmurs on you 65% Wrath crit rate on Rotface.), but I've seen and received countless messages about how someone was destroyed by RNG. This is not a problem with randomness. This is a problem with perception.
Second, the Solar Eclipse and Lunar Eclipse proc rates are fairly similar. Yes, Solar Eclipse is procs 100% of the time when you crit with Starfire, as apposed to 60% with Wrath, but Wrath is also a much faster cast time. In the time it takes you to cast one Starfire, you can cast 1.8 to 2 Wraths. I won't detail the math for you here, but at Naxx levels of gear the two proc rates are very close. At ICC levels of gear the Solar Eclipse proc rate is about 5% higher but that is primarily due to Starfire having a higher crit rate. If Blizzard wanted to equalize the proc rates, they could increase the Wrath rate to 70% on crit instead of 60%, but more then that would clearly favor Lunar Eclipse.
Eclipse is too powerful
The first problem with Eclipse is that it is now the 800 pound gorilla in the balance tree. For most moonkin Eclipse is responsible for about 14% of their DPS. To put this in perspective, most talents boost DPS by around 3%-5%. As you can see Eclipse is way over budget in comparison.
On the surface this may not seem like a big deal. It gives moonkin more DPS, and that is rarely a bad thing. However, if you look a little closer you will see it is a problem because it amplifies all of the other problems with the talent. If it was a just a minor DPS increase, then who would care that it has big limitations in some fights? So, unless the talent is designed perfectly, having a big talent also means having big problems.
Eclipse and scaling
The size of Eclipse not only amplifies other problems but it actually causes other problems as well. The prime example is how moonkin scale with crit rating. A lot of people assume that Eclipse increases the marginal value of crit rating for moonkin because Eclipse procs off of critical strikes. In reality Eclipse hurts the marginal value of crit rating relative to other stats more then it helps. This is due to diminishing returns felt as critical strike chance increases.
To put it in less mathy terms, increasing your critical strike chance increases the DPS of only part of your rotation. If you have a 50% crit chance and cast 100 spells, the 50 spells that would have crit anyway won't crit harder because your crit chance is increased to 51%. So, as you increase your critical strike chance, there are fewer and fewer spells that can be affected by increasing your crit rating. This decreases the marginal value of crit rating. When you have a buff like Lunar Eclipse that increases your crit chance by 40%, the marginal value of crit rating goes down a lot. In fact, the Lunar Eclipse buff is so large, that moonkin are able to cap their critical strike chance with normal ICC 25man gear, thus reducing the marginal value of crit rating to zero for Starfire while Lunar Eclipse is active. This is important, because it means other classes will improve with better gear more quickly then moonkin will.
There is one issue with Eclipse that the moonkin community understands very well. That is the problem of losing Eclipse procs due to movement or other fight mechanics that prevent moonkin from DPSing.
Every class has some issues with movement heavy fights, but for most classes, their key abilities and procs aren't necessarily rendered useless by poor timing. For example, a fire mage can still use Hot Streak while moving because it makes Pyroblast an instant spell. Warlocks can still get the full benefit of Backlash and Backdraft, because the buff duration is much longer then the time needed to used the charges.
Eclipse is different. Let me use Professor Putricide as an example. Phase 2 of the fight is like a ping pong match where moonkin are constantly run back and forth across the room. Usually it is possible to cast a few spells on each end, but rarely do moonkin have the opportunity to take full advantage of a 15 second long Eclipse. Therefore, the most powerful talent moonkin is dramatically diminished on any fight with significant interruptions, and few fights very few let you stand still and DPS.
The good news is that Blizzard agrees. In the recent Developer chat on Twitter, the developer's commented that "losing an important proc because it happened at the wrong time (say just before you moved) is a problem." They also commented that they have some ideas on how to fix the issue in the next expansion.
Limited opportunities for Eclipse
The last issue I will talk about has to do with the cooldown on Eclipse. Since neither Eclipse buff can be procced while either of the buffs are active, the amount of time moonkin have to proc Eclipse in a fight is quite a bit less then the actual fight length. For example, if a fight is 5 minutes long, a moonkin can proc can proc Eclipse a maximum of 20 times. If Eclipse procs 10 times, the moonkin had only 2.5 minutes to proc those Eclipse buffs. On the other had, lets say an elemental shaman has an average cast time of 1.5 seconds on his Lightning Bolt. In that same five minute fight he can cast Lightning Bolt 200 times and all of them have the potential to crit no matter what happened before. To put it in more mathy terms, moonkin have a smaller sample size for their procs then most other classes.
A lot of players confuse this problem with randomness, but it is really a problem with the variability of results. All classes can have a streak of bad luck. However, when a moonkin goes without an Eclipse proc for an extended period of time, it has a much greater impact on his DPS then when a Shaman goes through a similar dry spell. The cooldowns on Eclipse mean moonkin have less time to rebound while the shaman still has the full 5 minutes. This issue with a smaller sample means there is greater variability in moonkin DPS because outliers are more probable then they are for other classes.
Eclipse is broken as most moonkin believe, but not in the way that most of them think. Randomness is not the problem. All classes have to deal with randomness to some extent. They all have random procs or crits, and in the long run bad luck will balance out with good luck.
The problems with Eclipse are much more structural. First, Eclipse is way too powerful and dominates the moonkin DPS calculation. Second, the buffs are so large that they limit a moonkin's ability to scale with gear. Third, moonkin are unable to control when Eclipse procs, and are likely to lose a significant portion of the buff to movement or another fight mechanic. Finally, the cooldown on Eclipse limits a moonkin's ability to proc Eclipse, and makes moonkin DPS more variable then that of other classes.
Despite the wishes of some players Eclipse is likely a permanent part of moonkin DPS, and will continue to be based on random procs to some extent. The good news is that Blizzard realizes there is an issue, and have a plan to change Eclipse in the next expansion.
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