The Moon and Tides
Abstract
Have 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.
Objective
Use secondary data to investigate the connection between the phases of the Moon and changes in the
tides.
Introduction
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 centimetres (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



Tides
High tide
Low tide



Tidal range
Gravitational force
Tidal bulge



Phases of the Moon
Spring tide
Neap tide
Questions
1.
What causes tides in Earth's oceans?
2.
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?
3.
What is the difference between a spring tide and a neap tide?
4.
If you were at the seashore and saw the Moon overhead, would the tide be high or low?
Bibliography




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.
Science Buddies Staff. "The Moon and Tides." Science Buddies, 6 Apr. 2021,
https://www.sciencebuddies.org/science-fair-projects/project-ideas/Astro_p009/astronomy/moonand-tides. Accessed 17 Sep. 2023.
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 coloured pencils (or you can use an online program, like Create a Graph)
Experimental Procedure
1. 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)
Table 1. An example data table for recording dates, heights, and tidal ranges for neap tides (you will need to add more rows to
your data table).
Tides During Full Moons (Spring Tides)
Date
Height of High Tide (cm)
Height of Low Tide (cm)
Tidal Range (cm)
Table 2. An example data table for recording dates, heights, and tidal ranges for spring tides (you will need to add more rows
to your data table).
2. 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.
(For local content try: https://www.timeanddate.com/moon/phases/australia/brisbane) 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 for the
current year. In 2011, for example, there was a first quarter moon on January 12th and a full Moon on January 20th.
3. 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".
4. Write the dates of all of the full moons for the entire year in your data table titled "Tides During the
Full Moons".
5. Go to the http://www.bom.gov.au/oceanography/projects/ntc/qld_tide_tables.shtml website,
which predicts the timing and height of tides in a variety of places.
a. Click the location you would like to see predictions for. Any location will work, so pick one
that interests you.
b. Download the PDF.
Figure 5. Download tide predictions for the entire year by clicking on the blue text besides your chosen
location.
6. Use the predictions in the 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.
a. Find the dates on the tide table that match the dates of the first quarter moons you wrote in
your data table.
b. 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.
You only need the first high
tide and low tide of each 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 centimetres (cm) not feet (ft). On some days, high tide will happen before low
tide. On other days, 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, it just means the tide was very low that day.
7. 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."
8. 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.
Equation 1:
Tidal Range (cm) = Height of High Tide (cm) – Heigh to flow Tide (cm)
9. Now it is time to analyse 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 or Excel.
a. Use one colour to plot tidal range vs. the dates of the first quarter moon (the data points
from Table 1).
b. Use a different colour to plot tidal range vs. the dates of the full moon (the data points from
Table 2).
c. Remember to include a legend on your graph explaining what the colours 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.
10. 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?