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As a quite handsome man, ‘from a certain angle and in a certain light’, how did you pull Mrs Cave?

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I am thirteen. [ ] What is your favourite dinosaur? NICKLAS, STOCKHOLM, SWEDEN   Can you recommend a female poet I should read? TAMMY, ROME, ITALY   Do you think The Red Hand Files have been a success? JONATHAN, CHOBHAM, UK Dear Steven, Nicklas, Tammy and Jonathan,With the passing of the years, that certain angle […]

The post As a quite handsome man, ‘from a certain angle and in a certain light’, how did you pull Mrs Cave? appeared first on The Red Hand Files.

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ssorc
50 days ago
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Road Bike Gears Explained : Why the Right Choice Matters

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I often come across cyclists saying things such as,

I ran out of gears on the descend.

Only if I have 1 or 2 lower gears, I’d finish the climb strongly.

If I’d have another gear, I wouldn’t have cramped.

Look, gearing is a very important aspect in cycling but it’s often overlooked by many as it can quickly become very technical. After all, we all want to ride our bikes and not sweat much over the technicals and mathematics behind it.

Understanding the basics behind the gearing would help you make a better-informed decision the next time you think about replacing or upgrading your drivetrain.

Having the ideal gears would definitely make your riding much more enjoyable.

On this page, I’ll explain to you the various jargons and terminologies road cyclists used and how to determine the ideal gearing setup for your type of riding.

Anatomy of a Road Bike Drivetrain

Before I jump into more complicated topics, I’ll spend some time going through the various components that make up a bike’s drivetrain. 

If you’re already familiar with these, then you can skip this section.

  • Chainring. The chainring is the large pair of cogs at the front where your pedals connect to. Most road bikes have double chainrings (sometimes called 2X). Kids, cyclocross, gravel, and mountain bikes usually have just a single chainring (1X). Commuter bikes are likely to have up to triple chainrings (3X).
  • Cassette. The cassette is the smaller set of gears on the right side (also called the drive side) of the rear wheel. They can range from 7 all the way up to 13 cogs. More cogs mean a broader range of ears. Each cog can be made up of 9 up to 52 teeth. The more teeth, the lower the gearing is. For example, 11-50T means that the smallest cog has 11 teeth and the largest cog has 50 teeth.
  • Chain. The chain connects the chainring and cassette together and is the driving force that takes the energy from the pedals and turns it into forward motion. Typically made of steel or aluminum, the chain has holes that fit perfectly into the teeth of the cassette and chainring to avoid slipping.
  • Front Derailleur, sometimes referred to as just FD, is attached to the frame on the seat tube. It de-rails the chain from one chainring to the other, both up and down. A 1X bike will not have a front derailleur.
  • Rear Derailleur, also known as RD, hangs from the frame via a derailleur hanger from the chainstay. It moves the chain along the cassette, giving you access to a range of high and low gears.
  • Shifters. The shifters are the mechanism that uses cable tension or electrical signal to make the derailleurs change gear. These days, the shifters are integrated with the brake levers. Shimano calls this STI (Shimano Total Integration).

Mechanical vs Electronic Drivetrain

Did you know that Mavic was actually the first to introduce an electronic drivetrain way back in 1992?

It’s called the Mavic Zap, but unfortunately, it didn’t hit the mainstream.

It was not until 2009 that Shimano launched its first generation of Di2 (Digital Integrated Intelligence), the Dura Ace 7970. Then in 2012, Campagnolo introduced the EPS. 

SRAM was the last of the 3 big drivetrain manufacturers to introduce an electronic drivetrain. In 2015, SRAM introduced its first wireless, electronic drivetrain, the SRAM eTap.

Over the past few years, there are many discussions surrounding whether should one get a mechanical or an electronic drivetrain.

So, let’s take a look a the pros and cons of each.

