To whom is Sir Philip Sidney's "Sonnet 31" addressed?

Sir Philip Sidney’s “Sonnet 31,” as a perfect example of apostrophe (the figure of speech in which the speaker talks to an absent person or an inanimate object), is a direct address to the moon.  This is obvious even from the first line.  “With how sad steps, O Moon, thou climb’st the skies!”  The speaker makes his direct address to the moon even more obvious within lines 9-10.  “Then even of fellowship, O Moon, tell me / Is constant love deemed there but want of wit?”  It is interesting that Sidney makes the moon into a proper noun by capitalizing it, almost giving it human qualities in doing so.  The speaker's address to the moon isn’t his main point, of course.  The moon, which remains “silent” and “wan” and with a “languished grace,” simply looks to the speaker the way he feels.  The speaker, then, uses his observance of the moon as a springboard to talk about the negative aspects of love:  loyalty looks like lack of intelligence, beauties are too proud, lovers scorn their partners, and ingratitude is something to be prized.  Taking the analysis of the poem a bit further, one can consider the speaker to be Astrophel and the woman in question to be Stella.  "Sonnet 31" is one of the more famous sonnets in Sidney's Astrophel and Stella sonnet sequence.