1276: "Angular Size"

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Zdrak
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Re: 1276: "Angular Size"

Postby Zdrak » Mon Oct 14, 2013 10:39 pm UTC

I have to disagree with Mr. Munroe. If the entire sky was projected on the surface of the earth, a magnifying glass would not be enough to dabble in astronomy. You'd also have to move very fast.

Thing is, pretty much every object in the sky, or rather, the projection of said object on the Earth's surface, makes one full revolution around the earth in 24 hours. It's a bit different for the moon and planets, of course, but still, quite close to 24 hours. So, divide the Earth's circumference (40,000 km or 25,000 miles) by the length of one day, and we get [the projections of] those objects zipping at approximately 1,042 MPH, or 1,667 km/hr, or 463 m/sec.
Some exceptions:
- low-earth-orbit satellites are moving much faster, at ~8 km/sec
- geostationary objects aren't moving at all (that's what geostationary means)
- stars located closer to the north and south pole are moving much slower. The Northern Star (Polaris), for example, located less than 1' from the north pole, travels only at a speed of 21.3 km/hr (theoretically, a fast runner can catch up to it on foot. If he can run fast enough through the arctic ice floes, of course)

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Pfhorrest
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Re: 1276: "Angular Size"

Postby Pfhorrest » Mon Oct 14, 2013 11:53 pm UTC

Moose Anus wrote:
Pfhorrest wrote:How many is a brazilian?

http://answers.yahoo.com/question/index ... 833AACXZ7Q

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Coyoty
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Re: 1276: "Angular Size"

Postby Coyoty » Tue Oct 15, 2013 7:25 pm UTC

Did Shadowfacts help Randall with this?

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PinkShinyRose
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Re: 1276: "Angular Size"

Postby PinkShinyRose » Tue Oct 15, 2013 9:43 pm UTC

Coyoty wrote:Did Shadowfacts help Randall with this?

I don't think these are shadows, maybe shadowfacts annoyed Randall while making this... "It would leave a much larger shadow if you put a point light source behind it".

rmsgrey
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Re: 1276: "Angular Size"

Postby rmsgrey » Tue Oct 15, 2013 10:06 pm UTC

PinkShinyRose wrote:
Coyoty wrote:Did Shadowfacts help Randall with this?

I don't think these are shadows, maybe shadowfacts annoyed Randall while making this... "It would leave a much larger shadow if you put a point light source behind it".



Depends how far behind it...

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PinkShinyRose
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Re: 1276: "Angular Size"

Postby PinkShinyRose » Tue Oct 15, 2013 10:27 pm UTC

rmsgrey wrote:
PinkShinyRose wrote:
Coyoty wrote:Did Shadowfacts help Randall with this?

I don't think these are shadows, maybe shadowfacts annoyed Randall while making this... "It would leave a much larger shadow if you put a point light source behind it".



Depends how far behind it...

Would a point light source create a shadow smaller than that objects angular size if placed at a certain distance form the object?

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Re: 1276: "Angular Size"

Postby rmsgrey » Tue Oct 15, 2013 10:38 pm UTC

PinkShinyRose wrote:
rmsgrey wrote:
PinkShinyRose wrote:
Coyoty wrote:Did Shadowfacts help Randall with this?

I don't think these are shadows, maybe shadowfacts annoyed Randall while making this... "It would leave a much larger shadow if you put a point light source behind it".



Depends how far behind it...

Would a point light source create a shadow smaller than that objects angular size if placed at a certain distance form the object?


Only in non-physical cases.

Yeah, size of shadow is at least size of object, while angular size is smaller than that of equivalent objects actually on the Earth's surface...

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phlip
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Re: 1276: "Angular Size"

Postby phlip » Wed Oct 16, 2013 12:49 am UTC

Build an elliptical reflector behind the object, with the centre of the Earth as one focus, and put a point light source at the other. Then the shadow will be the correct size.

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Re: 1276: "Angular Size"

Postby Neil_Boekend » Wed Oct 16, 2013 5:56 am UTC

Quicksilver wrote:Disappointed in the lack of a "Your Mom" joke.

There is a PILF in there. Isn't that wrong enough?
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Klear
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Re: 1276: "Angular Size"

Postby Klear » Wed Oct 16, 2013 8:48 am UTC

I'm surprised after third page I'm the first one to point this out but... holy shit Betelgeuse is huge!

(so is R Doradus, though I didn't know it is a star until just now)

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PM 2Ring
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Re: 1276: "Angular Size"

Postby PM 2Ring » Thu Oct 17, 2013 1:39 am UTC

S Doradus is no slouch, either.

gph
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Re: 1276: "Angular Size"

Postby gph » Sat Oct 19, 2013 4:17 pm UTC

Former ISS commander Chris Hadfield just posted a link to this comic, on his Facebook page.

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Klear
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Re: 1276: "Angular Size"

Postby Klear » Sun Oct 20, 2013 1:04 am UTC

gph wrote:Former ISS commander Chris Hadfield just posted a link to this comic, on his Facebook page.


...which makes him a bit more awesome than I thought possible.

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orthogon
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Re: 1276: "Angular Size"

Postby orthogon » Sun Oct 20, 2013 4:17 pm UTC

rmsgrey wrote:
orthogon wrote:Warning: I'm an engineer not a physicist. My explanation is Newtonian. General Relativity may render it totally wrong.


But only in cases where the Newtonian approximation breaks down - if you're not dealing with high (relative) speeds or accelerations, nor intense gravitational fields, Newton's going to be close enough.

Thanks - I thought not, but with General Relativity I don't even know what I don't know; at least with Special I know I can ignore it if the speeds are <<c.

