to what does lily compare her heart.?

When Lily finds the tin box she lies down and looks at the things inside. Then she held her mother's things across her chest and looked at the sky above, where her skin felt like the sky and the moon was her "heart beating up their in the dark." She found solace in her mother's things, and the feeling feeling was all encompassing........... her heart was like a shining light in the sky. Page 23