Mechanical Drivetrain - The Good vs Bad

  • Cheaper. Mechanical drivetrains significantly cheaper than electronic gears while providing a smooth shifting experience.
  • Don’t use batteries. Remembering to keep your Di2 battery charged or having to wait while it charges before you ride is one more complication a rider doesn’t need.
  • Susceptible to cable wear and failure. Over time, the shifter cable will get worn out as it rubs against the cable housing when you shift. Also, as the derailleurs use cable tension to shift, the cable will be stretched. A good practice is to get a new set of cables yearly.
  • Needs constant indexing. As the cable stretches, you’ll need to perform micro-indexing to ensure the derailleurs shift accurately. 

Eletronic Drivetrain - The Good and Bad

  • Very smooth and accurate shifting. Electronic shifting is smooth and quiet and while a well-tuned mechanical groupset can do the same, once you try electronic you don’t want to go back. Once set up, you will probably never need to index the gears again (unless you get a new set of wheels).
  • Self-trimming – The front derailleur on an electronic groupset will automatically align itself to avoid chain rub. It does this by over-shifting to quickly move the chain and then self-trim to compensate for that over-shift.
  • Expensive. They’re not cheap and very often cost at least 2x the price of the equivalent mechanical drivetrain.
  • Batteries. Another battery to maintain and remember to keep charged. While most groupsets will warn you in advance of a flat battery, you need to remember to check before longer rides!

How Many Speed Do You Need?

One of the arguments I hear from people is that the more speed the drivetrain has, the better it is.

But, is it really?

Speed refers to the number of cogs in the rear cassette. Multiply the speed with the number of front chainrings, you’ll get the total available gears on the drivetrain.

For example, a 22-speed drivetrain is made up of 2 front chainrings with an 11-speed cassette. That makes 2 x 11 = 22 speed.

Or, 1x chainring with 12 cogs cassette, that would be 1 x 12 = 12 speed.

Confusing, I know.

These days, the minimum for road bikes is either 11 or 12-speed. Older bikes (pre-2012) could still be running 10-speed drivetrains.

Shimano road drivetrains are 11-speed, while SRAM and Campagnolo are 12-speed.

Regardless of whether it’s 22 or 24-speed, you’ll have enough gears for most terrain with some crossovers. Crossover is where a ratio is the same between the small and big chainring.

For example, 53×19 is the same gear ratio as a 39×14, meaning the big chainring at the front and 19T at the rear is the same gearing as a small chainring upfront and 14T if you have a 53/39 chainset.

So, what’s the difference between 11 and 12-speed then?

The more speed, the tighter the gear ratio is.

In other words, there’s a smaller jump between adjacent gears. It might sound like a trivial matter for most cyclists, but it can be greatly felt especially when you’re riding at your limits. A smaller jump allows you to maintain a more consistent cadence overtime.

Rather than sweating over the number of speed, I think the cassette choice is a more important factor, especially for recreational cyclists.

Choosing the Ideal Rear Cassette

Cassettes come in many configurations and understanding what they mean,  how they ride help you greatly over your gearing choices.

The numbers on the cassette represent the highest and lowest gears on it.

  • 11-23 : 11-12-13-14-15-16-17-18-19-21-23
  • 11-25 : 11-12-13-14-15-16-17-19-21-23-25
  • 11-28 : 11-12-13-14-15-17-19-21-23-25-28
  • 11-32 : 11-12-13-14-16-18-20-22-25-28-32
  • 12-25 : 12-13-14-15-16-17-18-19-21-23-25
  • 14-28 : 14-15-16-17-18-19-20-21-23-25-28

For example, a 11-23T cassette has 11T as the highest gear and 23T as the lowest gear.

A cassette with a closer gap (eg: 11-23T) will have a tighter ratio compared to 11-34T.

Notice the larger gaps?

The key to deciding the type of cassette depends a lot on the type of riding you do and the terrain.

Here’s a quick way to determine the type of cassette you need.

