Wikipedia:Reference desk/Archives/Science/2021 March 4

= March 4 =

Why is Nyiragongo a stratovolcano?
Given the fluidity of its lava, why isn't it a shield volcano? ZFT (talk) 04:27, 4 March 2021 (UTC)


 * That's a good question, the neighbouring Nyamuragira is a shield volcano, although it erupts high potassium basalts. I've looked through this thorough review and I can't find a clear explanation - apart from the earliest stage eruptions, the volcano is dominated by lava flows. Mikenorton (talk) 09:04, 4 March 2021 (UTC)
 * I've seen a discussion a while ago on another volcano that the eventual slope of a volcano depends on the median lava flow length, which in turn is a function of the median volume of each eruption. So even a volcano with very liquid lavas will grow steep if the median lava flow is short. Jo-Jo Eumerus (talk) 11:19, 5 March 2021 (UTC)

Do astronomical observatories really shoot laser into sky like this?
Do astronomical observatories really shoot laser into sky like this? Here is the picture taken from Optical amplifier Rizosome (talk) 06:28, 4 March 2021 (UTC)


 * See laser guide star for an explanation of the technique - why would you doubt it? Mikenorton (talk) 08:44, 4 March 2021 (UTC)


 * In usual terminology, space observatories are observatories located in outer space. Observatories observing space (usually from the ground, but sometimes from space or even a high-altitude balloon, aircraft or from below the Earth's surface) are called astronomical observatories. And yes, sometimes they shoot a laser into the sky. Apart from lunar ranging, it's sometimes done for adaptive optics. Adaptive optics uses a deformable mirror to correct for diffraction of light in moving bubbles of air of different density. To measure the required corrections, the system has to observe a sufficiently bright reference star (much brighter than the object you're interested in). If no sufficiently bright star is available close enough to the object you want to observe, a laser can be used to project one in the higher parts of the atmosphere. Of course, they have to make sure they don't hit an aircraft passing over, as those lasers can be very annoying to pilots (just as aircraft can be annoying to astronomers). PiusImpavidus (talk) 09:02, 4 March 2021 (UTC) I changed my title, still your answer supports my question? Rizosome (talk) 13:46, 4 March 2021 (UTC)
 * Yes, that was already the question I answered. The laser-equipped observatories are always ground-based, not in the air, in space or below ground. Except gravitational wave observatories, which are laser-equipped and can be below ground, but don't shoot their laser beams into the sky. PiusImpavidus (talk) 16:35, 4 March 2021 (UTC)


 * The Royal Observatory, Greenwich has a similar laser, The Meridian Laser, which only serves to show the location of the Greenwich Meridian and has no scientific purpose beyond education. It's more of a museum than a working observatory, but one of their historic telescopes has been brought back into use recently. Alansplodge (talk) 15:30, 4 March 2021 (UTC)
 * Yes, I once saw that, when leaving the Maritime Museum on a winter afternoon/evening (after the fire alarm went off). PiusImpavidus (talk) 16:35, 4 March 2021 (UTC)