When calculating composite midpoint charts, you have the option of basing the houses for the chart on either the Ascendant or the Midheaven. The reason this is important is because composite midpoint charts are not “real” charts. This means they don’t represent a time and place like most other charts (specifically radix/natal charts).
Since they are a calculated chart based on the midpoints of two (2) other charts, sometimes the calculations of the houses are incoherent. For instance, it’s possible the midpoint of the 2nd house doesn’t follow the midpoint of the 1st house in zodiacal order.
In order to account for this, LUNA needs to know which of the angles you want to use as the base of the houses:
Ascendant
The midpoint of the two (2) ascendants will be used as the ascendant of the composite chart. All other midpoints between the houses will follow in zodiacal order.
It’s possible the 2nd house (and all others) of the composite chart could actually be the composite midpoint of a totally different house.
Midheaven
The midpoint of the two (2) midheavens will be used as the midheaven of the composite chart. All other midpoints between the houses will follow in zodiacal order.
It’s possible the 11th house (and all others) of the composite chart could actually be the composite midpoint of a totally different house.
Good to Know
Please note that although it’s possible to calculate two (2) different composite midpoint charts based on this setting, very often you will see the same chart regardless of which setting you choose. You can experiment with this setting on all your composite midpoint charts to see which ones are affected by it.