  • 11-23T or 11-25T. If you ride mostly on flat terrains, you want a cassette with a tighter ratio.
  • 11-28T. If you’re after a versatile, all-rounder, this cassette is very popular among a lot of cyclists.
  • 11-30T. If you ride very hilly terrains, you might want to consider this, or even 11-34T for the steepest hills.

Standard, Semi-compact or Compact Chainring?

The typical road bike setup will be two chainrings upfront. They will generally come in different ratios called standard, compact, and semi-compact.

  • Standard would be 53/39T, which is 53 teeth on the big ring and 39 on the small. This is typical for road bikes for stronger or more experienced riders or for relatively flat terrain as they are geared for speed.
  • Compact would be 50/34T and would be suitable for hillier terrain. The smaller rings mean lower gearing, which is suited for long days in the saddle, new cyclists, or hills.
  • Semi-compact at 52/36T is the middle ground, suitable for sportive riders, long days in the saddle, or undulating terrain. The 52/36T sits between the compact and the standard, hence the name.

To make things more complicated, SRAM offers different chainring options for its 12-speed drivetrain. They’re available in 50/37T, 48/35T, and 46/33T. The principles of gearing are the same but the exact ratios differ slightly.

Finally, there is the TT chainring for flat and fast terrains. It will use either a 54 or 55 and a 39 tooth ring. A larger big ring means a faster top speed and for the average TT held on relatively flat terrain, helps maintain that speed over the distance.

These aren’t suitable for other types of riding though unless you have the power output of Fabian Cancellara or Tony Martin.

What Are Gear Ratios?

The ratio describes how many times the back wheel rotates for a single rotation of the pedals. 

For example, if you’re on the big ring on a standard chainring and in 12T at the rear, that’s 53/12 or a ratio of 52:12 which breaks down to 4.42. For every rotation of your pedals, your rear wheel will rotate 4.42 times.

Another example would be a 36T chainring using that same 12T rear cog. This gives a ration of 3:1. One full revolution of the pedals will rotate the rear wheel 3 times.

Ratios are further complicated by progression metres. This is used in Europe to express how far the bike would travel with one full revolution of the pedals. This also takes into account wheel size as this has an influence over how far the bike will travel.

In our example above, 53/12 using standard 700C wheels, the bike would travel 9.28 metres. In the 36/12 example, it would travel 6.3 metres.

Like I said, you don’t need to know gear ratios in this depth unless you really want to.

The post Road Bike Gears Explained : Why the Right Choice Matters appeared first on The Geeky Cyclist.

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ssorc
226 days ago
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Bootstrapping a minimal math library

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Sometimes you don’t have all the math functions available that you would like. For example, maybe you have a way to calculate natural logs but you would like to calculate a log base 10.

The Unix utility bc is a prime example of this. It only includes six common math functions:

  • sine
  • cosine
  • arctangent
  • natural log
  • exp
  • square root

Users are expected to know how to calculate anything else they need from there. (Inexplicably, bc also includes a way to calculate Bessel functions.)

This post collects formulas that let you bootstrap the functions listed above into all the trig and hyperbolic functions and their inverses.

Logarithms

Most programming languages provide a way to compute natural logs but not logs in bases other than e. If you have a way to compute logs in base b, you can compute logs in any other base via

\log_a(z) = \frac{\log_b z}{\log_b a}

So, for example, you could compute the log base 10 of a number by computing its natural log and dividing by the natural log of 10.

Exponents

If you have a way to calculate ex and natural logs, you can compute xy via

x^y = \exp(y \log x)

Since square roots correspond to exponent 1/2, you can use this to compute square roots.

