It's the Tides!

Sunday, I was on my section of the Appalachian Trail, cleaning out waterbars and removing some blowdowns from the trail (Flat Stanley was helping out). Two very interesting things happened while I was out there.

  1. I found live ammunition! In case you’re wondering where it came from, sections one and two of the AT in Georgia are frequently frequented by the Army Ranger trainees out of Camp Frank D. Merrill, which is the Ranger mountain training camp. Occassionally, they’ll be traipsing along and across the AT. And, apparently, leaving cardboard boxes full of rifle ammuntion (blanks) for me to find with my fire rake. Hmmmmm… After thinking about it for a bit, I decided to bury them a dozen feet off the trail. I couldn’t in good conscience leave them there as the next person who happens along would probably be a thru-hiker. I don’t remember getting explosives & demolition training at the GATC orientation!
  2. I learned new principles of gravitational physics! As background, let me explain: when cleaning out trail waterbars, you need to remove the accumulated loose debris and dig out any silt that has been laid down since the last time you cleaned them. While I was doing this at one of the waterbars, two backpackers ambled up. They were what I would (shallowly) label as “Redneck Backpackers” but would probably be more appropriately called “Hunters.” I call them this because they were covered in camoflage clothing and had huge knives in sheathes dangling from their shoulder straps. But that’s ok. It takes all kinds, right? As long as we’re all out enjoying the weather and the trail, yes? They asked what I was doing, and I explained about the waterbars, etc., and I also remarked that they’d be suprised how quickly the outlets fill up with dirt, which was why I was digging them out so much.

    One of them said*, “You should dig them on a full moon.” I stared at him. “Yeah, you know those times when you dig a hole and then fill it back in and you’ve got too much dirt.” His friend interjected, “Or not enough! I had that happen.” “Maybe it’s supposed to be on a new moon, then. It’s got something to do with the tides.”

    It is a good thing that there were two of them to talk with each other about this, because I honestly did not know what to say. I wanted to be polite and say “uh huh, yeah, me too” or something similar, but I just couldn’t lower my science-laden brain that far. My mind was going woo-woo-woo and I merely stared.

    Eventually, we got off that topic, they kept moving along, and I went back to cleaning waterbars.

What is the gravitational difference (i.e. tidal effect) on patch of dirt between the full moon and the new moon? Let’s calculate:

  • F=GMm/r^2. If we figure that the moon goes all the way ’round the earth in 28 days, then …

    Wait! The Earth rotates once per day, which means the moon is overhead once per day (or so) as well as opposite you on the other side of the Earth.

  • Let’s take the question in the spirit it is offered: What is the gravitational difference on a patch of dirt between the moon being overhead its being on the opposite side of the Earth?

    Gravitational force on the dirt = GM1m/r1^2. The gravitational constant won’t change, and neither will the masses. The only thing changing is the relative positioning of the moon, ie. ‘r’. I leave it as an exercise to the reader to do the figuring, as I cannot be bothered to write it down.

    My rough calculations, with numbers I’m pulling out of my butt for mass of earth and moon and radii, etc. shows a 0.004% change in the gravitational force. Definitely not enough to cause me problems if I’m digging holes.

*these are not direct quotations, but as close to them as I could remember. I swear that this conversation actually took place in a format very similar to what is written.

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2 Responses to It's the Tides!

  1. Pingback: Back to the Appalachian Trail | The Evil Eyebrow

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