Wikipedia:Reference desk/Archives/Science/2018 October 19

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

October 19

A(nta)rctic Circle

The Arctic and Antarctic Circles are defined as being the lowest latitudes "at which the sun can remain continuously above or below the horizon for twenty-four hours", and both events happen on each circle annually. But I'm missing something: since the Sun appears to be a disc, and not a point, why isn't it possible for there to be a location where a tiny part of the Sun is visible at midnight on the summer solstice and at noon on the winter solstice? I see from the articles that atmospheric refraction causes the midnight and noontime Sun to be visible at slightly lower latitudes, but it seems to me that the same should be true of the slightly lower latitude: you would have a spot where the refracted solar disc just barely peeps over the horizon at midnight and noon respectively. Where did I go wrong? Nyttend (talk) 22:46, 19 October 2018 (UTC)

The definitions I'm seeing in Google say that within the arctic circle, the sun does not set on the summer solstice, and likewise for the Antarctic circle and the winter solstice. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:37, 19 October 2018 (UTC)

"The time of sunset is defined in astronomy as the moment when the upper limb of the Sun disappears below the horizon." --Sunset.

Here is how you figure if you are in the arctic:

Just south of the arctic circle: on the longest day, the sun dispensers completely (assuming a flat horizon (a calm sea, for example) and you standing in a hole or treading water with your eyes exactly at sea level.

Just north of the arctic circle: same but the sun doesn't quite disappear. --Guy Macon (talk) 00:10, 20 October 2018 (UTC)

So once again: how is it possible for the lowest latitude of midnight sun and the lowest latitude of noon non-sun to be identical, since the center of the solar disk at sunrise is lower than the center of the solar disk at non-sun? Nyttend (talk) 03:45, 20 October 2018 (UTC)

That puzzles me, too. I would have thought that it was the centre of the sun that defines the arctic circle, and that the sun remains visible on the horizon for 24 hours some miles south of the arctic circle. Has anyone been there to make an observation? Dbfirs 07:19, 20 October 2018 (UTC)

Some details at Midnight sun may provide some clarity. ←Baseball Bugs ^{What's up, Doc?} carrots→ 07:41, 20 October 2018 (UTC)

Both are defined as a latitude at which the upper limb of the Sun in visible at noon or does not disappear at midnight. This is the same latitude. Ruslik_Zero 08:25, 20 October 2018 (UTC)

Midnight sun says it's the centre of the sun. I've edited the articles Arctic circle and Antarctic circle to prompt further discussion here. Dbfirs 12:54, 20 October 2018 (UTC)

What rule requires that more than half the sun below the horizon counts as the sun having "set"? ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:51, 20 October 2018 (UTC)

Sunset is clearly defined in terms of the last remaining sector of the disc of the sun disappearing below the horizon, but the arctic circle is defined by the geometry of Earth's orbit, so must refer to the centre of the sun. Dbfirs 15:51, 20 October 2018 (UTC)

The articles indicate it doesn't exactly coincide. In any case, by your definition of sunset, if you can see any part of the sun at the solstice, then it qualifies. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:37, 20 October 2018 (UTC)

That's why the arctic circle can't be defined in terms of sunset. Dbfirs 16:53, 20 October 2018 (UTC)

So in other words, the definition I originally quoted is a good approximation, but not completely accurate. That resolves the issue, it seems to me. Nyttend (talk) 18:01, 20 October 2018 (UTC)

If you see 66 degrees 34 minutes latitude that's a way to know they're talking about the center of the Sun with no air. Sagittarian Milky Way (talk) 16:19, 21 October 2018 (UTC)

Agreed. I've never seen the current arctic or antarctic circle marked at any other latitude. Dbfirs 16:36, 21 October 2018 (UTC)

On a human-sized globe or map, 66°34′ is fine, but on the earth, the arctic circle moves measurably from year to year. See here for an interesting project to track the exact article circle location over time. Because of a natural wobble in the earth's axial tilt, the arctic circle location fluctuates about 2.4 degrees over the course of 40,000 years, which is about 12 meters per year, or 1 meter per month, or about 3 centimeters per day. Currently, the Earth's wobble is such that the axis of rotation is "straightening out" relative to the sun, so the arctic circle is shrinking. --Jayron32 12:17, 23 October 2018 (UTC)

Landing a plane in high crosswinds and landing gear

When those planes land all sideways, does the landing gear change direction to match the direction of the runway? Anna Frodesiak (talk) 23:25, 19 October 2018 (UTC)

Can you find a video of a plane landing sideways? ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:33, 19 October 2018 (UTC)

I just saw this, but it's hard to tell. Anna Frodesiak (talk) 23:36, 19 October 2018 (UTC)

They're somewhat sideways from true alignment, but not 90 degrees sideways. And the wheels don't appear to be turning. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:39, 19 October 2018 (UTC)

Landing gear indicates that the side wheels, at least, do not turn. Straightening out a landing plane is accomplished by uneven application of the brakes. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:44, 19 October 2018 (UTC)

[ https://www.youtube.com/watch?v=-MsY7q5Ubtg ] --Guy Macon

How To Make A Perfect Crosswind Landing

--Guy Macon (talk) 00:36, 20 October 2018 (UTC)

Unfortunately, youtube is blocked where I am, but the other link works and is very interesting. Thank you. Anna Frodesiak (talk) 00:38, 20 October 2018 (UTC)

Very interesting, Baseball Bugs. Thank you. Anna Frodesiak (talk) 00:38, 20 October 2018 (UTC)

B52 in particular have steerable main gear to simplify crosswind landings.Greglocock (talk) 01:34, 20 October 2018 (UTC)

The short answer is that it depends on the type of aircraft. Some aircraft have zero, partial, or completely steerable wheels. Some very advanced aircraft have automatic computer control to steer wheels in accomodation of a cross-wind landing.

