Here's a problem from an astronomy book "the top of the mountain 1000 m in height can just be visible seen from a ship approaching the land where the mountain is situated. . If the observer's eye is 30 m above sea level. Then how far the ship is from the mountain? " The main problem is i don't know to measure the horizon distance from a mountain top when the observer itself has some altitude. . Plz suggest me what correction should i need here in the formula of horizon distance "root over 2RH " to determine this distance?