Wikipedia:Reference desk/Archives/Science/2020 July 26

= July 26 =

Moon question.
What part of the world sees the moon the least? Or has the hardest time seeing the moon, like if the moon can only be seen down circling the horizon. 166.137.83.64 (talk) 07:35, 26 July 2020 (UTC).
 * Perhaps you will want some constraints, eg the moon is invisible inside most caves, and inside many forests. The Aleutian Islands are nearly always covered in cloud, so they cannot see the moon either. Deep valleys would have reduced time of moon in the sky too. The south facing slopes in Antarctica would also get less sun and moonshine. Graeme Bartlett (talk) 07:45, 26 July 2020 (UTC)
 * In Graeme's response, for "could" read "cloud". 2A00:23C1:E101:D700:2C7F:D268:D5DE:B039 (talk) 10:48, 26 July 2020 (UTC)
 * As the orbit of the Moon is within a few degrees of the ecliptic, it follows that an observer in the Arctic or Antarctic will generally see it lower in the sky than is true for observers elsewhere. However, it would not be a case of "only down circling the horizon"; even at the poles it could be about 28° above the horizon at times, I believe. --174.89.49.204 (talk) 07:53, 26 July 2020 (UTC)
 * The angle of the Moon's orbital plane with the Earth's equatorial plane oscillates between about 18.5° and 28.5°, with a period of 18.61 years. Assuming a perfectly spherical cow Earth with no geological, architectural or atmospheric impediments to Moon gazing, the Moon should on the average be visible half the time – but near the Earth's poles a half synodic month of visibility will alternate with a half synodic month of invisibility. --Lambiam 11:28, 26 July 2020 (UTC)
 * The question was about different places, not different times. --174.89.49.204 (talk) 22:55, 27 July 2020 (UTC)
 * And my answer was that it is half the time everywhere. --Lambiam 20:50, 29 July 2020 (UTC)
 * That's not quite true. For example, moonrises and moonsets (like sunrises and sunsets) take longer at higher latitudes. If you count the moon as being visible if any part of it is above the horizon (as is usually done with the sun AFAIK), then I'd guess that the polar circles are a good bet for maximum moon time, while equator is a good bet for minimum. That's before taking into account all the cow non-sphericities: Earth's flattening, possible long-term correlations between Earth's rotation, month length and Saros cycle etc. If any of these effects exist they would further unbalance moon visiblity time. 93.136.62.103 (talk) 16:42, 30 July 2020 (UTC)
 * The extremes are 5.295 degrees plus ecliptic, either north or south. This occurs when Luna is far from Earth, syzygys, solstices or perihelions. Right now ecliptic is 23.438 degrees or less, decreasing slowly so maximum should be 28.733 degrees for the center, almost but not quite 29 degrees for the top and at least a minute of arc or so on top of that for atmospheric refraction. Wider moons but closer to the 4.995 degree minimum shouldn't improve that much if at all. But wait! There's parallax. So subtract about a degree for that and you get about 28 degrees for the top. The tropic of Cancer for moons is about Orlando. No parallax there the rare times you're right under it. Sagittarian Milky Way (talk) 01:22, 27 July 2020 (UTC)
 * And I'm wrong of course, the highest Moon center of 1988-2024 or more from the North Pole is 27.92357 degrees September 14, 2006 21:37 Eastern US time or so, but really more cause North Pole weather is usually colder than the Earth average 15 Celsius around the equinoxes which can refract light more. My error is that the North Pole is not the top of the Earth from a Moonar perspective when the Moon is also at almost 29 degrees North. Sagittarian Milky Way (talk) 02:02, 27 July 2020 (UTC)
 * Between the 60s and the 2060s exclusive the highest from any pole was 1987-Sep-15 27.9452 degrees. Far out man. Sagittarian Milky Way (talk) 02:53, 27 July 2020 (UTC)


 * Why a synodic month? Surely the nodes precess slowly enough that a sidereal month is more relevant here. —Tamfang (talk) 02:28, 27 July 2020 (UTC)
 * The fact that the length of a synodic month varies through the year is a giveaway that this is the wrong kind of month here. It should be the kind of month that lasts the period of time between two successive events in which the Moon passes through the Earth's equatorial plane from (say) the southern to the northern sky. Is that an anomalistic month? Or a tropical month? My head also starts spinning. --Lambiam 08:11, 27 July 2020 (UTC)
 * It's not one of the major 5 moonths, tropical month should be the long term average. Sagittarian Milky Way (talk) 13:16, 27 July 2020 (UTC)
 * If the ascending node is entering the southern hemisphere this moonth will be shorter than tropical, if it's on the other side of the sky it'll be longer. Sagittarian Milky Way (talk) 13:47, 27 July 2020 (UTC)

Olympus Mons
Would it make sense to have a launch site, maybe even an electromagnetic one, on the peak of Olympus Mons on Mars, to save fuel? Greetings, 2003:C3:EF2A:EC86:C085:C6A3:FBFE:686B (talk) 11:03, 26 July 2020 (UTC)


 * To a first approximation, the energy needed to make a given fixed mass escape from a planet is inversely proportional to the distance from the centre of the planet. The height of Olympus Mons is about 0.6% of the radius of Mars, so the saving would be about 0.6%. In practice it should be more for a fuel-driven rocket, since the fuel consumption needed to achieve the same thrust is higher at the surface, and therefore the energy needed in practice is not really inversely proportional. Whether the saving is worth the trouble of having to haul up everything to the high launch site depends on too many factors for a back-of-the-envelope calculation. In SF stories, larger space vessels typically remain in orbit, with smaller shuttles operating between orbit and the surface. The lighter the shuttle, the less interesting such savings. Presumably, most shuttles will operate from launch sites near settlements (operational bases), and other considerations about the most suitable locations for operational bases, such as the local availability of natural resources, may well outweigh considerations of fuel efficiency. Using not the peak but the slope of Olympus Mons for the track of an Electromagnetic Shuttle Launch System (ESLS), on the other hand, seems (IMO) worth serious consideration. I'm too lazy to do the calculation, but I guess one should comfortably be able to reach escape velocity given the track length that is possible. --Lambiam 12:44, 26 July 2020 (UTC)


 * Thank you very much! --2003:C3:EF2A:EC86:C085:C6A3:FBFE:686B (talk) 13:28, 26 July 2020 (UTC)


 * Another option for non-rocket spacelaunch would be a space elevator. One could be built on Mars with existing materials, as Mars has much lower gravity than Earth. --47.146.63.87 (talk) 06:19, 27 July 2020 (UTC)
 * The space elevator should be anchored at the Arean equator, thereby leaving Olympus Mons out of the picture. --Lambiam 07:41, 27 July 2020 (UTC)
 * Another thing to keep in mind is the rotational speed of the surface, wich is maximal at the equator, and a "free gift" for launches into equatorial orbits. Olympus Mons is close enough to the equator that that would not probably disqualify it, but it reduces the benefit. --Stephan Schulz (talk) 08:28, 27 July 2020 (UTC)
 * And the track of an electromagnetic launch system on the slope of Olympus Mons should use the western side (where East and West are with respect to the directions where the Sun rises and sets on Mars), so as to launch in the easterly direction and add rather than subtract from that free gift. --Lambiam 08:48, 27 July 2020 (UTC)


 * The rim of one of the great volcanos, I think Pavonis, crosses the equator, so that's the obvious site for an elevator (used by Kim Stanley Robinson and perhaps others). —Tamfang (talk) 00:01, 1 August 2020 (UTC)