Wikipedia:Reference desk/Archives/Science/2024 June 21

= June 21 =

Noctilucent clouds
How high (in angle) do noctilucent clouds have to be to be seen? Do they have to appear outside the lighter area of the sky during nautical twilight? I've been watching clouds before last few sunrises but I'm not sure if i'm just observing high altitude cirruses, they are whitish but not bright at all like in the pictures. Other (lower) clouds stay dark for a long time after that, almost until sunrise. I can't tell if they disappear after sunrise because there's a lot of humidity haze or maybe still Sahara's smog, and my phone is terrible at taking pictures at twilight. Please don't just tell me to read the page, it hasn't helped me with this. 31.217.31.107 (talk) 02:21, 21 June 2024 (UTC) They were visible from around 3.30 to 4.30 here (30min ago) in Zagreb, whatever they are. They weren't that sharp like on the pictures. 31.217.31.107 (talk) 02:57, 21 June 2024 (UTC)


 * Under the right conditions you should be able to see them at any angle, wherever they are hanging out, from the horizon to directly overhead. --Lambiam 05:48, 21 June 2024 (UTC)
 * I saw them at up to 45 degrees up (the 'height' of the polar star here). They didn't really look like the pictures in the article, they were more faint and blurry although they were lighter than the sky all along while low clouds are darker than the sky that close to sunrise. I think they could look that way because of heavy light pollution lately and high humidity but I'm still not sure if they weren't just cirruses because low clouds are always lighter than the sky near midnight, there's enough light pollution. Is there any way to be sure? 31.217.31.107 (talk) 05:58, 21 June 2024 (UTC)
 * On the only occasion I've seen noctilucent clouds, they were up to 4 degrees above the horizon (half the altitude of Capella at that time), almost straight north. At the same time, the sun was also almost straight north, 14 degrees below the horizon. Not nautical twilight, but the sky was still fairly bright and the noctilucent clouds were in the bright part. With those angles, the radius of the Earth and some high-school geometry, you should be able to calculate the minimum altitude of the clouds to be in sunlight. If that's more than about 12 kilometres, they can't have been ordinary cirrus clouds. Use a planetarium program like Stellarium (open source) to find the position of the sun at the time you observed the clouds. PiusImpavidus (talk) 08:11, 21 June 2024 (UTC)

Lunar Standstill
A lunar standstill is supposed to happen tonight. I've read the article's simplified description and I'm still confused. So, is this correct? In my own words...

During a lunar standstill, the Moon does not actually stand still, nor does it appear to. All this means is that tonight the Moon will rise at its most northeastern point and set at its most northwestern point. Period. It's not something you can go out at a specific time to observe, see something different, the event ends, and you go about your evening. It's an event that takes "all" (air quotes) night (the time the moon is out).

Do I have that right?

Thanks! † dismas †|(talk) 12:14, 21 June 2024 (UTC)
 * I wasn't aware of this, this is quite interesting (if you're into that sort of thing). "Standstill" refers to (the lack of) motion in declination, i.e. the north-south coordinate (equivalent to latitude on earth), and is equivalent to the solstices. Summer solstice was yesterday (sun at its northern-most point), we also have full moon tonight, and since the full moon is opposite to the sun this means that moon is at its southern-most point (not northern!). In addition the moon is at its lowest point below the ecliptic, so that in sum we have a major lunistice. In principle these are all point events but the changes over the course of a night are so small that it is effectively an all-nighter. Where I am, the moon will only be 12 degrees above the horizon when it culminates in the south around midnight. --Wrongfilter (talk) 13:31, 21 June 2024 (UTC)
 * According to this article, we're not at the absolute extreme tonight, though. The situation is quite complicated and the extremes differ for moonrise and moonset, compare the graphics in that article. --Wrongfilter (talk) 13:44, 21 June 2024 (UTC)
 * For about one year every 18.61 years your hometown Moon rise/peak/set paths are more extreme than any other time (north/south, rises and sets unusually left/starboard, also low/high if you're between latitudes 29 and 62 or so. North or south of 29 you'll never see a shorter hometown Moon shadow than one of these fortnightly standstills in the next year or so probably near one of the standstills near a half moon near one of the next 2 equinoxes (the Full Moons can be up to 1.2 Moon radii less extreme but the c. 19 year cycle is 20 radii to minus 20 radii) Sagittarian Milky Way (talk) 14:01, 21 June 2024 (UTC)
 * The cycle peaks early 2025 I believe but cause complexities records can be up to seasons away. Records like rightmost Full Moon rise for your location between about 2006 and 2043 or lowest Full Moon path of a night in that timeframe, might be tonight or about 355 days in the future. High Full Moon records are probably December 2024, some other records are probably near an equinox in 2025 or late 2024. If you want I can get each major record to the nearest millisecond (false precision?) for any location on Earth. From what might be the most accurate possible way (https://ssd dot jpl dot nasa dot gov/horizons/app.html#/, there's also other interfaces like email and telnet but for this you don't need the relatively few email/telnet-only tools) Sagittarian Milky Way (talk) 14:27, 21 June 2024 (UTC)
 * From 74W 40¾N 0ft the highest Moon center in the future of anyone alive is 77.753639° the year 2025 April 3rd 6:05:02.857pm (ellipsoidal coordinates). The highest before that was 2006 Mar 7th 18:56:52.857 77.753848°. The sky was pretty dark for this one as it's always near half moon so can't be too close to midnight. The highest before that was 1950 October 3 6:07:53.010am 77.782823°. These follow the obliquity Milankovitch cycle which is slow enough that a new record doesn't happen every 18.61 year cycle as you go back in time. So the highest since about 3X thousand BC was about 10,000 years ago. Sagittarian Milky Way (talk) 21:19, 21 June 2024 (UTC)
 * From Earth's center the northernmost Moon of Oct 2006 to Aug 2043 inclusive is March 7 2025 15:56 UTC 28°43.0'N. Thus longitudes of about 45E will get a slightly more northern Moon record than any other place on Earth. Sagittarian Milky Way (talk) 17:25, 22 June 2024 (UTC)
 * The cycle is dependent on the longitude of the ascending node.  This book  says the longitude of the ascending node (☊) = ☊o - 0.0529539De, where ☊o is the ecliptic longitude of the ascending node at the standard epoch J2000 and ☊ is the ecliptic longitude of the ascending node after De (an interval measured in days).   The ecliptic longitude of the ascending node at the standard epoch (12h 1 January 2000) was 125.044522°.
 * Now, the major standstill occurs when the longitude of the ascending node is 0°, which means that the total movement since J2000 is 360° + 125.044522° = 485.044522° (remember the movement is backwards).  The number of days since J2000 is therefore 485.0422522/0.0529539 = 9159.750689 days.   24 years contain 8766 days, so the standstill occurs (9160 - 8766) = 394 days after 1 January 2024, or around 29 January 2025.   I have just found out that the calendar of golden numbers (mentioned on the Reference desk from time to time) is (or was) mentioned elsewhere in Wikipedia, and the text given there has been augmented to embrace the matters raised here:

