Thuban, Polaris, Vega
March 5, 2019 8:53 AM Subscribe
It's clear that when Hoover dam was built, it was considered to be a new Wonder of the World like the Great Pyramid. When I visited it, I saw the old display model and film that visitors used to be shown, and it was filled with optimism and pride at what human-kind could accomplish. I wish I had known about this!
posted by acrasis at 9:44 AM on March 5, 2019 [7 favorites]
posted by acrasis at 9:44 AM on March 5, 2019 [7 favorites]
It's amazing to imagine a government willing to fund such a wonderful and arcane piece of art/science like this. I doubt anything like this would survive the howls of budget hawks today. More's the pity.
posted by Thorzdad at 9:53 AM on March 5, 2019 [5 favorites]
posted by Thorzdad at 9:53 AM on March 5, 2019 [5 favorites]
This is fascinating!
I wonder how much geological changes have affected/ will affect the accuracy of the astronomical clock?
posted by porpoise at 10:13 AM on March 5, 2019
I wonder how much geological changes have affected/ will affect the accuracy of the astronomical clock?
posted by porpoise at 10:13 AM on March 5, 2019
This is neat! I've never been, but now it's on my list of things to look for when I get there.
(The Long Now foundation is an amazing project - I think Neal Stephenson's Anathem is the piece of fiction that pairs most closely with it. Their home in San Francisco is also a cocktail bar, cafe, and museum - The Interval - and it's a marvelous spot for a drink.)
posted by RedOrGreen at 10:14 AM on March 5, 2019 [1 favorite]
(The Long Now foundation is an amazing project - I think Neal Stephenson's Anathem is the piece of fiction that pairs most closely with it. Their home in San Francisco is also a cocktail bar, cafe, and museum - The Interval - and it's a marvelous spot for a drink.)
posted by RedOrGreen at 10:14 AM on March 5, 2019 [1 favorite]
> I wonder how much geological changes have affected/ will affect the accuracy of the astronomical clock?
Not a geologist, but if I had to guess, it is minimal. The Earth is 4 billion years old; dinosaurs ruled the world during the Jurassic era, from 200,000,000 years ago to 145,000,000 years ago. The precession timescale is 26,000 years, and the dam construction era (1936) is barely an eyeblink away on these geological timescales.
On further browsing: Wikipedia says that the continents were essentially at their present day positions at the Pleistocene era, which ranges from 2,600,000 years ago to 11,700 years ago. That's actually more recent than I thought. But still, not relevant on 100-year timescales.
posted by RedOrGreen at 10:25 AM on March 5, 2019 [1 favorite]
Not a geologist, but if I had to guess, it is minimal. The Earth is 4 billion years old; dinosaurs ruled the world during the Jurassic era, from 200,000,000 years ago to 145,000,000 years ago. The precession timescale is 26,000 years, and the dam construction era (1936) is barely an eyeblink away on these geological timescales.
On further browsing: Wikipedia says that the continents were essentially at their present day positions at the Pleistocene era, which ranges from 2,600,000 years ago to 11,700 years ago. That's actually more recent than I thought. But still, not relevant on 100-year timescales.
posted by RedOrGreen at 10:25 AM on March 5, 2019 [1 favorite]
The 27,000 year period is a precession of the angle with the equatorial plane. So it also can be thought of as a precession of the seasons. If you stretch the calendar back to a January in the northern hemisphere 13,000 years ago - it would be the height of summer.
Regarding the celestial sphere, stars themselves are also moving across the sky with their own proper motion. By the time we swing back to Polaris in 27,000 years, Polaris itself will have moved.
posted by vacapinta at 10:27 AM on March 5, 2019 [5 favorites]
Regarding the celestial sphere, stars themselves are also moving across the sky with their own proper motion. By the time we swing back to Polaris in 27,000 years, Polaris itself will have moved.
posted by vacapinta at 10:27 AM on March 5, 2019 [5 favorites]
Wikipedia has a good graphic of the axial precession overlaid on a star chart. You can see where the pole passes over Thuban about 5000 years ago and where it lays today next to Polaris.
