Longing in a dream often reaches toward something specific and unmet -- a connection, a person, or a version of your life you wish were closer than it currently is. Unlike nostalgia, which looks backward, longing in a dream usually points at a present-tense need, something you're actively missing right now rather than simply remembering.
Make the Sun, or any of the superiors to signify princes and great persons. Scribes and rusticks, [are shown by] the inferior planets, and chiefly the Moon. - William Lilly (1602.-1681.)