The Moon does not sit still in space. It travels around Earth at about 1.02 km/s — roughly one kilometer every second.
- A burn that sends the spacecraft home is measured relative to the Moon, but the resulting path through the Earth-Moon system depends on the Moon's own velocity.
- If the departure direction lines up with the Moon's orbital motion, the Moon's velocity adds to the spacecraft's velocity in the Earth frame. If it points the other way, it subtracts.
- This is why timing matters: the same burn, fired at a different point in the lunar orbit, produces a different Earth-return path.
- Mission planners chose the departure window so the geometry worked in the crew's favor — the Moon's motion contributed to the escape rather than fighting it.
- The Moon's orbital speed is not a fuel source, but it is a free velocity contribution that the burn plan can exploit.
Key idea: The Moon's motion around Earth is part of the departure problem, not just background scenery.