Trig functions

If you have a way to calculate sine and cosine, you can calculate the rest of the six standard trig functions.

\begin{align*} \sec z &= 1/\cos z \\ \csc z &= 1/\sin z \\ \tan z &= \sin z / \cos z \\ \cot z &= \cos z / \sin z \end{align*}

Inverse trig functions

If you have a way to calculate inverse tangent, you can bootstrap it to compute the rest of the inverse trig functions.

\begin{align*} \text{arcsin}(x) &= \arctan(x / \sqrt{1 - x^2}) \\ \text{arccos}(x) &= \arctan(\sqrt{1 - x^2 }/ x) \\ \text{arccot}(x) &= \pi/2 - \arctan(x) \\ \text{arcsec}(x) &= \arctan(\sqrt{x^2 - 1}) \\ \text{arccsc}(x) &= \arctan(1/\sqrt{x^2 - 1}) \end{align*}

Also, you can use arctan to compute π since π = 4 arctan(1).

Hyperbolic functions

If you have a way to compute exponentials, you can calculate hyperbolic functions.

\begin{align*} \sinh(x) &= \frac{\exp(x) - \exp(-x)}{2} \\ \cosh(x) &= \frac{\exp(x) + \exp(-x)}{2} \\ \tanh(x) &= \frac{\exp(x) - \exp(-x)}{\exp(x) + \exp(-x)} \\ \end{align*}

Inverse hyperbolic functions

If you can compute square roots and logs, you can compute inverse hyperbolic functions.

\begin{align*} \text{arcsinh} (z) &= \log \left( z + \sqrt{1 + z^2} \right) \\ \text{arccosh} (z) &= 2 \log\left( \sqrt{(z+1)/2} + \sqrt{(z-1)/2} \right) \\ \text{arctanh} (z) &= \log \left( (1+z) \sqrt{1/(1 - z^2)}\right) \end{align*}

Complex branch cuts

Up to this point this post has implicitly assumed we’re only working with real numbers. When working over complex numbers, inverse functions get more complicated. You have to be explicit about which branch you’re taking when you invert a function that isn’t one-to-one.

Common Lisp worked though all this very thoroughly, defining arc tangent first, then defining everything else in a carefully chosen sequence. See Branch cuts and Common Lisp.

The post Bootstrapping a minimal math library first appeared on John D. Cook.
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Moving our privacy advocacy forward to protect Australia and you

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Moving our privacy advocacy forward to protect Australia and you

At FastMail, we care deeply about protecting your right to data privacy. Recently, we continued our advocacy to protect privacy rights by voicing our concerns about the Telecommunications and Other Legislation Amendment (Assistance and Access) Act 2018, also known as TOLA or "the AABill” to Parliament.

To bring you up to speed, the act, which focuses on services built with end-to-end encryption, allows law enforcement to compel companies to modify their services and intercept data from their customers in its unencrypted form. Our previous post discussed how FastMail uses encryption, and why the act does not change your privacy or data security with FastMail. However, we believe this act has real implications for the Australian tech industry and the potential to weaken the security of other technology products created or used in Australia.

Being a voice for privacy in Parliament

We believe it’s important to use our voice, even if we’re not being directly impacted. We have made submissions in each round of public consultation regarding this bill, most recently we submitted feedback to the Australian government’s Parliamentary Joint Committee on Intelligence and Security on Feb 21, 2019. In our submission, we cited possible impacts of the act on the local technology sector including creating a distrust of Australia and Australian companies, causing financial losses to tech business, and creating confusion and stress among technical talent.

Although the outcome is unclear, we are hoping for the best. The good news is that many groups within the technical community are working together toward change.

Trusted by customers

As we have stated in the past, FastMail’s business is not directly affected by this legislation and we won’t be making changes to our technology or policies in response to this act.

While we had a small number of customers tell us they were discontinuing service due to this legislation, as well as a handful of potential customers tell us it was impacting their evaluation of our services, the AABill did not have a material impact on our business. However, we shared these developments in our submission to Parliament as an example of the AABill’s potentially chilling effect on Australian businesses. Parliament deserves to know how the marketplace views their decisions.

