In Portuguese there is an expression that says:
"Essa pessoa é de lua."
Literally "this person's mood changes according to the moon", which means that nobody can predict that person's mood.
Is there an equivalent expression or similar to this in English?
When someone is angry one day and very peaceful the next day...
Such a person is said to be mercurial or capricious.
mercurial: likely to change your mood or opinion unexpectedly
capricious: suddenly and unexpectedly changing your opinion or behaviour without any good reason
An equivalent expression might be, "He/she is as changeable as the weather."
Though there are many individual words that might capture the Portuguese phrase, common English phrases for variability are:
an "unpredictable and volatile temperament" comes to mind.
volatile - adj. (of persons) disposed to caprice or inconstancy; fickle; mercurial.
Examples:
"Warburg grew up in a conservative home environment. Early on he demonstrated an unstable, unpredictable and volatile temperament."
"She's unpredictable as weather."
"Agrippa's kinsman already alluded to above, did somehow run afoul of Nero's unpredictable and volatile temperament"
" you may need to wear protective gear if your dog has an unpredictable and volatile temperament"
There is also a traditional comparison (credit to tchrist) which fits perfectly: "as changeable as the moon" - "Her mood is changeable as the moon"
- The frequent and obvious changes in the position of the moon, in the area of its enlightened surface, and in the quantity of light reflected there from the earth, are so obvious that they are proverbial; and to say of anything that it is as changeable as the moon, is to mark it with a note of instability in the extreme.
To add a gloss to the excellent answers offered by members of our community of wordsmiths, I propose to offer the word moonstruck which seems to cover the OP's request for a luna reference to the Portuguese expression.
moonstruck (Merriam-Webster): affected by or as if by the moon: as
a: mentally unbalanced b: romantically sentimental c: lost in fantasy or reverie (Merriam-Webster)
Note: As an additional answer for the OP's consideration, I offer the words lunatic and lunacy.
From M-W's definition of moonstruck one might infer that such a person is likely to exhibit mood behavioral patterns of the type the OP characterizes as "unstable and [changeable] at random". The M-W's definition of moonstruck enables us to link the OP's reference to moon and then cross-reference it to the word lunatic which Wikipedia informs us is an "informal term referring to a person who is considered mentally ill, dangerous, foolish or unpredictable (my emphasis), conditions once attributable to lunacy...[which] derives from [Latin] lunaticus meaning 'of the moon' or moonstruck (my emphasis)".
The word fickle means
likely to change your opinion or your feelings suddenly and without a good reason (Cambridge)
There seem to be a few similes using it, and the best one may depend on the culture of your hearers. As a British English speaker, I like
as fickle as the weather
(7.8k ghits) but people from English-speaking countries which aren't small temperate islands may have difficulty associating fickleness with the weather.
Such a person can be called moody/temperamental.
M-W:
moody adjective
: having moods that change often
She's a moody woman—she can be happy one minute and angry the next.
temperamental adjective
: unpredictable in behavior or performance
The actor is known for being temperamental.
The Jargon File, a dictionary of slang used in certain English-speaking information technology subcultures, defines "phase of the moon" similarly to the Portuguese usage that you describe:
Used humorously as a random parameter on which something is said to depend. Sometimes implies unreliability of whatever is dependent, or that reliability seems to be dependent on conditions nobody has been able to determine. “This feature depends on having the channel open in mumble mode, having the foo switch set, and on the phase of the moon.”
So in English, one may say “Whether he'll be in a mood to accept that deal depends on how well he slept, what he ate last night, the phase of the moon...”
But when applying moon metaphors to women, take care to avoid negative stereotypes about the menstrual cycle.
If the changes are with regard to commitment, resolve, opinion, etc, then "flaky" is what you're looking for (an unreliable person).
The closest adjectives which reflect "this person's mood changes according to the moon" are probably lunatic and moonish.
Lunatic is probably much stronger than your intented meaning, but moonish, according to the Merriam-Webster, simply means:
influenced by the moon
It should be noted, however, that the Oxford Dictionary reports moonish as obsolete.
I would say that their mood changes with the tide. That (indirectly) keeps the lunar reference.
One saying I’m familiar with is something similar to:
His mood changed like quicksilver.
because of the properties of liquid metal mercury, also known as quicksilver.
For example, the Longman Dictionary of Contemporary English writes in its definition of quicksilver:
literary something that is like quicksilver changes or moves quickly in a way that you do not expect
His mood changed like quicksilver.
If extreme changes were what you had in mind, perhaps werewolf would suit:
noun were·wolf \ˈwir-ˌwu̇lf, ˈwer-, ˈwər-\
in stories : a person who sometimes changes into a wolf especially when the moon is full
Werewolves become notoriously disagreeable in the light of the full moon.
