The Moon and Tides
AbstractHave you ever wondered what causes the tides in Earth's oceans? In this astronomy project, you will discover the answer for yourself! You will investigate how the Sun and Moon control tides in Earth's oceans.
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ObjectiveUse data from the U.S. Naval Observatory and National Oceanic and Atmospheric Administration (NOAA) to investigate the connection between the phases of the Moon and changes in the tides.
If you have been to the seashore, you may have noticed that the water level changes throughout the day. Sometimes the water is high, and sometimes the water is low. This pattern of rising and falling sea level is known as the tides. When the water level is high, we call it high tide. When the water level is low, we call it low tide. In between high tide and low tide, tidal currents move water toward or away from the shore. Scientists have a special name for the difference in water level between high tide and low tide. They call it the tidal range. For example, if the water is 75 centimeters (cm) deep at high tide and 25 cm deep at low tide, then the tidal range is 50 cm (75 cm - 25 cm = 50 cm). Figure 1, below, shows high tide and low tide at the Bay of Fundy, which is located between the Canadian provinces of New Brunswick and Nova Scotia.
Figure 1. The Bay of Fundy has one of the largest tidal ranges in the world. The photo on the left shows the coast at high tide. The photo on the right shows the exact same place at low tide. The tidal range is the difference in water height between the left picture (high tide) and right picture (low tide). (Image credit: Wikimedia Commons user Samuel Wantman, 2013)
Tides result from a gravitational tug-of-war between Earth, the Moon, and the Sun. The Moon's gravitational force is slightly stronger on the side of Earth that is closer to the Moon than it is on the side of Earth that is farther from the Moon. This small difference in the strength of the Moon's gravitational force pulls Earth's oceans into an elliptical shape. As a result, water "piles up" in the oceans on opposite sides of Earth, as shown in Figure 2, below. The two long ends of the stretched-out, elliptical shape are called the tidal bulge. High tides happen close to where each end of the tidal bulge is located. Low tides happen in between, where the ocean is thinner, because of the pile-up of water at the tidal bulge.
Figure 2. Without the Moon, Earth's outline would look like a circle if you were looking down on the North Pole. But because Earth does have a moon, differences in the gravitational forces exerted by the Moon stretch the oceans into an elliptical shape, creating a tidal bulge that sticks out beyond where the ocean would be if Earth didn't have a moon. Note: This image is not to scale. In reality, the tidal bulge is much smaller than shown here. The Moon is also much farther from Earth.
The tidal bulge always points near an imaginary line connecting Earth and the Moon. As Earth rotates, different parts of Earth are affected by the tidal bulge. This is what makes the tides rise and fall. In many places, there are two high tides and two low tides each day. Some places have two high tides and two low tides, but one of the pairs of high tides and low tides has a smaller tidal range than the other. In other places, there is only one high tide and low tide each day. These differences are often controlled by the shape of the seafloor and coastline in a particular place.
The Sun also creates a tidal bulge because the Sun's gravitational force is a little bit stronger on the side of Earth that faces the Sun than it is on the side of Earth that faces away from the Sun. Depending on where the Moon is in its orbit around Earth, the tidal bulges made by the Moon and the Sun may line up in the same direction, or partially cancel one another. We can tell where the Moon is in its orbit by the phases of the Moon. The Moon has four primary phases: new, first quarter, full, and third quarter. As shown in Figure 3, below, the tidal bulges created by the Sun and the Moon line up in the same direction at the new moon and full moon, when the Sun, Earth, and Moon form a line. This creates a spring tide. Spring tides have a larger-than-normal tidal range (higher high tides and lower low tides). When the Moon is at first quarter or third quarter, the Sun, Earth, and Moon form an "L" shape, and the tidal bulges of the Moon and the Sun make the shape of a plus sign (+). This creates a neap tide. Neap tides have a smaller-than-normal tidal range (lower high tides and higher low tides).
Figure 3. (Top) Spring tides happen when the Sun, Earth, and Moon form a line, so the tidal bulge created by the Sun (shown in light yellow) and the tidal bulge created by the Moon (shown in light blue) both line up with each other. Spring tides have a bigger tidal range than normal. (Bottom) Neap tides happen when the Sun, Earth, and Moon form an "L" shape. Neap tides have a smaller tidal range than normal. Note: This image is not to scale. The Moon and Sun are much farther away from Earth than shown here. The Sun is also much larger.
In this astronomy science project, you will compare the tidal range at the full moon and first quarter moon to see just how much the phases of the Moon affect the tides in Earth's oceans.
Terms and Concepts
- High tide
- Low tide
- Tidal range
- Gravitational force
- Tidal bulge
- Phases of the Moon
- Spring tide
- Neap tide
- What causes tides in Earth's oceans?
- When do the tidal bulges created by the Moon and Sun line up and increase tidal range? When do they form a "+" shape and partially cancel each other?
- What is the difference between a spring tide and a neap tide?
- If you were at the seashore and saw the Moon overhead, would the tide be high or low?
- Stuart, F.L. (2000). Boat Safe Kids! How Does the Moon Affect Tides? Nautical Know How, Inc. Retrieved August 16, 2014.
- Moon Connection Staff. (n.d.). Understanding the Moon Phases. MoonConnection. Retrieved August 16, 2014.
