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.

Study the image to see how the distances between the Earth and the moon and sun change over the course of a month and a year, then answer the questions below.

Orbital Shape

The orbital paths of the moon around the earth and the earth around the sun are both elliptical. The point in the moon’s orbit where it is farthest from the earth is called apogee, while it’s closest approach is known as perigee. Earth is at its maximum distance from the sun at aphelion, and at its minimum distance at perihelion. These orbits produce cyclical changes in the height of the tides.


  1. Why does the distance from the earth to the moon or the sun affect the tides?
    Check Answer
    The tide generating force depends on the mass of an object and its distance. As the distance between the earth and the moon or sun changes, the tidal force each produces will vary.
  2. When will lunar tides be higher, at perigee or apogee? How often does each occur?
    Check Answer
    The lunar tides will be highest when the moon is closest to the earth, a point in the moon’s orbit that is called perigee. It takes the moon a month to complete an orbit, so perigee will happen about once a month.
  3. When will solar tides be higher, at perihelion or aphelion? How often does each occur?
    Check Answer
    The solar tides will be highest when the sun is closest to the earth, a point in the earth’s orbit that is called perihelion. It takes the earth a year to complete an orbit around the sun, so perihelion will happen once a year.