Watching the moon and tides
05/03/23 02:04
I’m beginning to pay a little more attention to the moon. I’ve been aware of the phases of the moon for as long as I can remember. And I know that the moon’s orbit around our planet is elliptical, meaning that it is closer at some times and farther away at others. Although it has been overcast for a couple of days, I know that the moon is nearly full. The full moon is a couple of days from now, so it would appear to be full if I could see it through the clouds. My attention to the moon is heightened by my growing awareness of the tides.
The tidal variation in our little bay is approximately 9 feet, but that varies with the distance between the earth and the moon. 9 feet of variation is enough that Terrell Creek, which empties into the bay a little over a mile from our house, flows both directions, depending on the height of the tides and the time of the day. The creek is nearly flat as it makes its way alongside the shore before making the final turn into the ocean. When the tide is high, the creek flows inland with the tide for all the way to a marshy area three miles from where it empties into the ocean. The water in the creek above the marsh is fresh. The water below the marsh is saltier. Ocean creatures such as shrimp and crabs make their way up the stream, providing excelling fishing for gulls and other birds. The saltwater loosens some of the plants that grow in the marsh, washing them downstream, making our end of the creek a good feeding area for ducks and geese. The result is that the creek is a great place for birdwatching.
Right now the highest tide is occurring around 5:30 am and the lowest around 10:30 pm. These times will get later each day. In between the morning high tide and the evening low tide is another low tide at around 11 am and a high tide around 3:30. The midday high is a couple of feet lower than the early morning one and the midday low is probably about 6 feet higher than the one late in the evening. It sounds a bit complicated when I try to describe it in writing, but it seems pretty natural when we are walking alongside the bay nearly every day and watching the flow in the creek.
There are times when the variation between the low and high that occur midday right now is so much less than the current variation that the bay only sees one high and one low in a 24-hour period. That is fairly rare, and since we mostly walk during daylight hours we aren’t always aware of it unless we are looking at the online tide chart.
Interestingly, the tides are a bit higher than they were thousands of years ago. The orbit of the moon is getting slightly bigger than it used to be. The moon is moving farther away from the earth. The process is known as “lunar recession.” Scientists now have a method for accurately measuring the distance between the moon and the earth. The astronauts of the Apollo missions, placed reflectors on the surface of the moon. Scientists can now aim lasers at those reflectors and measure the amount of time it takes for the light to be reflected back to the earth. Since the speed of the light’s travel is constant they can use that information to accurately measure the distance between the moon and the earth. The moon is currently moving away from the earth at a rate of about 1.5 inches per year. The rate of lunar recession varies over time, but a recent study suggests that around 3.2 billion years ago the moon was about 170,000 miles from earth. Now it is around 238,855 miles. That’s a big difference.
Since the reflector and laser method of measuring the distance between the moon and earth are relatively new, you might wonder how scientists have come to an estimate of the distance between the two bodies over 3 billion years ago. Their clue is that there is geological evidence of the ebb and flow of tides billions of years ago. And tidal variation was smaller billions of years ago. Scientists also can determine from the fossil record that the length of days was also shorter at that time. Those two phenomena are both related to the moon.
The farther the distance between the earth and the moon, the higher the tidal variation. The water on the surface of the earth sloshes with the pull of the moon’s gravity. The farther away the moon the higher the slosh. (You can see that I’m using highly technical terms like slosh here.) The bigger the slosh the more friction the water places on the rotation of the earth. Over billions of years this results in slowing the rate of rotation of the earth. The slower the rotation the longer the day. There was a time when the earth was rotating so fast that the distance between sunrise and sunset was only slightly over 5 hours, giving two sunrises and two sunsets in 24 hours. The rotation rate has slowed to the point where the length of a day is now around 24 hours. And that rate of rotation continues to slow as the moon continues to move farther from the earth. Our days are getting slightly longer with each passing year, though by such a small amount that one would never notice in the span of a lifetime.
The rate at which the rotation of the earth changes is affected by continental drift. The current configuration of the continents on the surface of the planet leaves the distribution of water in the earth’s oceans nearly balance with the spin of the planet. This means that the sloshing is fairly regular. It would be much different if the continents were wider or narrower.
