The Vikings could find their longitude once they had made landfall.
Here are the simple tools needed.

1. A pendulum about 14 1/2 inches long.

2. Four different strings of counting beads. So you could count 30, 60, 60, 24.

3. A Viking shield with five holes that line up with the five brightest stars in the night sky. And 365 marks around the edge to indicate the days of the year.

4. They would need to keep count of what day of the year it is.

Here are the steps to find longitude when on land.

1. The first night, using the north star, set up two plumb bobs on strings such that the two strings line up with north.

2. The next day watch the suns shadow on the two strings until the shadow of one of the strings hits the other.

3. Using the pendulum, count the number of seconds until noon tomorrow. Use the three strings 60, 60, 24.

4. Using the same pendulum and starting at noon on the second day, count half the number of seconds of a full day. The way to do this is to use the 30, 60, 24 strings and count the same number of minutes and hours as a full day. This gives you local midnight.

5. At local midnight, you line up the five bright stars with the holes in the shield. Noting the day of the year marked around the edge of the shield.

6. The difference between the day of the year as read off from the edge of the shield, and the day of the years as found by counting the days, gives you the number of days from home you are. In modern times we call this degrees from home.