You can also see how Julius Caesar contains an anachronism in Caesar's famous line: "I am constant as the northern star / Of whose true-fix'd and resting quality / There is no fellow in the firmament". In Caesar's time there was no pole star, the closest was Beta Ursae Minoris, ladle end of Ursa Minor, but it was about ~9 degrees away from the pole. Alpha Ursae Minoris (aka "Polaris") was accepted as the pole star in Shakespeare's time, but not Caesar's.
posted by peeedro at 12:48 PM on March 5, 2019 [32 favorites]
You can also see how Julius Caesar contains an anachronism in Caesar's famous line: "I am constant as the northern star / Of whose true-fix'd and resting quality / There is no fellow in the firmament". In Caesar's time there was no pole star, the closest was Beta Ursae Minoris, ladle end of Ursa Minor, but it was about ~9 degrees away from the pole. Alpha Ursae Minoris (aka "Polaris") was accepted as the pole star in Shakespeare's time, but not Caesar's.
posted by peeedro at 12:48 PM on March 5, 2019 [32 favorites]
> Julius Caesar contains an anachronism in Caesar's famous line: "I am constant as the northern star [...]"
This is utterly fantastic! I did not know this, and now I'll never forget...
posted by RedOrGreen at 12:59 PM on March 5, 2019
This is utterly fantastic! I did not know this, and now I'll never forget...
posted by RedOrGreen at 12:59 PM on March 5, 2019
It's a tangent, but there's another more well-known anachronism in Act II, scene ii of Julius Caesar:
Striking clocks? Very much not a thing in ancient Rome, much like Aaron the Moor in Titus Andronicus.
posted by Mr. Bad Example at 1:49 PM on March 5, 2019 [2 favorites]
CAESAR
What is 't o'clock?
BRUTUS
Caesar, 'tis strucken eight. Striking clocks? Very much not a thing in ancient Rome, much like Aaron the Moor in Titus Andronicus.
posted by Mr. Bad Example at 1:49 PM on March 5, 2019 [2 favorites]
If you stretch the calendar back to a January in the northern hemisphere 13,000 years ago - it would be the height of summer.
I was curious about this and it looks like with current definitions this wouldn't happen -- we futz with leap seconds and whatnot to keep northern-hemisphere winter in December. What would be different is the constellations visible at night in January.
posted by GCU Sweet and Full of Grace at 1:59 PM on March 5, 2019 [1 favorite]
I was curious about this and it looks like with current definitions this wouldn't happen -- we futz with leap seconds and whatnot to keep northern-hemisphere winter in December. What would be different is the constellations visible at night in January.
posted by GCU Sweet and Full of Grace at 1:59 PM on March 5, 2019 [1 favorite]
This is awesome! Here's another site, with more pictures, which also lays out how he uses the moon, Jupiter, and Mercury in the chart to give the exact date. Any sufficiently advanced stargazing civilization of the future ought to be able to work out when we built it with good accuracy!
(Like I'm pretty sure medieval folks could do it, given adequate time and paper to work out the calculations. I don't know a whole lot about pre-Gregorian astronomical math though, so possibly much earlier.)
posted by Eyebrows McGee at 3:21 PM on March 5, 2019 [1 favorite]
(Like I'm pretty sure medieval folks could do it, given adequate time and paper to work out the calculations. I don't know a whole lot about pre-Gregorian astronomical math though, so possibly much earlier.)
posted by Eyebrows McGee at 3:21 PM on March 5, 2019 [1 favorite]
Very cool story.
I admit that the creator's prose is tricky, but this line stands alone: "There is an angle for doubt, for sorrow, for hate, for joy, for contemplation, and for devotion."
posted by doctornemo at 4:42 PM on March 5, 2019
I admit that the creator's prose is tricky, but this line stands alone: "There is an angle for doubt, for sorrow, for hate, for joy, for contemplation, and for devotion."
posted by doctornemo at 4:42 PM on March 5, 2019
...But that point near Polaris, which we call the North Star, is actually slowly moving and tracing a circle through the night sky. While Polaris is our North Star, Hansen’s terrazzo floor points out that the North Star of the ancient Egyptians, as they built the great pyramids, was Thuban. And in about 12,000 years, our North Star will be Vega.
And at that future time, the Vegans will probably Contact us again, but with a new message containing the last Trump campaign rally.
posted by cenoxo at 6:29 PM on March 5, 2019
And at that future time, the Vegans will probably Contact us again, but with a new message containing the last Trump campaign rally.
posted by cenoxo at 6:29 PM on March 5, 2019
I was curious about this and it looks like with current definitions this wouldn't happen -- we futz with leap seconds and whatnot to keep northern-hemisphere winter in December. What would be different is the constellations visible at night in January.
