# When the time of day for a certain ship at sea is 12 noon, the time of day at the prime meridian (0° longitude) is 5 P.M. What is the ship’s longitude?

##### 1 Answer

#### Explanation:

The trick with this problem is to figure out the position of the ship **in relation to** the Prime Meridian, that is, *on which side* of the Prime Meridian, **East** or **West**, you can expect to find the ship.

As you know, **longitude** expresses the position of a point on Earth's surface in terms of how many *degrees* East or West relative to the Prime Meridian that point is located.

The Prime Meridian is assigned the value of **complete rotation**, i.e.

This means that you can find Earth's rotation angle **per hour** by using

#1color(red)(cancel(color(black)("hour"))) * (360^@)/(24color(red)(cancel(color(black)("hours")))) = 15^@"/hour"#

So, the difference between the time at the Prime Meridian, which is given as

This means that the Earth rotated by a total of

#5color(red)(cancel(color(black)("hours"))) * (15^@)/(1color(red)(cancel(color(black)("hour")))) = 75^@#

But what is the ship's longitude, **East** of the Prime Meridian or **West** of the Prime Meridian?

To figure this out, you can use the fact that the Sun *rises* in the **East** and *sets* in the **West**, which is equivalent to saying that the Earth **rotates** from West to East.

Notice that the time in the ship's position is **behind** the time at the Prime Meridian, i.e. the position of the sun in the sky at the ship's location corresponds to what the position of the sun at the Prime Meridian was **five hours earlier**.

This means that the Earth will rotate for another five hours until the position of the sun in the sky see by the ship will match that of the sun in the sky at the Prime Meridian **five hours earlier**.

Since the Earth is rotating from West to East, it follows that the ship must be **west** of the Prime Meridian, at a longitude of