FastMail continues to enjoy a strong, loyal and growing customer base due to our relentless focus on the privacy of your personal information, a robust and standards-compliant technical platform, and a superior customer experience. We thank you for your ongoing support, and we are happy to be able to advocate on your behalf in legal, political and technical spheres!

Standing up for you

We are a proud Australian company committed to our home. All advocacy work happens on the ground and we’ll continue to fight for you and your privacy rights. We remain optimistic and inspired when we see our colleagues in the tech sector also working for change.

Privacy is central to our commitment to you. We have taken—and will continue to take—a public position on the AABill because it has raised concerns from technology companies and privacy supporters around the world. We will continue our work to advocate for privacy, for Australian citizens and for customers of Australian businesses everywhere.

What you can do

We hope you will join us in advocating for privacy rights, in Australia and your own home countries. While we are talking about Australia today, legal challenges to privacy rights come up in different countries all the time.

Here are the actions you can take to join our efforts in Australia:

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936 days ago
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Facehugger sleep apnea mask, Jared Gray

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Facehugger sleep apnea mask, Jared Gray

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965 days ago
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Earth-Moon Fire Pole

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Earth-Moon Fire Pole

My son (5y) asked me today: If there were a kind of a fireman's pole from the Moon down to the Earth, how long would it take to slide all the way from the Moon to the Earth?

Ramon Schönborn, Germany

First, let's get a few things out of the way:

In real life, we can't put a metal pole between the Earth and the Moon.[1]For one, someone at NASA would probably yell at us. The end of the pole near the Moon would be pulled toward the Moon by the Moon's gravity, and the rest of it would be pulled back down to the Earth by the Earth's gravity. The pole would be torn in half.

Another problem with this plan. The Earth's surface spins faster than the Moon goes around, so the end that dangled down to the Earth would break off if you tried to connect it to the ground:

There's one more problem:[2]Ok, that's a lie—there are, like, hundreds more problems. The Moon doesn't always stay the same distance from Earth. Its orbit takes it closer and farther away. It's not a big difference,[3]You may occasionally see people get excited about the "supermoon," a full Moon that appears slightly larger because it happens at the time of the month when the Moon is closest to Earth. But really, the full Moon always looks surprisingly large and pretty when it's near the horizon, thanks to the Moon illusion. In my opinion, it's worth going outside and looking at the Moon whenever it's full, regardless of whether it's super or not. but it's enough that the bottom 50,000 km of your fire station pole would be squished against the Earth once a month.

But let's ignore those problems! What if we had a magical pole that dangled from the Moon down to just above the Earth's surface, expanding and contracting so it never quite touched the ground? How long would it take to slide down from the Moon?

If you stood next to the end of the pole on the Moon, a problem would become clear right away: You have to slide up the pole, and that's not how sliding works.

Instead of sliding, you'll have to climb.

People can climb poles pretty fast. World-record pole climbers[4]Of course there's a world record for pole climbing. can climb at over a meter per second in championship competition.[5]Of course there are championship competitions. On the Moon, gravity is much weaker, so it will probably be easier to climb. On the other hand, you'll have to wear a spacesuit, so that will probably slow you down a little.

If you climb up the pole far enough, Earth's gravity will take over and start pulling you down. When you're hanging onto the pole, there are three forces pulling on you: The Earth's gravity pulling you toward Earth, the Moon's gravity pulling you away from Earth, and centrifugal force[6]As usual, anyone arguing about "centrifugal" versus "centripetal" force will be put in a centrifuge. from the swinging pole pulling you away from Earth.[7]At the distance of the Moon's orbit and the speed it's traveling, centrifugal force pushing away is exactly balanced by the Earth's gravity—which is why the Moon orbits there. At first, the combination of the Moon's gravity and centrifugal force are stronger, pulling you toward the Moon, but as you get closer to the Earth, Earth's gravity takes over. The Earth is pretty big, so you reach this point—which is known as the L1 Lagrange point—while you're still pretty close to the Moon.