Um... so I thought that leap seconds and days and stuff were to keep Earth's calendar in synch with its location in its orbit... but a precession of the angle with the equatorial plane would mean that earth's tilt would be changed, and so what is now a Northern Hemisphere Winter tilt in January would be opposite in 13,000 years and would be a Northern Hemisphere Summer tilt.
Or am I missing something entirely here?
posted by hippybear at 6:52 PM on March 5, 2019
Um... so I thought that leap seconds and days and stuff were to keep Earth's calendar in synch with its location in its orbit... but a precession of the angle with the equatorial plane would mean that earth's tilt would be changed, and so what is now a Northern Hemisphere Winter tilt in January would be opposite in 13,000 years and would be a Northern Hemisphere Summer tilt.
Or am I missing something entirely here?
posted by hippybear at 6:52 PM on March 5, 2019
I gathered from the limited bits I could quickly find that one of the things they're doing (or at least have plans to do) with leap seconds and what not is very slowly moving January 1 around Earth's orbit, so that it keeps pointing the northern hemisphere away from Sol.
It could all be horseshit ultimately founding in one guy's time-cube-esque rantings for all I know. I didn't look hard or for very long, but the few sites I glanced at all said that the timekeeping boffins had this covered.
posted by GCU Sweet and Full of Grace at 7:05 PM on March 5, 2019
It could all be horseshit ultimately founding in one guy's time-cube-esque rantings for all I know. I didn't look hard or for very long, but the few sites I glanced at all said that the timekeeping boffins had this covered.
posted by GCU Sweet and Full of Grace at 7:05 PM on March 5, 2019
> I thought that leap seconds and days and stuff were to keep Earth's calendar in synch with its location in its orbit...
Yep.
> but a precession of the angle with the equatorial plane would mean that earth's tilt would be changed, and so what is now a Northern Hemisphere Winter tilt in January would be opposite in 13,000 years and would be a Northern Hemisphere Summer tilt.
It's not immediately obvious that the inclination of the axis is the same angle as the angle of precession, but apparently it is, so you're exactly right. The Wikipedia article on axial precession sums it up - in particular, look at this image.
That said, I'm really not sure whether the leap seconds do in fact add up to keeping the equinoxes on the correct dates. I didn't think so - especially since we were about to abolish leap seconds - but now I'm not sure...
posted by RedOrGreen at 7:33 PM on March 5, 2019
Yep.
> but a precession of the angle with the equatorial plane would mean that earth's tilt would be changed, and so what is now a Northern Hemisphere Winter tilt in January would be opposite in 13,000 years and would be a Northern Hemisphere Summer tilt.
It's not immediately obvious that the inclination of the axis is the same angle as the angle of precession, but apparently it is, so you're exactly right. The Wikipedia article on axial precession sums it up - in particular, look at this image.
That said, I'm really not sure whether the leap seconds do in fact add up to keeping the equinoxes on the correct dates. I didn't think so - especially since we were about to abolish leap seconds - but now I'm not sure...
posted by RedOrGreen at 7:33 PM on March 5, 2019
It's a tangent, but there's another more well-known anachronism in Act II, scene ii of Julius Caesar:
CAESAR
What is 't o'clock?
BRUTUS
Caesar, 'tis strucken eight.
Striking clocks? Very much not a thing in ancient Rome
From the Wikipedia page on water clocks [emphasis mine]:
posted by hippybear at 7:45 PM on March 5, 2019
CAESAR
What is 't o'clock?
BRUTUS
Caesar, 'tis strucken eight.
