a subscript to variables in mathematical equations, such as π☽ (lunar parallax)
1803, Robert Patterson, The Papers of Thomas Jefferson, volume 40 (in English), Princeton University Press, published 2013, page 71:
Suppose the apparent angular distance of the sun & moon's nearest limbs [...] to be 110°.2′.30″ the app. alt of ☉'s lower limb measuring 20°.40′ and that of ☽'s lower limb 35°.24′