The sun goddess Amaterasu and her brother, Susanowo, had what might generously be called a rocky relationship. Susanowo would not listen to reason, he caused problems for the mortals on earth and the other gods in heaven constantly, and he influenced his children to take part in his trouble making. He also had a habit of barging into his sister's palace, demanding her hospitality, trying to pick a fight with her, and terrorizing her attendants. Amaterasu's job ruling over the heavens would have been difficult enough without having to deal with her unruly little brother, and she always dreaded the times when he would show up at her palace gates.
And so, when he started one of his visits by bursting directly into the room where she was working on a thunderbolt, clearly in the mood to start a fight even more than usual, she decided that she truly wasn't in the mood to deal with it. She left to hide in a cave and give her brother time to get over whatever had caused his little temper tantrum. Hopefully seeing the consequences of a few days without sunlight would make him hesitate to be so rude to her in the future.
Amaterasu couldn't say exactly how long she stayed in her cave. It was hard to judge the passage of time when the inside of the cave was always bright with her radiance and the outside was perpetually dark. It couldn't have been more than a week at the very most when the sound of beautiful music began drifting into the cave.
Going to the mouth of the cave, Amaterasu saw the goddess Uzume doing a celebratory dance outside. Amaterasu had always been very close with Uzume, and the other goddess was a talented and lovely dancer, so Amaterasu sat in the cave's entrance to watch.
When Uzume's dance ended, Amaterasu called out to her. "What are you celebrating, Uzume?"
"Oh, haven't you heard?" Uzume called back, a mischievous smile pulling on her lips. "I have found a sun goddess to light our days and lead the heavens. Isn't that wonderful?"
Amaterasu narrowed her eyes at this strange and confusing answer. "Have you? I suppose that's good, then, though I can think of no one suited to the job."
Uzume's smile grew and she walked over to sit across from Amaterasu. "Oh, she's perfectly suited," she said. "Not to mention beautiful. The most beautiful goddess of them all, in fact."
Amaterasu raised an eyebrow. "So, you're appointing yourself to the position?"
Uzume laughed. "Oh no, not me. Here, I'll show you who it is. Truly, you'll be amazed."
Uzume pulled an ornate gold frame from her robes and held it out for Amaterasu to see. Amaterasu's own face stared back at her, the sun goddesses light reflected dazzlingly off of a mirror.
Amaterasu chuckled and rolled her eyes. "You're a terrible flirt," she said, standing up and offering a hand to Uzume. "If you wanted me to return you could have just asked."
Uzume grinned as she took Amaterasu's hand and pulled herself to her feet. "Where's the fun in that?" she asked. Her smile faded slightly before she spoke again. "You will come back, won't you? Things are not going well without you."
Amaterasu nodded and began walking back toward her palace, hand in hand with Uzume. "Yes, I'm coming back. I was always planning to, and if Susanowo hasn't learned his lesson by now, I doubt he will."
Uzume's smile returned in full force. "Wonderful! Everyone will be so happy to have you back." With that, she ran toward the palace, pulling Amaterasu along with her.

(
Amaterasu emerging from her cave)
Author's Note: This story is based on The Miraculous Mirror. In the original, Amaterasu flees from Susanowo because she is afraid of him. Then Uzume tells her that the god's have found a new sun goddess who is more beautiful than Amaterasu and shows her a mirror. Amaterasu thinks her reflection is her replacement, rushes out of the cave to fight it, and the cave is sealed behind her. I thought that it didn't make any sense for Amaterasu to fall for a trick like that, so I turned the mirror thing into a joke between her and Uzume. I also had her leave because she was fed up with Susanowo instead of afraid of him, because I don't think the literal queen of heaven would be that easily frightened.