What Kind of Tides Occur When the Moon & Sun Are at Right Angles?


Hi. I'm Sean Tvelia, and we're here at Suffolk County Community College to determine what tides occur when the moon is 90 degrees away from the sun relative to the Earth. Now, to understand tides, you need to understand a little bit about gravity. Gravity is an attractive force that is caused by mass. And, simply stated, the more massive you are, the more gravity you have. As a result of the gravity of the moon, the Earth's surface actually rises and falls as the Earth rotates beneath the moon. Now, although the moon is a relatively large object, there is a much larger object in our solar system and that's the sun. And, because it is so massive, the gravitational influence of the sun is also experienced on the Earth. So, when the moon is in line with the sun, what we would call inferior conjunction in astronomy, we deal with the influence of not only the moon but also the sun, causing a spring time, which is when the high tide is much larger than low tide. The moon revolves around the Earth when it is about 90 degrees away from the sun. That means the gravitational influence from the moon in one direction is canceled by the gravitational influence of the sun in another direction, meaning the difference between high tide and low tide is much less dramatic than it is when the moon is directly in front of the sun. This produces what we call a neap tide. A neap tide occurs when the moon is 90 degrees away from the sun relative to the earth.

Professor Sean Tvelia is an Associate Professor of Geology at Suffolk County Community College.