What is high tide and low tide?

As a marine scientist with a focus on oceanography, I can explain the phenomenon of high tide and low tide. Tides are the regular rise and fall of sea levels caused by the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth. High tide is when the sea level is at or near its highest point, occurring roughly every 12 hours and 25 minutes. This happens because the gravitational pull of the Moon (and to a lesser extent, the Sun) causes a bulge in the ocean on the side facing the Moon. The diagram you mentioned illustrates this concept well. The Moon's gravity pulls water toward it, creating a bulge or high tide on the side of the Earth facing the Moon. Low tide, on the other hand, is when the sea level is at or near its lowest point, also occurring roughly every 12 hours and 25 minutes. There are two low tides for every high tide. Low tide occurs on the opposite side of the Earth from the Moon because the centrifugal force from the Earth's rotation counteracts the Moon's gravitational pull, causing a bulge on the opposite side as well. Additionally, a third bulge is created due to the combined effects of the Moon and the Sun when they are aligned, which can lead to higher high tides or lower low tides, known as spring tides.

The following diagram shows how the moon causes tides on Earth: In this diagram, you can see that the moon's gravitational force pulls on water in the oceans so that there are "bulges" in the ocean on both sides of the planet. The moon pulls water toward it, and this causes the bulge toward the moon.