Striking clocks? Very much not a thing in ancient Rome
From the Wikipedia page on water clocks [emphasis mine]:
Between 270 BC and AD 500, Hellenistic (Ctesibius, Hero of Alexandria, Archimedes) and Roman horologists and astronomers were developing more elaborate mechanized water clocks. The added complexity was aimed at regulating the flow and at providing fancier displays of the passage of time. For example, some water clocks rang bells and gongs, while others opened doors and windows to show figurines of people, or moved pointers, and dials. Some even displayed astrological models of the universe. The 3rd century BC engineer Philo of Byzantium referred in his works to water clocks already fitted with an escapement mechanism, the earliest known of its kind.[35]So, um... yeah.
posted by hippybear at 7:45 PM on March 5, 2019
Ack, I missed the edit window -- Wikipedia on water clocks.
posted by hippybear at 7:50 PM on March 5, 2019
posted by hippybear at 7:50 PM on March 5, 2019
sorry, when I wrote "leap seconds and whatnot" I just meant "timekeeping stuff I don't immediately understand" and should have written "meow meow timekeeping meow."
posted by GCU Sweet and Full of Grace at 8:12 PM on March 5, 2019 [2 favorites]
posted by GCU Sweet and Full of Grace at 8:12 PM on March 5, 2019 [2 favorites]
much like Aaron the Moor in Titus Andronicus.
I hate to keep being That Guy, but I'm just doing basic googling. Moors were in ancient Rome. One of them became a Roman emperor.
posted by hippybear at 8:45 PM on March 5, 2019
I hate to keep being That Guy, but I'm just doing basic googling. Moors were in ancient Rome. One of them became a Roman emperor.
posted by hippybear at 8:45 PM on March 5, 2019
To keep the seasons in synch with the calendar, we add leap days. One every four years is too many, so we leave off the leap day in century years like 1900 or 2100. But leaving off a leap day every century gives too few leap days, so we add them back every fourth century, as you may recall if you were around on February 29, 2000. The special treatment of leap days in century years is the difference between the Julian and (modern) Gregorian calendars, and switching from J to G required skipping several calendar days in the which had erroneously been celebrated as leap days in previous centuries. The driving force behind this computational effort was to keep the spring equinox at March 21, to simplify the accurate prediction of the correct date for Easter.
Earth's precession is the reason that the tropical year --- that is, the mean time between spring equinoxes --- is twenty minutes shorter than the sidereal year, which is the interval over which Earth and Sun make the same angle with respect to the stars. Twenty minutes is 1/26,000th of a year. To follow a sidereal calendar, so that "March" continues to name when the Sun is in Aries even when that no longer corresponds to Spring, we would have a few more leap days in century years, but still not every century as in the Julian calendar. I think. I figure this out every few years and I mix up a "longer/briefer" every time.
Leap seconds aren't related to the seasons. They're inserted because Earth's daily rotational period isn't exactly 24 hours. Without leap seconds, the Sun would eventually not be overhead at noon. But with a leap second needed only once every few years, that's a much smaller correction than Earth's processional motion. Leap seconds, unlike leap days, are unpredictable: Earth's rotation changes due to stochastic events like earthquakes.
posted by fantabulous timewaster at 8:47 PM on March 5, 2019 [2 favorites]
Earth's precession is the reason that the tropical year --- that is, the mean time between spring equinoxes --- is twenty minutes shorter than the sidereal year, which is the interval over which Earth and Sun make the same angle with respect to the stars. Twenty minutes is 1/26,000th of a year. To follow a sidereal calendar, so that "March" continues to name when the Sun is in Aries even when that no longer corresponds to Spring, we would have a few more leap days in century years, but still not every century as in the Julian calendar. I think. I figure this out every few years and I mix up a "longer/briefer" every time.
Leap seconds aren't related to the seasons. They're inserted because Earth's daily rotational period isn't exactly 24 hours. Without leap seconds, the Sun would eventually not be overhead at noon. But with a leap second needed only once every few years, that's a much smaller correction than Earth's processional motion. Leap seconds, unlike leap days, are unpredictable: Earth's rotation changes due to stochastic events like earthquakes.
posted by fantabulous timewaster at 8:47 PM on March 5, 2019 [2 favorites]
But does that keep the SEASONS in synch with the calendar, or does it keep the equinoxes/solstices in synch with the calendar? Because since the development of our modern calendar, we haven't had enough precession take place to even notice the here-hypothesized seasonal shift.
posted by hippybear at 8:51 PM on March 5, 2019
posted by hippybear at 8:51 PM on March 5, 2019
Solstices and equinoxes mark seasons, regardless. Whether the calendar syncs up with either of those is another question, but those things are surely identical.
posted by sjswitzer at 8:57 PM on March 5, 2019
posted by sjswitzer at 8:57 PM on March 5, 2019
But when the June 21 Solstice marks the peak of the northern hemisphere being tilted away from the sun... that's surely a seasonal marker, but it isn't what we experience now, and that it still falls on June 21 is at best a mathematical achievement.
posted by hippybear at 9:01 PM on March 5, 2019
posted by hippybear at 9:01 PM on March 5, 2019
The equinoxes and solstices, which are notable points in the annual variation of how sunlight is distributed across Earth's surface, determine the seasons.
