Dark Side of the Moon

That title sounds cool, but should be amended to read "Far Side of the Moon". The moon rotates around the earth and is "locked" so that we only see one face of it. My sister and bro-in-law were wondering what the explanation was for this. I took a crack at it, and had a tough time convincing them. So, I've studied up and am writing it here for them.

Why is the moon locked facing the earth? I looked this up years ago and learned that it is basically due to what's called "tidal locking". From the wikipedia article on tidal locking, "A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner." Tides basically cause an spherical body (like the moon and earth) to be stretched in the direction of it's orbital partner.

How is it that something related to tides could cause a celestial dance between giant objects like the earth and moon? Don't tides just make the oceans go up and down? The image below gives some idea of what's happening on earth.


Tidal bulges. Earth's oceans are stretched toward the moon on the side facing the moon and away from the moon on the opposite side. The diagram above also shows earth's rotation dragging the stretched shape slightly counterclockwise such that the bulge "leads" the moon.

The moon is also stretched like the earth, and the stretched direction always points to the earth. This is because when the stretched direction is not pointed at the earth, there is a correcting force that rotates the stretched direction to point at earth.

When the moon was liquid, it was bulged, and rotated "under the bulge" just like the earth does. As it cooled and froze, it's rotational energy must've tried to move the bulge from its orientation toward earth, but the corrective force mentioned won and we have our tidally locked moon.

Cool, huh? My dad asked the tough question of why the stretched shape is formed. For example, why doesn't earth just have a single bulge on the side facing the moon? Why another bulge on the opposite side? How the devil does this really work? Ike Newton was one of the first (if not the first) to understand this scientifically. The wikipedia page on tidal force is excellent. A summary follows.

Tidal forces diagram. Moon's gravitational forces (red arrows) are acting on the earth (the black sphere).

The red arrows in the top pane shows the moon's gravity which is weaker farther from the moon. The earth (the black sphere) is in free fall. Thus, it makes sense to subtract off the average gravitational force acting on the earth. This is done in the lower pane. Now you can see the forces that cause the tidal stretching.

Keep in mind that the tides are only ~10 feet. This is a tiny bulge for an object with a radius of 20 million feet.

Another cool thing I learned in the article is that the debris that makes up Saturn's rings would have condensed into moons if not for tidal forces exerted by Saturn.

Cassini image of Saturn.

Tidal forces are stronger closer to the source of gravity. Moons formed around Saturn when the debris was far enough away that tidal forces didn't disturb the formation.

windward side rain / orographic lifting

The issue of rain on the windward side of mountains arose in my trip last week. My dad attributed the phenomenon to cooling of the rising air mass. The reasoning is that air is cold at high elevation, so as the air gets higher, it cools, reaches dewpoint, and starts releasing water. After further study, fundamentally, it is correct that the air cools and reaches dewpoint. However, the cause for the cooling is kinda ... cool. It's called orographic lifting. Basically, the air expands as it goes to high altitude. As it expands, the air does work on the air that it displaces. The work energy had to come from somewhere. It came from the energy of the air molecules. Temperature is, basically, a measure of the average speed of the air molecules.

For further reference, see
http://en.wikipedia.org/wiki/Orographic_lift

On that page, there is a beautiful picture of clouds being formed at the crests of waves, called gravity waves. Orographic lifting can cause air at the crests of the waves to reach dewpoint and form clouds.

Clouds are created at the crests of gravity waves due to orographic lifting. These waves are created as air flows over an island in the southern Indian Ocean.

For more on gravity waves, see
http://en.wikipedia.org/wiki/Gravity_wave

horseback trip into Gilbert Basin

I just spent a week in Utah, visiting Tess, Tim, and Zoey in SLC on either side of a 3 day horsepacking trip into the innermost valleys of the Uinta mountains.

Visiting Tess, Tim, and Zoey in SLC was nice. Zoey is 8 months old now, and is getting quite animated in her interaction with people.


Isn't she cute?! We went for a hike up Big Cottonwood Canyon. Tim and I played some golf in a serious windstorm. We also got in a few hands of bridge. Their pups, Howie and Sadie, are doing very well too and are as sweet as ever. We had a lot of fun one night doing accents. Tim has me beat on his Snoop Dog accent, but I think I've got him in the Aussie category.

Next stop was Roosevelt to get ready for our 3 day ride. The hosses were in good shape. My parents just got a new horse named Coco in addition to their old hands, Boss and Mischief. I was to ride Boss, who, true to his name, is the head horse, nipping the others as required to exert his authority. Steve had drawn up a route for us -- head up and over Fox Queant Pass and down in to the Uinta River drainage. Then up to Gilbert Basin. I've ridden my parents' horses on and off over the past several years, but never for more than an hour at a time. This time, I was gonna ride 8 hours a day for three days! I hoped my bike seat had me saddle ready.

