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March 23

Did any cultures or parts of history not mentioned in International Date Line also have a stationary line where new dates are born?

Which had at least a little currency in at least one culture. Anyone who didn't comment due to interpreting international strictly is welcome to comment here. Sagittarian Milky Way (talk) 18:26, 24 March 2018 (UTC)[reply]

Gegenschein

Is the bright spot in this image Gegenschein? I would like to add the image to the Wikipedia article for Gegenschein if so, but I want to be sure. Thanks! Steevven1 ^{(Talk) (Contribs) (Gallery)} 04:22, 23 March 2018 (UTC)[reply]

It looks too low to for it to not be twilight and ruin the picture, it's probably the zodiacal light. Sagittarian Milky Way (talk) 04:34, 23 March 2018 (UTC)[reply]

If I'm reading zodiacal light correctly, the first question to ask is whether the sun is immediately below the horizon in this image (in which case, it's zodiacal light) or is behind the photographer (in which case, it's gegenschein). Matt Deres (talk) 14:16, 23 March 2018 (UTC)[reply]

It can't be too immediately below the horizon, it has to be 40% of the way towards diagonally down or almost to be as dark as that photo. Sagittarian Milky Way (talk) 23:51, 23 March 2018 (UTC)[reply]

As an aside it is amusing that astronomers whinge on about light pollution but apparently their radio dishes are afraid of the dark and have to be illuminated at night. Greglocock (talk) 17:08, 23 March 2018 (UTC)[reply]

Joking apart, I'd guess that during the long exposure used for the sky, the photographer (or an assistant) walked up to each telescope in turn and illuminated it with a flash. [Which I now see is mostly confirmed by the photo's Summary data – duh!]

I concur that this is the Zodiacal light; one can clearly see its pyramidal shape, whereas the Gegenschein is discernably oval, and also less bright compared to the background sky, although the long exposures necessarily used for both can make this less obvious than to the naked eye. {The poster formerly known as 87.81.230.195} 2.218.14.51 (talk) 21:45, 23 March 2018 (UTC)[reply]

Thanks everyone! I'll add this image to the Zodiacal light article in a bit! Steevven1 ^{(Talk) (Contribs) (Gallery)} 00:41, 24 March 2018 (UTC)[reply]

Ultrasound repellent devices

Do these devices work? Could they work? Maybe insects are immune due to their lack of ears. But couldn't an ultrasound siren ward off at least cats and dogs? Dogs get normally scared by loud noises (like those of firecrackers), but would a 100-150 dB ultrasonic boom (not perceptible by humans) also scare them? Does ultrasound spread as well as sound in terms of range or intensity/distance? That is, would producing ultrasound through a tweeter be equivalent to producing normal sound through a loudspeaker? — Preceding unsigned comment added by Doroletho (talk • contribs) 23:08, 23 March 2018 (UTC)[reply]

I was going to direct you to our article, located at Electronic pest control, but it's pretty bad. Matt Deres (talk) 12:34, 24 March 2018 (UTC)[reply]

The article scratches the topic. But I don't mean concrete devices, or anything that you can find regularly in late night infomercials. As a matter of principle, could a burst of loud ultrasound scare some animals away? Specially dogs and cats, I imagine, are less likely to ignore noise. I suppose ultrasound is just noise for them. --Doroletho (talk) 13:03, 24 March 2018 (UTC)[reply]

It is the general consensus that animals rapidly grow used to such devices, limiting their effectiveness. Abductive (reasoning) 13:15, 24 March 2018 (UTC)[reply]

Such devices at a slightly lower frequency (15 to 18 KHz) have been used to deter teenagers from congregating in shop doorways. See The Mosquito and Acoustic harassment device. Dbfirs 17:34, 24 March 2018 (UTC)[reply]

@Dbfirs: what sort of unit is the Kelvin Hz? Klbrain (talk) 00:20, 25 March 2018 (UTC)[reply]

Thank you for drawing my attention to the typo. I meant kHz of course. Why can't SI units use consistent capitalisation? I was brought up on Imperial. Dbfirs 07:16, 25 March 2018 (UTC)[reply]