Unfortunately for you, space is big, so "pretty close" is still a long way. Even if you climb at better-than-world-record speed, it will still take you several years to get to the L1 crossover point.

As you approach the L1 point, you'll start to be able to switch from climbing to pushing-and-gliding: You can push once and then coast a long distance up the pole. You don't have to wait to stop, either—you can grab the pole again and give yourself a push to move even faster, like a skateboarder kicking several times to speed up.

Eventually, as you reach the vicinity of the L1 point and are no longer fighting gravity, the only limit on your speed will be how quickly you can grab the pole and "throw" it past you. The best baseball pitchers can move their hands at about 100 mph while flinging objects past them, so you probably can't expect to move much faster than that.

Note: While you're flinging yourself along, be careful not to drift out of reach of the pole. Hopefully you brought some kind of safety line so you can recover if that happens.

After another few weeks of gliding along the pole, you'll start to feel gravity take over, speeding you up faster than you can go by pushing yourself. When this happens, be careful—soon, you'll need to start worrying about going too fast.

As you approach the Earth and the pull of its gravity increases, you'll start to speed up quite a bit. If you don't stop yourself, you'll reach the top of the atmosphere at roughly escape velocity—11 km/s[8]This is why anything that falls into the Earth hits the atmosphere fast enough to burn up. Even if an object is moving slowly when it's drifting through space, when it gets close to the Earth it gets accelerated up to at least escape velocity by that final segment of the trip down into the Earth's gravity well.—and the impact with the air will produce so much heat that you risk burning up. Spacecraft deal with this problem by including heat shields, which are capable of absorbing and dissipating this heat without burning up the spacecraft behind it.[9]People often ask why we don't use rockets to slow down, to avoid the need for a heat shield. You can read this article for an explanation, but the bottom line is that changing your speed by 11 km/s takes either a tank of fuel the size of a building or a tiny heat shield, and the tiny heat shield is a lot easier to carry. Thanks to heat shields, slowing down is much easier than speeding up—which requires the aforementioned giant fuel tank. (For more on this, see this What If question).

Heat shields only work for slowing down; if there were a way to use the same heat shield mechanism to speed up, space travel would get a lot easier. Sadly, no one's figured out a practical way to build a "reverse heat shield" rocket. However, while the idea seems silly, in a sense it's sort of the principle behind both Project Orion and laser ablation propulsion.
Since you have this handy metal pole, you can control your descent by clamping onto it and controlling your rate of descent through friction.

Make sure to keep your speed low during the whole approach and descent—and, if necessary, pausing to let your hands or brakepads cool down—rather than waiting until the end to try to slow down. If you get up to escape velocity, then at the last minute remember that you need to slow down, you'll be in for an unpleasant surprise as you try to grab on to the pole. At best, you'll be flung away and plummet to your death. At worst, your hands and the surface of the pole will both be converted into exciting new forms of matter, and then you'll be flung away and plummet to your death.

Assuming you descend slowly and enter the atmosphere in a controlled manner, you'll soon encounter your next problem: Your pole isn't moving at the same speed as the Earth. Not even close. The land and atmosphere below you are moving very fast relative to you. You're about to drop into some extremely strong winds.

The Moon orbits around the Earth at a speed of roughly one kilometer per second, making a wide circle[10]Yes, I know, orbits are conic sections which in the case of the Moon is technically not exactly a circle. It's actually a pentagon. every 29 days or so. That's how fast the top end of our hypothetical fire pole will be traveling. The bottom end of the pole makes a much smaller circle in the same amount of time, moving at an average speed of only about 35 mph relative to the center of the Moon's orbit:

35 miles per hour doesn't sound bad. Unfortunately for you, the Earth is also spinning,[11]I mean, unfortunately in this specific context. In general, the fact that the Earth spins is very fortunate for you, and for the planet's overall habitability. and its surface moves a lot faster than 35 mph; at the Equator, it can reach over 1,000 miles per hour.[12]It's common knowledge that Mt. Everest is the tallest mountain on Earth, measured from sea level. A somewhat more obscure piece of trivia is that the point on the Earth's surface farthest from its center is the summit of Mt. Chimborazo in Ecuador, due to the fact that the planet bulges out at the equator. Even more obscure is the question of which point on the Earth's surface moves the fastest as the Earth spins, which is the same as asking which point is farthest from the Earth's axis. The answer isn't Chimborazo or Everest. The fastest point turns out to be the peak of Mt. Cayambe, a volcano north of Chimborazo. And now you know.[13]Mt. Cayambe's southern slope also happens to be the highest point on Earth's surface directly on the Equator. I have a lot of mountain facts.

Even though the end of the pole is moving slowly relative to the Earth as a whole, it's moving very fast relative to the surface.

Asking how fast the pole is moving relative to the surface is effectively the same as asking what the "ground speed" of the Moon is. This is tricky to calculate, because the Moon's ground speed varies over time in a complicated way. Luckily for us, it doesn't vary that much—it's usually somewhere between 390 and 450 m/s, or a little over Mach 1—so figuring out the precise value isn't necessary.

Let's buy a little time by trying to figure it out anyway.

The Moon's ground speed varies pretty regularly, making a kind of sine wave. It peaks twice every month as it passes over the fast-moving equator, then reaches a minimum when it's over the slower-moving tropics. Its orbital speed also changes depending on whether it's at the close or far point in its orbit. This leads to a roughly sine-wave shaped ground speed:

Well, ready to jump?

Ok, fine. There's one other cycle we can take into account to really nail down the Moon's ground speed. The Moon's orbit is tilted by about 5° relative to the Earth-Sun plane, while the Earth's axis is tilted by 23.5°. This means that the Moon's latitude changes the way the Sun's does, moving from the northern tropics to the southern tropics twice a year.

However, the Moon's orbit is also tilted, and this tilt rotates on an 18.9-year cycle. When the Moon's tilt is in the same direction as the Earth's, it stays 5° closer to the Equator than the Sun, and when it's in the opposite direction, it reaches more extreme latitudes. When the Moon is over a point farther from the equator, it has a lower "ground speed," so the lower end of the sine wave goes lower. Here's the plot of the Moon's "ground speed" over the next few decades:

The Moon's top speed stays pretty constant, but the lowest speed rises and falls with an 18.9-year cycle. The lowest speed of the next cycle will be on May 1st, 2025, so if you want to wait until 2025 to slide down, you can hit the atmosphere when the pole is moving at only 390 m/s relative to the Earth's surface.

When you do finally enter the atmosphere, you'll be coming down near the edge of the tropics. Try to avoid the tropical jet stream, an upper-level air current which blows in the same direction the Earth rotates. If your pole happens to go through it, it could add another 50-100 m/s to the wind speed.

Regardless of where you come down, you'll need to contend with supersonic winds, so you should wear lots of protective gear.[15]For aerodynamic reasons, this gear should probably make it look like you're wearing a very fast airplane. Make sure you're tightly attached to the pole, since the wind and various shockwaves will be violently battering and jolting you around. People often say, "It's not the fall that kills you, it's the sudden stop at the end." Unfortunately, in this case, it's probably going to be both.[17]If it helps, people have survived supersonic ejections before—and even a supersonic aircraft disintegration—so there's hope.

At some point, to reach the ground, you're going to have to let go of the pole. For obvious reasons, you don't want to jump directly onto the ground while moving at Mach 1. Instead, you should probably wait until you're somewhere near airline cruising altitude, where the air is still thin, so it's not pulling at you too hard—and let go of the pole. Then, as the air carries you away and you fall toward the Earth, you can open your parachute.

Then, at last, you can drift safely to the ground, having traveled from the Moon to the Earth completely under your own muscle power.

(When you're done, remember to remove the fire pole. That thing is definitely a safety hazard.)

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