Getting the horses loaded up is no mean feat. Saddle blanket, saddle, front and back girth hitches. Front saddle bags. Back saddle bags. Bridle. Breast strap. Boots. Yes, rather than regular metal horseshoes, nailed into hooves, my parents prefer rubber boots which snugly fit around the hoof and are strapped around the ankles. Of course, the usual camping gear has to be organized too.

The horses were cruising slowly on the first day, not in any rush to get far away from their trailer which they knew would bring them back to their comfortable pasture, and a steady supply of hay and water. We were a little off the pace, but made it to Fox Queant Pass some time around 2 pm. The trail is rocky and the horses had to take it slow. Most horseback riders probably wouldn't even consider taking their horses on a trail like this. It's a touch hiking trail! But Mischief and Boss are expert trail horses and Coco is well on his way.

Looking NW from Fox Queant Pass

From the Pass, we cruised to our campsite past some pretty lakes and gorgeous meadows formed by beaver dams.

I'm reclining on the saddle trying to rest my back which got rather sore. Beaver-created meadow to the right of my head.

The campsite location (near the trees above Steve's hat) was gorgeous. Gunsight pass is just below the sunlit clouds. See the low, sharp peak? Maybe the pass got its name because this sharp peak is like the front post in a common gunsight.

On our first night, we watched the full moon rising.

The spot I slept is behind me in the photo. I was rather chilly that night. Daisy hung with me a while, then retreated to the tent with my parents. Frost covered my bag. Really, I wasn't too cold except for my toes. But I did squeeze into the tent the next night.

Horses grazing in the morning. This pretty stream ran right past our campsite.

The next day, I was up at the crack (partly to try to warm up). Looked out in the meadows, but didn't see any game. Of course, the horses had cow bells around their necks and game probably could hear us miles away... The good thing is that bears could too and probably stayed far away. We got going pretty early. Terrain was again quite rocky. We reached our destination, though: Gilbert Basin. The basin sits just below Gilbert Peak (13,442 ft). My dad and I had been to this basin back in 2003. On that trip, we saw lots of elk. This time, it was still glorious, but no elk that we could see. They must've been laying low.

Gilbert Basin panorama in 2003.

Mom and Dad at our high point in the middle of Gilbert Basin.

The ride back down that day got pretty scary when Coco blew out a rear horse shoe. The arabs (Coco and Mischief are partly Arabian) tend to freak out when a rear horse shoe comes off their foot and hangs by the ankle strap. They start kicking wildly trying to get the thing off. Steve dismounted, asked me to stop Boss, and started working on the rear boot. I tried, but Boss.. well, he's the Boss, not me. Maybe if I was a more experienced rider, I could've halted him. But he walked, encouraging Coco to walk instead of stay still. Steve got knocked under Coco and the horse spooked and ran. Fortunately, Coco didn't step on him. Stopping is tough for these horses. Maybe it has to do with their insatiable appetites. They want to go, go, go until they get to some grass. Then, maybe they're willing to stop and munch. Any chance they get, really, they try to stop and eat. They are eating machines which makes sense for 1100-pound animals.

Sleeping in the tent was nice and warm. The next morning, we started back. After straightening out my stirrups, it seemed we were ready to make some good time. A few minutes down the trail, though, my saddle started sagging sideways. I knew this was bad news -- when the saddle slides sideways, the horse flips out. Boss started trotting, cantering... my mom yelled for me to bail. I did. That's where the crash skills from the snow sport (skiing / snowboarding) days come in handy. No problemo. We realized we hadn't tightened the saddle cinch!! Doh! Another lesson learned.

Navigating the rocky trail back on the 3rd day of riding was pretty tiring. We managed though. And the scenery was amazing.

My favorite from the trip: the hairbell.

Another nice flower: Daisy. :)

Looking back SW from near Fox Queant. The animals are all lined up behind Steve .. well, he's an animal too, really :)


Cowboy Eric. Anita thinks I look a little like the Marlboro man in this photo.

This trip was tres enjoyable overall. In general, I think Boss was a great trail horse for a beginner like me. Quite an experience to get out on a 40 mile horseback ride. Always fun to spend time with my family.

We had a few science/physics-related discussions that I plan to blog about soon. Teasers: Why does the moon always face the earth? Why does rain tend to fall on the windward side of mountains.