As for how long it would take to figure out if the modern calendar were tracking the seasons correctly: it took about sixteen centuries, accumulating error in the dates of equinoxes at approximately one day per century. Plus a delay of a few centuries before the improved calendar was adopted worldwide, for political reasons.
posted by fantabulous timewaster at 9:04 PM on March 5, 2019
As for how long it would take to figure out if the modern calendar were tracking the seasons correctly: it took about sixteen centuries, accumulating error in the dates of equinoxes at approximately one day per century. Plus a delay of a few centuries before the improved calendar was adopted worldwide, for political reasons.
posted by fantabulous timewaster at 9:04 PM on March 5, 2019
So you're saying that in 13,000 years June will occur in what we now call December, at the opposite point in our orbit around the Sun, in order to keep our months in synch with the seasons, even though our orbit is always the same length of time (more or less)?
posted by hippybear at 9:07 PM on March 5, 2019
posted by hippybear at 9:07 PM on March 5, 2019
So you're saying that in 13,000 years June will occur in what we now call December, at the opposite point in our orbit around the Sun, in order to keep our months in synch with the seasons, even though our orbit is always the same length of time (more or less)?
AFIK, that has been the goal of the calendrical adjustments so far. We can't predict the future, but if present trends continue...
posted by sjswitzer at 9:14 PM on March 5, 2019
AFIK, that has been the goal of the calendrical adjustments so far. We can't predict the future, but if present trends continue...
posted by sjswitzer at 9:14 PM on March 5, 2019
That is... fascinating to me, and I'd like to read material on this which might change my obviously stated views on this subject.
posted by hippybear at 9:16 PM on March 5, 2019
posted by hippybear at 9:16 PM on March 5, 2019
What I'm getting from a lot of this is a vast conspiracy to ensure that Australians never actually get to celebrate a cold Quonsmas.
posted by AnhydrousLove at 9:31 PM on March 5, 2019 [2 favorites]
posted by AnhydrousLove at 9:31 PM on March 5, 2019 [2 favorites]
The risk with saying things like "June will occur in what we now call December" is that you're trying to use the same names of the months to mean two different things in the same sentence. It's pretty easy to get flustered this way.
The Earth's orbit around the Sun lies pretty much on a plane in space. This plane is called the ecliptic, because when the Moon crosses it, we see eclipses. If you imagine extending the ecliptic plane out into space, it would pass through some constellations with familiar names: Aries, Taurus, Gemini, and the other "zodiac" constellations. When this was figured out 2500 years ago, the summer solstice happened when the Earth was on the Capricorn side of the orbit, so that a person standing on Earth (as people do) and pointing at the Sun was also pointing in the direction of Cancer. The spring equinox, which happens when the Sun crosses from the southern to the northern half of the sky, was around the time that the Sun appeared to cross from Pisces into Aries, and in old literature the date of the spring equinox is known as the "first point in Aries." Keeping track of when this happened was a big part of what astronomers were for in medieval Europe. (I remember that for technical reasons it's easier to exactly date the equator-crossing than the solstices, but my memory is murkier on practically what those reasons are.)
But in the 2500 years since astrologers (who at the time weren't different people from astronomers) figured out these connections between the skies and the seasons, we've undergone about 10% of a precession cycle, and the relationship between the zodiac and the seasons has shifted by more than a month. Nowadays, the "first point in Aries," when the Sun crosses from the southern to the northern part of the sky, happens closer to when the Sun moves from Aquarius into Pisces. We've dealt with the change by not talking about the "first point in Aries" any more, unless you are in a deep-in-the-weeds discussion like this one.
At the June 2019 solstice, the Sun will appear to lie near the boundary between Taurus and Gemini, and during the brief summer nights, observers in the northern hemisphere will see Scorpius and Sagittarius low in the south. At the December 2019 solstice, we'll be on the opposite site of the Sun: the Sun will lie between Scorpius and Sagittarius, and Taurus and Gemini will be high overhead on the long, cold nights.
