Study the first 3 graphics and then answer the questions that follow each. Once you are sure you understand how the lunar day, the position of the sun and the moon, and orbital variations affect the tides, then try to apply this knowledge, and Be the Captain.

Lunar Day

Position of the
Sun and Moon

Orbital Shape

Be the Captain

Knowing when and how much the tides will rise and fall each day is important to beachcombers, mariners, fishermen, and the people who operate seaside industrial and commercial facilities. Miscalculating the arrival and size of the tides can have expensive, even deadly, consequences.

Predicting the tides is complicated because the tides are affected by many factors. In this activity, you’ll have a chance to consider some of the most important and universal influences, those due to the movements of the sun, the moon, and the earth. In reality, a number of local and transient factors, such as the shape of the coastline, the flow of currents, and the weather, must also be taken into account.

Play the animation and observe the relationship between the lunar day and the time, then answer the questions below.

Lunar Day

Over the course of what is called a lunar day, a spot on earth that is directly beneath the moon rotates once until the moon is again exactly overhead. Because the earth spins in the same direction that the moon orbits the planet, the cycle is slightly longer than a regular day -- it takes 24 hours and 50 minutes.


  1. How much time passes between one high tide and the next?
    Check Answer
    The tides are caused mainly by the gravitational attraction between the moon and the earth, so the spacing between tides will depend on the length of the lunar day. Since the lunar day is 24 hours and 50 minutes long and the earth rotates through two tidal bulges in that time, high tides will be spaced 12 hours and 25 minutes apart.
  2. How much time passes between low tide and the next high tide?
    Check Answer
    High tides are 12 hours and 25 minutes apart and are separated by a low tide. So low tide must come 6 hours and 12.5 minutes after one high tide and before the next.