The Moon and Tides—Lab Write-up

Abstract Have you ever wondered what causes the tides in Earth’s oceans? In this astronomy lab, you will discover the answer for yourself! You will investigate how the Sun and Moon control tides in Earth’s oceans. Objective Use 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. 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 centimeters (cm) deep at high tide and 25 cm deep at low tide, then the tidal range is 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 that it is on the side of Earth that is farther away 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 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 away 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 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 lab, 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 • What causes tides in Earth’s oceans? • When do 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? Credit Science Buddies Staff. (2017, July 28). The Moon and Tides . Retrieved December 1, 2017 from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Astro_p009/astronomy/moon- and-tides Name: Date: Data Tables 1. In this lab, you will gather data from two different sources. To keep your data organized, use the two tables on the following page. The first data table is for tides during the first quarter moon (neap tides). The second data table is for tides during the full moon (spring tides). You may or may not use all of the provided rows, depending on the number of neap tides and spring tides in a given year. If a row isn’t necessary, please type “N/A” in each text box in that row.