In 15019 there will be two solstices, too. There will be a solstice where the Sun lies between Taurus and Gemini, so that Scorpius and Sagittarius are visible at night --- but because the Earth will have precessed, for northern hemisphere observers the "S" constellations will be high instead of low, and that night will be long and cold. There will be a solstice where the Sun is in Scorpius and Sagittarius, and Gemini and Taurus will be low on the horizon and the night will be short and warm. If we do the leap days correctly, the northern-hemisphere-warm solstice will still be the one that happens in June, and we will have used "June" as the name of the month of every northern-hemisphere-warm solstice in the interim, as the locations of the background stars have slowly changed.
Any good introductory astronomy course (and textbook) will have a section about this history, and you can make your own predictions using a tool like Stellarium. I have great respect for the folks who figured out in the 1500s that the spring equinox was migrating towards February, using nothing but their eyes (telescopes came later), pen and paper, and moldering handwritten histories.
posted by fantabulous timewaster at 11:18 PM on March 5, 2019 [7 favorites]
The Earth's orbit around the Sun lies pretty much on a plane in space. This plane is called the ecliptic, because when the Moon crosses it, we see eclipses. If you imagine extending the ecliptic plane out into space, it would pass through some constellations with familiar names: Aries, Taurus, Gemini, and the other "zodiac" constellations. When this was figured out 2500 years ago, the summer solstice happened when the Earth was on the Capricorn side of the orbit, so that a person standing on Earth (as people do) and pointing at the Sun was also pointing in the direction of Cancer. The spring equinox, which happens when the Sun crosses from the southern to the northern half of the sky, was around the time that the Sun appeared to cross from Pisces into Aries, and in old literature the date of the spring equinox is known as the "first point in Aries." Keeping track of when this happened was a big part of what astronomers were for in medieval Europe. (I remember that for technical reasons it's easier to exactly date the equator-crossing than the solstices, but my memory is murkier on practically what those reasons are.)
But in the 2500 years since astrologers (who at the time weren't different people from astronomers) figured out these connections between the skies and the seasons, we've undergone about 10% of a precession cycle, and the relationship between the zodiac and the seasons has shifted by more than a month. Nowadays, the "first point in Aries," when the Sun crosses from the southern to the northern part of the sky, happens closer to when the Sun moves from Aquarius into Pisces. We've dealt with the change by not talking about the "first point in Aries" any more, unless you are in a deep-in-the-weeds discussion like this one.
At the June 2019 solstice, the Sun will appear to lie near the boundary between Taurus and Gemini, and during the brief summer nights, observers in the northern hemisphere will see Scorpius and Sagittarius low in the south. At the December 2019 solstice, we'll be on the opposite site of the Sun: the Sun will lie between Scorpius and Sagittarius, and Taurus and Gemini will be high overhead on the long, cold nights.
In 15019 there will be two solstices, too. There will be a solstice where the Sun lies between Taurus and Gemini, so that Scorpius and Sagittarius are visible at night --- but because the Earth will have precessed, for northern hemisphere observers the "S" constellations will be high instead of low, and that night will be long and cold. There will be a solstice where the Sun is in Scorpius and Sagittarius, and Gemini and Taurus will be low on the horizon and the night will be short and warm. If we do the leap days correctly, the northern-hemisphere-warm solstice will still be the one that happens in June, and we will have used "June" as the name of the month of every northern-hemisphere-warm solstice in the interim, as the locations of the background stars have slowly changed.
Any good introductory astronomy course (and textbook) will have a section about this history, and you can make your own predictions using a tool like Stellarium. I have great respect for the folks who figured out in the 1500s that the spring equinox was migrating towards February, using nothing but their eyes (telescopes came later), pen and paper, and moldering handwritten histories.
posted by fantabulous timewaster at 11:18 PM on March 5, 2019 [7 favorites]
I suppose another way of saying all this is that many people think that the calendar is based on the sidereal year, or the time taken for the Earth to return to the same place in its orbit based on the position of the stars.
But the calendar is in fact based on the tropical year, which keeps the season cycle constant. So I was wrong above. "June" 27,000 years ago would still be summer but it would occur at a different point in the Earth's orbit - as measured against the stars. Axial precession means the occurrence of the seasons migrates around to different positions in the Earth's orbit.
