Twice the number of hours in a typical earth day is 24*2=<<24*2=48>>48 hours.
Eight hours less than twice the number of hours in a typical earth day is 48-8=<<48-8=40>>40 hours.
If the distance between these two planets is 4,000 parsecs, then the average speed on this voyage was 4000/40=<<4000/40=100>>100 parsecs per hour.
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