DM's Guide to Immortals: Home World and System

The BECMI home world is modeled after our own planet, roughly 135 million years ago.  The land masses depicted on the map of the Known World in the Master DMs book are based on the supercontinent of Pangea, once it started to break apart.*

*see also this post on Thorfinn Tait's blog

Earth during the Late Jurassic

This permits us to assume a 24 hour day (although days were actually 23 hours long in the Late Jurassic, resulting in closer to 381 days per year), four seasons in temperate regions, and 12 months per year, based on the 12 phases of a single moon.

Although the planet was envisioned as a predecessor to our own, during a bygone "Age of Magic", this results in obvious discrepancies.  It might instead be considered a parallel world, located in an alternate reality.

Similarities to our own world, including names from ancient myths for various Immortals and specific cultures, could be explained by a magical connection with our own world's ancient past.

Home System:

The star and other bodies of the solar system containing the PCs homeworld are similar to that with which we are familiar, with a few exceptions:

A planet named Damocles orbits the Sun between the orbits of Mars and Jupiter.  Damocles is fated to be destroyed, creating the asteroid belt, the planets Mercury and Pluto, and the retrograde moons of Jupiter and other planets.

Beyond the orbit of Neptune, lies another planet, slightly larger than Mars, named Charon.

The Milky Way:

The stars closest to man's are Alpha Centauri (4.3 LY), Sirius (8.6 LY), Epsilon Eridani (10.7 LY), Procyon (11 LY), 61 Cygni (11.1 LY), Tau Ceti (11.2 LY), and Altair (15.7 LY).

The closest civilization able to use magic (counting only those outside the home system of humanity) is on several planets orbiting Epsilon Eridani.