Either GNU Public License v2 or later, or Swiss Ephemeris Professional License.

The choice must be made before the software developer distributes software
containing parts of Swiss Ephemeris to others, and before any public service
using the developed software is activated.

If the developer chooses the GNU GPL software license, he or she must fulfill
the conditions of that license, which includes the obligation to place his or
her whole software project under the GNU GPL or a compatible license.
See http://www.gnu.org/licenses/old-licenses/gpl-2.0.html

If the developer chooses the Swiss Ephemeris Professional license, he or she must
follow the instructions below and purchase the Swiss Ephemeris Professional
Edition from Astrodienst and sign the corresponding license contract.

For more information about licensing, see the Astrodienst website.
<http://www.astro.com/swisseph/>