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 Information
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)reply
"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.
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)reply
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?
Dbfirs07:19, 20 October 2018 (UTC)reply
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_
Zero08:25, 20 October 2018 (UTC)reply
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.
Dbfirs15:51, 20 October 2018 (UTC)reply
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)reply
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. --
Jayron3212:17, 23 October 2018 (UTC)reply
Landing a plane in high crosswinds and landing gear
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.
"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...
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)reply
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.
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)reply
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)reply
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)reply
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 Information
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)reply
"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.
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)reply
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?
Dbfirs07:19, 20 October 2018 (UTC)reply
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_
Zero08:25, 20 October 2018 (UTC)reply
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.
Dbfirs15:51, 20 October 2018 (UTC)reply
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)reply
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. --
Jayron3212:17, 23 October 2018 (UTC)reply
Landing a plane in high crosswinds and landing gear
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.
"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...
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)reply
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.
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)reply
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)reply
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)reply