Quoting directly from the Airplane Flying Handbook: Section 8-14: Crosswind Approach and Landing

"There are two usual methods of accomplishing a crosswind approach and landing - the crab method and the wing-low (sideslip) method. Although the crab method may be easier for the pilot to maintain during final approach, it requires a high degree of judgment and timing in removing the crab immediately prior to touchdown. The wing-low method is recommended in most cases, although a combination of both methods may be used."

In almost all airplanes, crosswind landing techniques involve a slip.

In the wing-down, or "side" slip - the wheels are always pointing directly forward, aligned to the runway. The aircraft counteracts the crosswind using the horizontal component of lift. At the last minute, the pilot banks (rolls) to touch down on all wheels at the same instant - unless the aircraft can safely place its entire weight on just some of the gear - in which case the pilot can execute a wheel landing.

In the "forward"-slip, or "crab," the wheels are not pointing toward the runway. The aircraft counteracts the crosswind using the horizontal component of thrust. At the last minute, the pilot turns (yaws) them to the correct alignment. When executed properly, this is a smooth and gentle maneuver timed exactly to coincide with touchdown. At the last moment, the pilot swings the nose to line up directly with the runway as weight is placed on to the wheels. This is a great skill that requires careful timing - because if there's even a slight timing error, the landing gear (wheels) feel a strong lateral force on touchdown.

This last-minute maneuvering can be imprecise, and in some airplanes, the equipment can help the pilot by providing a little tolerance by way of engineering design. For example, Diamond Aircraft have a castering nose-wheel. Large modern airliners may execute computer-assisted wheel alignment during the crosswind approach to landing.

Correct procedure is to touch down with the wheels perfectly aligned - so all these small details ultimately come down to how much tolerance for imperfection each pilot (and aircraft) can actually accept in a real-world scenario. In every case, the ultimate option is to abort the landing and to divert to a different airport (or runway) with a less severe crosswind condition.

One last thing: just because the wheels have steered toward the runway does not mean the landing has no side-load. If the wheel or landing gear doesn't absorb the impact, then the fuselage surely will...

Nimur (talk) 17:45, 20 October 2018 (UTC)

Interestingly: 14 C.F.R. §23.483 One-wheel landing conditions used to be a federal regulation... as recently as 2017. But when I went looking for it, I can't find it in the 2018 F.A.R.s... In fact, most of Part 23 "Airworthiness Standards... for Normal Airplanes..." is just ... gone. I wonder if we've started to see the actual effect of the current administration's Executive Order 13771... congratulations, America - we've deregulated safe airplane landings!

Nimur (talk) 18:05, 20 October 2018 (UTC)

• No. Unless it's a B-52, one of the few aircraft that does steer its main undercarriage, not just the nosewheel.

OTOH, aircraft tyres get a hard life, much of which is sideways tyre scrub from crosswind landings. Andy Dingley (talk) 21:46, 20 October 2018 (UTC)

Yes, here's an interesting source, with a picture of a B-52. And here's a more academic treatment by Dennis Vechte, Ute Marita Meissner and Klaus-Uwe Hahn, of the German Aerospace Center (DLR) at the Institute of Flight Systems, Brunswick, Germany, in the CEAS Aeronautical Journal (2014): [1] Martinevans123 (talk) 21:50, 20 October 2018 (UTC)

Weight does not matter if you have enough wheels and a B-52 is a bad example for your assumption "at some weight you need steered ones" on top. The maximum takeoff weight of a B-52 is 220 tons. The maximum takeoff weight of an A 380 is 569 tons (A 380plus even 578 tons) and it does not have turning wheels except the nose ones and the last pair under the two middle 6-wheel columns, which only turn for small radius maneuvering at low speed. The A 380 has 22 wheels/tires in total btw.

Tires are simply build to take extreme friction and Deformation forces beyond what many would judge as dangerous "abuse" and they still take it with high reliance. Their only weak spots is perforation. So tires dont have to be lined up for landing but landing strips have to be checked for sharp objects as frequent as possible.

Check out hard crosswind A 380 landing with all wheels visible (youtube). --Kharon (talk) 08:48, 21 October 2018 (UTC)

Sorry, Kharon, but youtube is blocked here. But thanks anyhow. Anna Frodesiak (talk) 02:54, 22 October 2018 (UTC)

Is there a video hosting site that isn't blocked for you? For obviously non-political videos like these I could try searching and seeing if an approved site has the same video. --Guy Macon (talk) 03:21, 22 October 2018 (UTC)

Probably iQiyi and Youku would be the major ones. Nil Einne (talk) 07:24, 22 October 2018 (UTC)

Actually I think iQiyi is more of a commercial video hosting site than a video sharing one, although possibly more open than something like Netflix, Hulu or Amazon Prime Videos. Nil Einne (talk) 07:58, 22 October 2018 (UTC)

Thank you, Nil Einne. Yes, those work. Anna Frodesiak (talk) 08:02, 22 October 2018 (UTC)

Wow, I never used those sites, Nil Einne. I just typed in an item to see if they have non-Chinese stuff. I searched for an old video I like that is unavailable here due to no Youtube, and they have it. That means they should have lots of things. Thank you! Anna Frodesiak (talk) 08:15, 22 October 2018 (UTC)

Hi, Guy Macon. Yes, the two above work, but I doubt they have such videos. But thank you anyhow. Anna Frodesiak (talk) 08:02, 22 October 2018 (UTC)

Thank you all very, very much!!! Anna Frodesiak (talk) 02:54, 22 October 2018 (UTC)