Another point for qanuta: intuitively you can see that the result is in the right direction, since a football-sized rock a few metres away would subtend the same angle as the moon, but you wouldn't expect to detect its gravitational attraction; by contrast the moon can lift the oceans several metres.

On the other hand, if all the celestial bodies were discs of the same thickness, then the gravitational force would be the same for a given subtended angle (though I think someone said on another thread that such a disc wouldn't be able to support itself under its own gravity).

PinkShinyRose wrote:
LtBrenton wrote:The M25 is definitely a motorway, 'freeway' implies too much movement :p It's more like the world's largest collection of dunderheads, where only the most seasoned and brass-balled lane hoppers can actually get anywhere in the same day :lol:

Don't Americans also use the word highway for motorways? Would that have been better?

I would argue that a "motorway" is a specific type of highway, with certain features such as being dual carriageway, having a central crash barrier, a hard shoulder, emergency telephones every mile, markers every 100m, minimum lengths of "slip road" (=entry/exit ramp), no roundabouts or traffic lights, nationally consistent markings and signage conventions and special regulations for what types of vehicles are allowed. It's similar to types of road found elsewhere in Europe, and the closest parallel in the US is probably an Interstate Freeway, though I'm not convinced that Interstates don't go down to one lane each way in particularly deserted areas. But every country is different, and where possible I'd refer to a road using the local word, so that I might take the motorway to Dover, get on a ferry, pick up the Autoroute and eventually end up on the Autobahn. The latter, with its legendary default speed limit of 3x108m/s, is particularly distinctive and should not be confused with something like the M25. (I must admit that I don't know offhand what I would be driving on through the Netherlands :oops: ).
xtifr wrote:... and orthogon merely sounds undecided.

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PinkShinyRose
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Re: 1276: "Angular Size"

Postby PinkShinyRose » Sun Oct 20, 2013 10:08 pm UTC

orthogon wrote:On the other hand, if all the celestial bodies were discs of the same thickness, then the gravitational force would be the same for a given subtended angle (though I think someone said on another thread that such a disc wouldn't be able to support itself under its own gravity).

So a very large pizza can only be a calzone?
orthogon wrote:But every country is different, and where possible I'd refer to a road using the local word, so that I might take the motorway to Dover, get on a ferry, pick up the Autoroute and eventually end up on the Autobahn. The latter, with its legendary default speed limit of 3x108m/s, is particularly distinctive and should not be confused with something like the M25. (I must admit that I don't know offhand what I would be driving on through the Netherlands :oops: ).

The Dutch word is "snelweg", I think the official word is "autosnelweg" but very few people use that word in normal conversation.

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orthogon
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Re: 1276: "Angular Size"

Postby orthogon » Mon Oct 21, 2013 12:34 pm UTC

PinkShinyRose wrote:
orthogon wrote:On the other hand, if all the celestial bodies were discs of the same thickness, then the gravitational force would be the same for a given subtended angle (though I think someone said on another thread that such a disc wouldn't be able to support itself under its own gravity).

So a very large pizza can only be a calzone?

I fear the calzone would be a short-lived intermediate spacebread, and pretty soon you'd have a celestial doughball.
I forgot to mention that you'd have to have a "true view" of the pizza, i.e. be looking at it normally, for the gravity to depend only on subtended angle; also the subtended angle would need to be small enough that the whole enchilada is at approximately the same distance. (Space enchiladas are a completely different matter, of course).
xtifr wrote:... and orthogon merely sounds undecided.

AndrewGPaul
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Re: 1276: "Angular Size"

Postby AndrewGPaul » Mon Oct 21, 2013 4:12 pm UTC

orthogon wrote:I would argue that a "motorway" is a specific type of highway, with certain features such as being dual carriageway, having a central crash barrier, a hard shoulder, emergency telephones every mile, markers every 100m, minimum lengths of "slip road" (=entry/exit ramp), no roundabouts or traffic lights, nationally consistent markings and signage conventions and special regulations for what types of vehicles are allowed. It's similar to types of road found elsewhere in Europe, and the closest parallel in the US is probably an Interstate Freeway, though I'm not convinced that Interstates don't go down to one lane each way in particularly deserted areas. But every country is different, and where possible I'd refer to a road using the local word, so that I might take the motorway to Dover, get on a ferry, pick up the Autoroute and eventually end up on the Autobahn. The latter, with its legendary default speed limit of 3x108m/s, is particularly distinctive and should not be confused with something like the M25. (I must admit that I don't know offhand what I would be driving on through the Netherlands :oops: ).



In the UK (or possibly only England & Wales) a highway is any route, path, road, etc over which the general public may pass at will. Anything from a deer path to a ten-lane road. A motorway is a road on which only certain classes of vehicles are permitted. Pedestrians, animals, bicycles and low-powered vehicles such as invalid carriages and scooters/mopeds are not permitted on a motorway. There are some associated engineering/construction standards, but it's the legal status that's important. Various A-roads look like motorways, but you can still walk or ride a horse or bicycle down them (even if it's unwise, and you'll likely get wiped out by a 40-tonne artic).

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mathmannix
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Re: 1276: "Angular Size"

Postby mathmannix » Mon Oct 21, 2013 4:49 pm UTC

cellocgw wrote:Hey -- what's the difference between a British Soccer Field, a USA Soccer Field, and for that matter a Brazilian Soccer Field? :twisted:


Well, the Brazilian soccer field has had all its grass temporarily removed.
I hear velociraptor tastes like chicken.

androidbleepboop
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Re: 1276: "Angular Size"

Postby androidbleepboop » Tue Nov 05, 2013 1:17 am UTC

Oh man, I loved this one. I now love looking out at Orion's right shoulder, reaching out my arms and touching a cone connecting the center of the Earth to that distant star. Holy god Betelgeuse is enormous.


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