Composite
Two charts read as one
Two charts merged into one by taking the midpoint of every planet, read as the chart of the relationship rather than of either person. Different question from synastry, and most people use both.
What a midpoint composite is doing
Every planet in the composite sits exactly halfway between where that planet falls in each of the two charts, going the short way round the circle. That is all it is: an average, computed rather than interpreted. What makes it interesting is that the result behaves like a chart, so the ordinary rules about aspects apply to it.
If you want the other half of the picture, the aspects between the two charts rather than the merge of them, that is compatibility.
Both people need birth details
A composite needs two charts, so it needs two sets of details. Times are optional, but without both of them the angles and houses are withheld rather than guessed.
Questions about composite charts
The four that come up most.
One chart made from two, by taking the midpoint of each pair of planets. Your Sun and their Sun give a composite Sun, and so on through the chart. It is read as a chart of the relationship itself rather than of either person in it.
Synastry compares two charts and looks at the aspects between them: your Venus to their Mars. A composite merges the two into a single chart and reads that. They answer different questions, and most astrologers use both.
Because a composite Ascendant is the midpoint of two Ascendants, and an Ascendant cannot be computed without a birth time. If either person’s time is missing, the composite angles would be the midpoint of a guess, so they are withheld rather than shown. The planets do not depend on the time and are always given.
No, and anything that tells you otherwise has made an error. A midpoint of two positions is a point, not a moving body, so it has no speed and cannot be direct or retrograde. This calculator leaves the field empty rather than inventing one.