What two birth dates can tell you
A birth date is enough to place nine of the ten planets astrologers use. The Sun moves about a degree a day, Venus and Mercury a little more or less, and everything from Jupiter outward barely moves at all. So two dates already give you the main threads of a synastry chart: whether your Suns get along, how Venus and Mars line up for attraction, and whether Mercury makes conversation easy.
What it can’t tell you
Without a birth time there is no Ascendant, which is the sign rising in the east at the moment of birth and changes every two hours or so. There are no houses either, so the calculator skips the overlays that show where one person’s planets land in the other’s life. The Moon is placed for noon and could be several degrees off. These are real gaps, and the result says which aspects they affect.
Same sign, different answer
People are often surprised that two friends who share a sign get different results with the same partner. Sun signs span 30 days; the planets underneath don’t line up the same way across that month. A Venus that changed sign on the 10th, or a Mars that was retrograde for part of it, changes the picture. That is the main thing this calculator adds over a sign-by-sign table.
When to use the full chart
If both of you know your birth times, the main calculator on the home page uses them along with birth places to add the Ascendant, an exact Moon and both directions of house overlays. If only one of you knows, it still uses that one time and fills in the rest at noon.
Questions people ask
How can a birthday tell you anything about compatibility?
Your birth date fixes where the Sun, Mercury, Venus, Mars and the slower planets were that day, to within a degree or so. Comparing those positions between two dates gives most of a synastry chart. Only the Moon, the Ascendant and the houses really need the time.
Is this just zodiac sign compatibility?
No. Sign compatibility compares your two Sun signs. This compares nine planets each at their actual degrees, so a Taurus born on April 22 and one born on May 18 get different results with the same partner.
How accurate is it without a birth time?
The Sun is within half a degree, Mercury through Pluto within a degree or less. The Moon moves about 13° a day, so it can be off by up to 7°; its aspects are shown but treat them as approximate.
What does adding the birth time change?
You gain the Ascendant, a precise Moon and the house overlays, which show where each person’s planets land in the other’s life. Use the full calculator on the home page when you have times and places.