In astronomy and astrodynamics (and I presume many other physical sciences), ⊕ denotes "Earth", or "of the Earth". For example:
This usage probably has its roots in astrology - much as the circle with a cross on the bottom (female) is the symbol for Venus, circle with an arrow (male) for Mars, and circle with a dot in the middle for the Sun.
Here's what one source (http://www.awarenessshop.com/KnowledgeBase/stellarsymbolsth.html) had to say about the symbolic Earth in a magical and astrological context:
"EARTH - Though this symbol is not often used in magickal work, its meaning should not be overlooked. Our global home is often represented with a cross circumscribed by a circle: matter surrounded by spirit."
Also, in Susan Cooper's The Dark is Rising sequence of novels, the Earth symbol is used as a powerful talisman for harnessing ancient Welsh magic.