- OneGeology Kids Staff. (n.d.). Tides. Retrieved August 16, 2014.
For help creating graphs, try this website:
- National Center for Education Statistics, (n.d.). Create a Graph. Retrieved June 25, 2020.
Materials and Equipment
- Computer with internet access
- Lab notebook
- Graph paper and colored pencils (or you can use an online program, like Create a Graph)
- In this science project, you will gather data from two different sources. To keep your data organized, make two data tables like Tables 1 and 2, below, in your lab notebook. One data table will be for tides during the first quarter moon (neap tides). The second data table will be for tides during the full moon (spring tides). You will need about 13 rows in each data table.
|Tides During First Quarter Moons (Neap Tides)|
|Date||Height of High Tide (cm)||Height of Low Tide (cm)||Tidal Range (cm)|
|Tides During Full Moons (Spring Tides)|
|Date||Height of High Tide (cm)||Height of Low Tide (cm)||Tidal Range (cm)|
- It is time to start collecting data! Search online for a "moon phases calendar" for the current year. You will find many websites that display a table of moon phases and dates throughout the year. Figure 4 shows an example.
Figure 4. Phases of the Moon calendar. For this project, you will need the dates of the first quarter and full moons. In 2014, for example, there was a first quarter moon on January 8th and a full Moon on January 16th.
- Write the dates of all of the first quarter moons for the entire year in your data table titled "Tides During the First Quarter Moons".
- Write the dates of all of the full moons for the entire year in your data table titled "Tides During the Full Moons".
- Go to the NOAA Tide Predictions website, which predicts the timing and height of tides in a variety of places.
- Click the state or island you would like to see predictions for. Any state or island will work, so pick one that interests you.
- Next, click on the name of a station. This will open a page with predicted tide heights and times for that particular station.
- Download the tide predictions by clicking the orange "Annual PDF" button in the "Published Tide Tables Formats" box (outlined by a red box in Figure 5, below).
On the upper right-side of the NOAA tide predictions website there is a link to download the annual tide predictions table.
Figure 5. Download tide predictions for the entire year by clicking on the orange "Annual PDF" button under the "Published Tide Table Formats" heading in the upper right corner of the website.
- Use the predictions in the "Annual PDF" to fill in the "Height of High Tide (cm)" and "Height of Low Tide (cm)" columns of your "Tides During First Quarter Moons" data table.
- Find the dates on the tide table that match the dates of the first quarter moons you wrote in your data table.
- Write the height of the first high tide and first low tide for each of the dates in your data table. See Figure 6, below, for an example of where to find this data.
For this experiment, you'll only need to record the first high and low tide of each day from the annual tide predictions table. These can usually be found at the top of the cell for a specific day.
Figure 6. In your data table, write the height of the first high tide and first low tide of each day with a full moon or first quarter moon. Be sure to record height in centimeters (cm) not feet (ft). On some days (like February 6, 2014), high tide will happen before low tide. On other days (such as March 16, 2014), low tide will happen first. So make sure you write tide heights in the correct columns of your data table. Do not worry if you see a negative number for a low tide (such as -9 cm on March 16, 2014); it just means the tide was very low that day.
- Repeat the procedure in step 6 to fill in the "Height of High Tide (cm)" and "Height of Low Tide (cm)" columns of your second data table, "Tides During Full Moons."
- All of the columns in your data tables, except for the last one, should now be filled in. Finish your data tables by calculating the tidal range for each day, using Equation 1, below. Write the number in the "Tidal Range (cm)" columns.
- Now it is time to analyze your data. Make a graph with the date on the x-axis (horizontal) and tidal range on the y-axis (vertical), as shown in Figure 7, below. You can make a graph by hand, or use a program such as Create a Graph.
- Use one color to plot tidal range vs. the dates of the first quarter moon (the data points from Table 1).
- Use a different color to plot tidal range vs. the dates of the full moon (the data points from Table 2).
- Remember to include a legend on your graph explaining what the colors mean.
Figure 7. Example of a graph that plots the date of the first quarter or full moon on the x-axis and tidal range on the y-axis.
- What differences do you observe between the tidal ranges on days with first quarter moons and the days with full moons? What causes these differences?
Ask an Expert
- This experiment uses data generated by mathematical models of the tides. You could also do this project using your own observations. If you live near a marina, see if they have a tide gauge that you can use to make readings. You could also use a piece of wood secured to a bulkhead to track the height of the tide each day. To track the phases of the moon, keep a lunar logbook. Go outside each evening and draw the moon's appearance. How closely do your recorded measurements match the predicted data?
- If you do not live near the ocean, but want to use tide measurements (not predictions), you can use historical tide measurements as the source of data for your project. An internet search for historical tide measurements will bring up sources where you can find that type of information.
- In addition to the gravitational pulls of the Moon and Sun, the shape of the seafloor and local geography affect the tides. Try doing this project with data from a few different places, such as an island in the Pacific Ocean, a state that borders the Gulf of Mexico, and a state that borders the Pacific Ocean. What differences in tidal range do you observe among these different locations? How might you explain the patterns at the different locations you selected?
- In this science project, you tracked tide height during the first quarter moon and full moon. How do the other phases of the moon affect the tide? Try an experiment comparing tidal range and lunar phase every day for a full lunar month. Can you observe a cycle or pattern?
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