Rotation of the Earth and Motion of the Earth Around the Sun

You may have already determined the length of the day and the length of the year in your world. If not, these are the first things to determine in creating the astronomy of your world. It is best to stay close to the values of the present day Earth if you want an Earth-like world. Throughout the Quick Start guide I will assume that an Earth-like astronomy is being generated, since picking values far from those of the real world may require a lot of astronomical knowledge to ensure a realistic outcome.

The first value to record is the length of the day in your world. How long is it from noon on one day until noon on the next day? If you have elliptical orbits (as we have on our Earth) this value will vary throughout the year, so pick the average value. You should also give some thought to how time will be measured in your world (keep in mind that sticking to a system close to ours will probably be easier on your players). You might want to read about historical timekeeping devices like sundials and waterclocks.

Next record the duration of one orbit of your Earth around the Sun relative to the stars. Assuming the direction of the rotation axis of the Earth moves very slowly (the Earth wobbles like a spinning top, with a circuit being made every 26,000 years or so) relative to the length of the year, this duration will correspond to 1 year in civic life since the seasons will repeat with this duration.

If your year length has a fractional day, you will need to modify some year lengths to keep your calendar in step with the seasons. The calendar we use today inserts a leap day every 4th year (most of the time - not always!) to keep the seasons in step with the calendar, but that is not the only way this can be done. Extra days can be inserted into the calendar when needed or at the whim of a ruler. These extra days may or may not be part of the count of days in a month. Multiple days can be added at once.

Now you should decide what the inclination of the Earth's axis is. Using a value close to the 23.4 ° value of the actual Earth's axis is probably best unless you want a climate very different from Earth's. It is this tilted axis that accounts for the seasons on Earth, with northern summer occuring when the north pole of the Earth is pointing toward the Sun, and northern winter occuring when the north pole of the Earth is pointing away from the Sun (the southern seasons are reversed since the south pole points in the direction opposite to the north pole). This value will be used later to find the path that the Sun, Moon and planets will appear describe in the sky of your Earth.

At this point you have enough information to begin working on a solar calendar, which will be very useful for societies to regulate agriculture, determine when it is safe to sail, roughly predict the weather and so on. You might want to wait until after you generate the data for one (or more) Moons to create your calendar in detail, but it is not too soon to start thinking about month names and deciding what you will do to keep the calendar synchronized with the seasons on your Earth. You can also think about having a unit of time less than a month but greater than a day, but again you might want to wait until you have thought about a Moon or Moons before determining this.