This universe is an incredibly complex system with many interrelated parts. We cannot perceive all of it. But at this phase of my life, I am able to look up to the sky and observe the phases of the moon and walk along the shore and observe the ebb and flow of the tides. It is enough to be fascinating for me and to get me to thinking, which is a good thing.
The tidal variation in our little bay is approximately 9 feet, but that varies with the distance between the earth and the moon. 9 feet of variation is enough that Terrell Creek, which empties into the bay a little over a mile from our house, flows both directions, depending on the height of the tides and the time of the day. The creek is nearly flat as it makes its way alongside the shore before making the final turn into the ocean. When the tide is high, the creek flows inland with the tide for all the way to a marshy area three miles from where it empties into the ocean. The water in the creek above the marsh is fresh. The water below the marsh is saltier. Ocean creatures such as shrimp and crabs make their way up the stream, providing excelling fishing for gulls and other birds. The saltwater loosens some of the plants that grow in the marsh, washing them downstream, making our end of the creek a good feeding area for ducks and geese. The result is that the creek is a great place for birdwatching.
Right now the highest tide is occurring around 5:30 am and the lowest around 10:30 pm. These times will get later each day. In between the morning high tide and the evening low tide is another low tide at around 11 am and a high tide around 3:30. The midday high is a couple of feet lower than the early morning one and the midday low is probably about 6 feet higher than the one late in the evening. It sounds a bit complicated when I try to describe it in writing, but it seems pretty natural when we are walking alongside the bay nearly every day and watching the flow in the creek.
There are times when the variation between the low and high that occur midday right now is so much less than the current variation that the bay only sees one high and one low in a 24-hour period. That is fairly rare, and since we mostly walk during daylight hours we aren’t always aware of it unless we are looking at the online tide chart.
Interestingly, the tides are a bit higher than they were thousands of years ago. The orbit of the moon is getting slightly bigger than it used to be. The moon is moving farther away from the earth. The process is known as “lunar recession.” Scientists now have a method for accurately measuring the distance between the moon and the earth. The astronauts of the Apollo missions, placed reflectors on the surface of the moon. Scientists can now aim lasers at those reflectors and measure the amount of time it takes for the light to be reflected back to the earth. Since the speed of the light’s travel is constant they can use that information to accurately measure the distance between the moon and the earth. The moon is currently moving away from the earth at a rate of about 1.5 inches per year. The rate of lunar recession varies over time, but a recent study suggests that around 3.2 billion years ago the moon was about 170,000 miles from earth. Now it is around 238,855 miles. That’s a big difference.
Since the reflector and laser method of measuring the distance between the moon and earth are relatively new, you might wonder how scientists have come to an estimate of the distance between the two bodies over 3 billion years ago. Their clue is that there is geological evidence of the ebb and flow of tides billions of years ago. And tidal variation was smaller billions of years ago. Scientists also can determine from the fossil record that the length of days was also shorter at that time. Those two phenomena are both related to the moon.
The farther the distance between the earth and the moon, the higher the tidal variation. The water on the surface of the earth sloshes with the pull of the moon’s gravity. The farther away the moon the higher the slosh. (You can see that I’m using highly technical terms like slosh here.) The bigger the slosh the more friction the water places on the rotation of the earth. Over billions of years this results in slowing the rate of rotation of the earth. The slower the rotation the longer the day. There was a time when the earth was rotating so fast that the distance between sunrise and sunset was only slightly over 5 hours, giving two sunrises and two sunsets in 24 hours. The rotation rate has slowed to the point where the length of a day is now around 24 hours. And that rate of rotation continues to slow as the moon continues to move farther from the earth. Our days are getting slightly longer with each passing year, though by such a small amount that one would never notice in the span of a lifetime.
The rate at which the rotation of the earth changes is affected by continental drift. The current configuration of the continents on the surface of the planet leaves the distribution of water in the earth’s oceans nearly balance with the spin of the planet. This means that the sloshing is fairly regular. It would be much different if the continents were wider or narrower.
This universe is an incredibly complex system with many interrelated parts. We cannot perceive all of it. But at this phase of my life, I am able to look up to the sky and observe the phases of the moon and walk along the shore and observe the ebb and flow of the tides. It is enough to be fascinating for me and to get me to thinking, which is a good thing.
