Everyone always says that Watson is the sun and Sherlock is the moon, but they are WRONG and here is why -
To start with the most obvious reason - Watson is, as Sherlock puts it, a conductor of light. He presents Sherlock Holmes to the world like an artist would a painting - if Watson was an illusrator he'd put a picture of Sherlock on every page of his books - and he basically already does in his descriptions.
Watson is also just very moon coded in general - his colours feel like a more warmish grey, and he has that relaxed old man vibe to him from time to time. Watsons character feels tied to how he cares for people and interacts with them, and I personally believe that to be a very moon-like quality.
Sherlock is also very sun coded - he shines brightly, and everyone notices him, but people can't truly see him with their bare eyes, he's misunderstood by many and can seem quite jarring. Sherlock accels in many areas, and people offen put him in a higher possision of importance compared to the moon. (But just like the moon, Watson is never fully forgotten in day-to-day conversation).
Sherlock cannot help but be the centre of attention due to his brightness.
People make the mistake of thinking sun = whoever is more bubbly and confident and moon = the shy, slightly grumpy one. And as much as I would ague this isn't always the case, I would like to mention that by that standard Sherlock still feels more sun-like and Watson still feels more moon-like.
People often forget that Sherlock isn't just a thinking mechine, he can be cheeky, theatrical, and Watson can often doubt his abilities and hide himself behind Sherlock's genius.
But either way you look at it - Sherlock amd Watson cannot truely shine to their full potential without each other.