To add realism to our space adventures, we should consider that all destinations are orbiting something. Moons orbit planets, planets orbit suns, and solar systems orbit their galaxy’s center. If two objects are orbiting the same body, such as planets orbiting the same sun, the closer planet is orbiting faster. This means that the distance between two planets will change. They could be on the same side of the sun, opposite sides, or somewhere between.
This is different than fixed locations on a world’s surface, and as a result, we have considerable leeway when announcing how far apart two places are at a particular moment, and therefore how long the trip will take. Due partly to the considerable time such travel takes, a destination is chosen not because that’s where the target is now, but because that’s where it will be when the ship also gets there.
This sort of thing can cause believable problems for characters. What if they realize a critical event is taking place on another planet in a week, but due to the worlds’ current locations, it will take longer than that to get there? Or maybe they only have enough fuel to reach the destination if the trip is short, but they need to go now and don’t have funds to get more fuel. Now maybe they hatch a plan to earn some money. Such details make stories better than ones where everyone just gets on a ship and goes whatever distance they need without much comment or impact on their situation.
Should we decide how far apart our locations are first and then invent propulsion systems according to our story needs, or invent propulsion systems and then alter how far away locations are based on story needs? The latter seems sensible because locations in space aren’t fixed. If we need two locations closer or farther apart for a story, this needs no explanation. Conversely, it makes little sense to devise propulsion systems to go between certain distances when those will change anyway or you haven’t decided how far the characters need to travel.
The amount of time to travel between different objects in space has so much leeway to it that we don’t need the same sort of consistent precision that terrestrial travel may require. On land, we can fudge our numbers by writing, “Not drawn to scale” on a map. But in space, this isn’t even needed. Nothing is in a fixed place except each planet’s distance to the sun, and even that changes a bit depending on how circular its orbit is. And the technologies are imaginary, unlike the wooden ships from the previous chapter or the actual vehicles or animals from the chapter before that.
If you were hoping for a calculator like those in the previous chapters, there isn’t one because most world builders will get along fine without worrying about this. It also involves a level of mathematics that is admittedly beyond me, and possibly you. And no one can come along and say that it would really take fifty-eight hours, not fifty, to travel between two invented planets in two imaginary solar systems, at a given time of the year (from the starting planet), at warp seven, especially since warp drives don’t exist. This is one area where SF beats fantasy handily.