By Joseph Kramer
As we continue to explore the relationship between the Moon and the Earth, I want to talk about actual physical impacts that we get to experience. One of the most prevalent examples is the tides but another one we will explore is the Moons orbit and rotation. Most people associate tides with water. Tides are most measurable in water due to its liquid characteristics, but the same gravitational relationship that causes tides in these large bodies of water has impacts on solids and gasses as well.
Every piece of matter has a gravitational relationship with every other piece of matter. It is fun to think about how our bodies have this attraction to distant objects such as Jupiter, even though it is not strong enough to have any significance from our perspective. It is as equally creepy as it is romantic to acknowledge there is a gravitation relationship between yourself, and your significant other or your crush. The strength of a gravitational force between two objects can be measured by dividing the product of the two masses by the square of the distance that separates them. Gravity does not just work on objects as a whole on the scale that we live in. Gravity works on all scales of size including the atomic level. Right now, as I type this, the gravitational relationship between the atoms in my hands and the keyboard is greater than the gravitational relationship between the atoms in my face, or the back of my head and the keyboard. This concept is what creates tides.
So we can do the math figure out the gravitational relationship between the Earth and the Moon and the Earth and the Sun using the formula I mentioned. When doing so, we will notice that the Earth/Moon system has a gravitational relationship that is 2.2 times greater than the Earth/Sun system despite the significantly larger mass of the Sun due to the greater distance. On a side note, the center of gravity between the Earth and the Sun is actually inside the Sun itself.
So as the Moon orbits the Earth, its gravitation pulls on the waters that are on the side of Earth facing the Moon. This creates a tide on that side. Then working its way through the Earth, the Moons gravity pulls the Earth away from the waters on the opposite of the Earth than the Moon which is what gives us a high tide on that side. To compensate for this rise in water, the water on the sides of the Earth that is perpendicular to those high tides recedes. During a New Moon, when the Sun and Moon are on the same side of the Earth, the gravitational relationship between the Earth and the Moon is exaggerated by the gravitational relationship with the Sun and the Earth giving us larger than normal tides. This normally happens in the Fall and Spring so they are called, ‘spring tides’. On opposite spectrum, when the Moon is quartering, or perpendicular to the Sun, their gravitational relationships counteract each other and the tides are relatively low. These are referred to as ‘neap tides’.
The Moons orbit around the Earth is not perfectly circular, just as the Earth’s orbit around the Sun is not perfectly circular. They are slightly elliptic with their host as one of the ‘foci’. So there is a time throughout their orbit where they are closest to their host, and a time where they are farthest from their host. For the Moon orbiting around the Earth, these distances occur once a month as that is the time the Moon takes to orbit the Earth. For the Earth orbiting the Sun, these distances occur once a year. (It may be weird considering that the Earth is closest to the Sun during the winter for the Northern hemisphere). While this elliptical shape is very near perfectly circular, at the large distances we are discussing, even a small degree of ellipse will result in a large difference of distance between the closest approach and the farthest approach to the host. It is undetectable to the unaided human eye, but the angular measurement of a full moon at its closest approach to the Earth is much larger than a full moon and its most distant approach. At this point it is important to mention that the phases of the Moon do not align with the distance of the Moon from the Earth in its orbit. Sometimes the coincidentally coincide and we get to see a full moon on its closes approach.
Now that you know, all matter has a gravitational relationship with all other matter, maybe you can tell me how the two colliding galaxies of Messier 51 ‘feel’. It is hard to comprehend because we don’t experience it on our level every day, but it does exist, it is measurable. Next week will discuss orbit a little bit, and finally I will complete this series with some of the popular theories about the formation of the Moon in our early solar system. It is our history.
Joseph Kramer is an Army Aviation Officer. He is an amateur astronomer and contributing member of Central Arkansas Astronomical Society.