The Unconscious World of Dream
(1) Contrasted with the sky, earth may symbolize a need to 'keep your feet on the ground' or pay attention to 'earthly' aspects of your being.
(2) It may be 'Mother Earth', symbolizing your own mother. Bear in mind that earth, as well as giving birth to things, also swallows them up again (when they die). Similarly, your mother - or your attachment to her - may have a 'devouring' aspect: one that hinders your individual development.
Mother Earth may also represent your unconscious. Life-giving energy comes up from the unconscious region. This is the 'womb' that contains potential for further development. Again, this may present a negative effect: the unconscious may seem to be threatening the conscious ego's attempts to maintain order in your life. However, the menacing appearance of the unconscious is a sign that such 'order' is precarious, having been achieved at the expense of neglecting important parts of your psyche. Wholeness is what counts.
(3) The planet earth seen as a sphere floating in space could be a symbol of your true self, as yet unrealized (i.e. not yet translated from idea or ideal into act and fact); wholeness, integration; or a weightless, relaxed 'floating' with the life-giving currents that will carry you to fulfillment of your individual 'destiny'.
Reference: Eric Ackroyd
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