Celestial Navigation: An Example

At 14^hr GMT, Sirius is on the meridian at 0^o longitude.  Earth rotates 15^o/hr. The navigator sees Sirius just rising on the Eastern horizon. Therefore Sirius is 90^o from the meridian, as seen from the ship.  The ship is located 90^o west of Greenwich, England, and therefore is at a longitude of 90^o west.  It will take 90^o/15^o/hr, or 6 hours for Sirius to reach the meridian as seen from the ship. The ship, at Lat. -30^o, Long. 90^o west, is closer to Easter Island: Lat. -28^o, Long. 100^o west than to Valparaiso: Lat. -33^o, Long. 72^o west.