LUNAR CALENDAR 1 MARCH 1900 - 28 FEBRUARY 2200

The lunar date for 29 February of a leap year is normally the same as that of the preceding day - thus the lunar date for 28 and 29 February 2028 is 3 Ronan. For use of the letters A - g to find the day of the week see Dominical letter. The months are: (1) Harriet, (2) Ronan, (3) Miri, (4) James, (5) Eloise, (6) Thomas, vii, (8) Nicholas, (9) Catherine, (10) Richard, (11) Emma, (12) Paul. Paul II, a 30-day month, is added between Paul and Harriet 7 times in 19 years. When the golden number is 19, Richard has 29 days instead of 30. See Saltus (third bullet point).

The numbers move down one day on 1 March of years which, although divisible by four without remainder, are not leap years. They move up one day eight times in 2500 years. The next eight movements will be on 1 March of 2100, 2400, 2700, 3000, 3300, 3600, 3900 and 4300. Sometimes the movements cancel out - thus in 2100 the numbers stay where they are. The numbers mark the first days of the lunar month. Each year's golden number is found by adding 1, dividing by 19 and taking the remainder. If the remainder is 0, the golden number is 19. Until 2099 Orthodox Easter falls from 19 - 25 Miri, on whichever day is Sunday. From 2100 to 2399 the range is 20 - 26 Miri, and so on. So to find Orthodox Easter in 2025:
 * 2025 + 1 = 2026. 2026/19 = 106 remainder 12. From table, 1 Miri is 31 March. The Sunday letter is E. 20 April (21 Miri) is an "E" day. Orthodox Easter is 20 April.

To calculate the date of occidental Easter, proceed as follows:

1. In the calendar, locate the date of 14 Miri

2. If 14 Miri falls on or before 17 April, Easter is the Sunday following. If 14 Miri falls on 18 April and no golden number is marked against 6 April, again Easter is the Sunday following. If 14 Miri falls on 18 April and a golden number is marked against 6 April, Easter falls on 18 April (if Sunday), and if 18 April is not Sunday Easter falls on the following Sunday.

3. If 14 Miri falls on 19 April, Easter falls on 19 April (if Sunday), and if 19 April is not Sunday Easter falls on the following Sunday.

4. If 14 Miri falls on 20 April or later, the date is to be treated as a day of March, and Easter falls on the day after the Saturday following that date.

The calendar may be used to locate the moon at any given time.

Example

It is 9 PM Greenwich Mean Time in London on 15 February 2024. The golden number is 11, which is printed against 11 February. This is the lunar new year (1 Harriet) and 15 February is therefore 5 Harriet. At the end of the previous month (as at the end of every lunar month) the sun and moon are together in the sky, but the lunar day is on average 4/5 hour longer than the solar day. Thus at 9 PM it is only 5 PM by the moon. Whether the moon is visible at that time may be determined using the fact that the moon moves through the zodiac at the rate of 13.2° per day (compared to 1° per day for the sun). On 15 February (5 Harriet) the moon will have advanced (5 x (13.2 - 1)) = 5 x 12.2 = about 61° ahead of the sun. So it will be where the sun will be about 61 days later, i.e. around 16 April.

But there is another factor. The moon's ascending node (where the plane of its orbit crosses the ecliptic in a northerly direction) moves backwards, completing a circuit relative to the equinox in 18.6 years. When the longitude is 0° (which it will reach around 29 January 2025) it reaches a maximum of 5° further from the celestial equator than does the sun (the major standstill). Half a revolution later (the minor standstill) it reaches a maximum of 5° nearer to the celestial equator than does the sun.

So considering the moon's position at 9 PM on 15 February 2024 we look whewre the sun would be at 5 PM on 16 April and (since the date is fairly close to the major standstill) a little higher in the sky. The moon was thus looked for (and was seen) high in the west.

The "establishment" of a port is the number of hours high tide is reached there after the moon crosses the meridian (i.e. 12 noon or midnight by the moon). The state of the tides may thus be predicted using the method above, remembering that tides are highest at the middle and the end of the lunar month ("spring tides") and their amplitude is greatest at the equinoxes. For accurate predictions consult specialist tables.