How to do a synastry chart when you do not know their birth time

You have your own birth time. You do not have theirs, and asking is either awkward or impossible. This is the most common situation in relationship astrology and almost nothing written about it is honest.

What a birth time actually buys

Three things, and only three.

The rising sign, which changes roughly every two hours as the whole wheel turns about one degree every four minutes. The twelve houses, which are set by the rising sign and decide which area of life every planet lands in. And the Moon's exact degree, because the Moon moves about thirteen degrees a day and can change sign entirely between breakfast and bedtime.

Everything else in a chart is fixed for the whole day. The Sun barely moves. Venus, Mars, Jupiter and Saturn barely move.

So what still holds

More than you would think, and specifically the parts that matter most for a relationship.

Their Venus sign and their Mars sign are solid, which means the whole attraction picture holds. Their Sun is solid. Their Mercury is solid, so how they think and argue holds. Saturn, Jupiter and the outer planets are solid. Contacts between your chart and any of those are real, and they carry most of what synastry is for.

You lose the house overlay and you lose the Moon's precision. You do not lose the relationship.

What you should refuse to accept

Any tool or astrologer that gives you their rising sign without their birth time is making it up.

The standard convention, and what this site does, is to compute an unknown time as noon. That is a reasonable default and it keeps the slow placements honest. But a rising sign derived from a guessed noon is not an approximation, it is fiction, and so is any house placement built on top of it. The correct behaviour is to say so and leave it out.

The same applies to a Moon sign near a boundary. If someone was born on a day when the Moon changed sign, the honest answer is that it could be either and here is what each would mean, not a confident single sign.

How to ask for it, if you want to

Worth saying, because people assume this is a strange thing to ask and it usually is not.

What time were you born is a surprisingly easy question. Most people either know or find it funny that they do not. If you would rather not ask directly, birth certificates carry it in most countries, and a parent's memory of late evening is genuinely workable as long as you treat it as approximate rather than exact.

And if none of that is available, run the reading anyway. A Venus and Mars picture with no houses is still a great deal more than a sun sign.