If the direction of the earth’s rotation (at a speed depending on latitude) was from the destination towards the source, then the destination would move fractionally towards the source between the times of the particles leaving and arriving, ie. the destination would be moving towards the particles while they were in transit. So they would have to travel less far than the simple physical measurement between the source and destination would suggest.
Therefore the particles could arrive sooner and so seem to travel faster than would otherwise be the case.
JOHN SLEE
Hopetoun Terrace,
Gullane, East Lothian