The ‘balcony scene’ in Romeo and Juliet is fake news. ‘O, swear not by the moon, the inconstant moon’ is one of the most famous lines to appear in this scene, Act 2 Scene 2, but it’s questionable whether Romeo is actually looking up at Juliet on her balcony. We’ll return to this issue of the balcony-that-wasn’t in a moment; but first, let’s attend to this line, ‘O, swear not by the moon, the inconstant moon’, and analyse its significance in this iconic scene from Romeo and Juliet.
To recap what has happened in Act 1 of the play, leading up to this talk of inconstant moons: Romeo of the Montague family has attended the masked ball held by his family’s sworn enemies, the Capulet family. His friend Mercutio tells Romeo about a fairy named Queen Mab who enters young men’s minds as they dream, and makes them dream of love and romance. At the masked ball, Romeo spies Juliet and instantly falls in love with her; she also falls for him.
They kiss, but then Tybalt, Juliet’s kinsman, spots Romeo and recognising him as a Montague, plans to confront him. Old Capulet tells him not to do so, and Tybalt reluctantly agrees. When Juliet enquires after who Romeo is, she is distraught to learn that he is a Montague and thus a member of the family that is her family’s sworn enemies.
In the scene that concerns us here, Act 2 Scene 2, Romeo breaks into the gardens of Juliet’s parents’ house and speaks to her at her bedroom window. The two of them pledge their love for each other, and arrange to be secretly married the following night. In the course of their conversation, Juliet addresses Romeo:
But trust me, gentleman, I’ll prove more true
Than those that have more cunning to be strange.
I should have been more strange, I must confess,
But that thou overheard’st, ere I was ware,
My true love’s passion: therefore pardon me,
And not impute this yielding to light love,
Which the dark night hath so discovered.
Romeo replies:
Lady, by yonder blessed moon I swear
That tips with silver all these fruit-tree tops –
But Juliet’s having none of this off-the-peg poetic rhapsodising and performative heart-on-sleeve wearing:
O, swear not by the moon, the inconstant moon,
That monthly changes in her circled orb,
Lest that thy love prove likewise variable.
Romeo asks what he should swear on, if he shouldn’t swear by the moon. Juliet bids him:
Do not swear at all;
Or, if thou wilt, swear by thy gracious self,
Which is the god of my idolatry,
And I’ll believe thee.
‘O, swear not by the moon, the inconstant moon’: but why is the moon inconstant? Juliet’s point is that there is a ‘new’ 
moon every month, as it passes through its different phases of waxing and waning. (Indeed, ‘month’ is derived from ‘moon’: ‘month’ is thus like ‘ninth’ or ‘tenth’, only with the moon replacing a numerical figure.)
But she’s not merely trying to stave off some superstitious astrological link between the thing sworn on and the topic of the pledge being sworn. True, swearing on something prone to inconstancy is probably not the best omen for a faithful long-term relationship (and it’s ironic that Romeo, perhaps the most constant and true male lover in all of Shakespeare once he meets Juliet, should now be a byword for a lothario or ladies’ man). But there’s a poetic as well as superstitious reason why Juliet wishes Romeo to refrain from swearing on ‘the inconstant moon’.
One of the key aspects of Romeo’s character development – perhaps the key aspect – is his transformation from a youth who is in love with the idea of being in love into a young man who truly is in love. He swaps the posturing for the real thing when he claps eyes on Juliet. And the moon was a common romantic symbol used by poets to reflect their love.
Indeed, around a decade before Shakespeare wrote Romeo and Juliet, Sir Philip Sidney, in his remarkable sonnet sequence Astrophil and Stella, addressed the moon about his own hopeless love for the woman, Stella:
With how sad steps, O Moon, thou climb’st the skies!
How silently, and with how wan a face!
What, may it be that even in heav’nly place
That busy archer his sharp arrows tries!
Sure, if that long-with love-acquainted eyes
Can judge of love, thou feel’st a lover’s case,
I read it in thy looks; thy languish’d grace
To me, that feel the like, thy state descries.
Then, ev’n of fellowship, O Moon, tell me,
Is constant love deem’d there but want of wit?
Are beauties there as proud as here they be?
Do they above love to be lov’d, and yet
Those lovers scorn whom that love doth possess?
Do they call virtue there ungratefulness?
It’s been suggested that this sonnet is tongue-in-cheek, and Sidney is sending up certain old tropes and associations which earlier courtly poets had made between lovers and moons. In other words, Sidney already knew what old hat this sort of thing was.
One suspects that Juliet feels the hollowness of such hand-me-down comparisons between lovers and the moon, and knows Romeo is better than this. He’s only just met Juliet, but the two have bonded instantly. But it’s a two-way relationship: indeed, their first great speech which they deliver to each other is actually a joint effort, which (pleasingly and cleverly) forms a sonnet, that most romantic verse form, when its lines are taken together. They develop each other’s imagery together, and inspire great poetry from each other. In the ‘swear not by the moon’ exchange, Juliet is once again tempering Romeo’s well-read (and well-worn) romantic sensibilities by making him see past the love poets’ words and to the truth of love. The fact that she tells him not to swear by anything shows that it’s not just the moon that’s the problem: it’s all poetic symbols being used as stand-ins for love.
But why is the ‘balcony scene’ a misnomer? Well, Shakespeare’s original audience wouldn’t have viewed it as such – in either sense of ‘viewed’. In the stage directions for Romeo and Juliet, Shakespeare writes that Juliet appears ‘above’, but he doesn’t mention a balcony. It would have been difficult for him to do so, since – perhaps surprisingly – Elizabethan England didn’t know what a ‘balcony’ was.
As Lois Leveen has noted, when the Jacobean travel-writer Thomas Coryat described a balcony in 1611, he drew attention to how foreign and exotic such a thing was to the English at the time. The balcony was most probably the invention of Thomas Otway in 1679, when the Venice Preserv’d author took Romeo and Juliet and moved its action to ancient Rome, retitling the play The History and Fall of Caius Marius. It was hugely popular, and, although Otway’s version is largely forgotten now, it did leave one lasting legacy: the idea of the ‘balcony’ scene.