Observational astronomers mostly care about Sidereal time, which is a clock fixed to the position of the stars. A Sidereal day is shorter than a Solar day (the time taken to see the Sun at the same position) because we are revolving around the Sun and so its position in the sky is moving at a different rate than the distant stars.
posted by vacapinta at 1:19 AM on March 6, 2019 [3 favorites]
But the calendar is in fact based on the tropical year, which keeps the season cycle constant. So I was wrong above. "June" 27,000 years ago would still be summer but it would occur at a different point in the Earth's orbit - as measured against the stars. Axial precession means the occurrence of the seasons migrates around to different positions in the Earth's orbit.
Observational astronomers mostly care about Sidereal time, which is a clock fixed to the position of the stars. A Sidereal day is shorter than a Solar day (the time taken to see the Sun at the same position) because we are revolving around the Sun and so its position in the sky is moving at a different rate than the distant stars.
posted by vacapinta at 1:19 AM on March 6, 2019 [3 favorites]
Moors were in ancient Rome.
The water clock thing I might grant, although it's a pretty safe bet Shakespeare had a medieval or later clock in mind--I love the man's work, but he wasn't big on historical details he didn't crib from Plutarch's Lives or Holinshed's Chronicles.
Not really any Moors in ancient Rome, though, at least if you go with the definition of "Moor" as (I'm cribbing myself here, from the OED) "[a] member of a north-western African Muslim people of mixed Berber and Arab descent"--or if we're sticking with Wikipedia, "[Moor] refers primarily to the Muslim inhabitants of the Maghreb, the Iberian Peninsula, Sicily, and Malta during the Middle Ages."
It's a little iffy dealing with the timeline of Titus Andronicus since it's not historical and its actual chronological setting is hazy, but the latest it could be set is by the fall of the western Roman Empire in the late fifth century. Islam (and subsequently Moors) didn't show up until the early seventh century.
posted by Mr. Bad Example at 1:46 AM on March 6, 2019
The water clock thing I might grant, although it's a pretty safe bet Shakespeare had a medieval or later clock in mind--I love the man's work, but he wasn't big on historical details he didn't crib from Plutarch's Lives or Holinshed's Chronicles.
Not really any Moors in ancient Rome, though, at least if you go with the definition of "Moor" as (I'm cribbing myself here, from the OED) "[a] member of a north-western African Muslim people of mixed Berber and Arab descent"--or if we're sticking with Wikipedia, "[Moor] refers primarily to the Muslim inhabitants of the Maghreb, the Iberian Peninsula, Sicily, and Malta during the Middle Ages."
It's a little iffy dealing with the timeline of Titus Andronicus since it's not historical and its actual chronological setting is hazy, but the latest it could be set is by the fall of the western Roman Empire in the late fifth century. Islam (and subsequently Moors) didn't show up until the early seventh century.
posted by Mr. Bad Example at 1:46 AM on March 6, 2019
Jupiter, and Mercury in the chart to give the exact date. Any sufficiently advanced stargazing civilization of the future ought to be able to work out when we built it with good accuracy!
The idea that a bunch of shiny inlaid metal and pretty tile fragment will survive long term strikes me as a little odd? But then again we uncover Roman mosaics, so I guess it depends what kind of audience we are expecting in 10,000 years... Primitive humans will tear that stuff apart for arrowheads or jewelry. If we all die off completely, I guess it might still be there for alien archaeologists?
posted by Meatbomb at 1:49 AM on March 6, 2019
The idea that a bunch of shiny inlaid metal and pretty tile fragment will survive long term strikes me as a little odd? But then again we uncover Roman mosaics, so I guess it depends what kind of audience we are expecting in 10,000 years... Primitive humans will tear that stuff apart for arrowheads or jewelry. If we all die off completely, I guess it might still be there for alien archaeologists?
posted by Meatbomb at 1:49 AM on March 6, 2019
thank you for hoping us, timewaster
posted by GCU Sweet and Full of Grace at 4:29 AM on March 6, 2019 [1 favorite]
posted by GCU Sweet and Full of Grace at 4:29 AM on March 6, 2019 [1 favorite]
We're currently closest to the sun (at perihelion) when the north pole is tilted furthest away from the sun. Does this mean that in 13,000 years we'll be closest to the sun when the north pole is tilted toward the sun?
(And does it mean that the same was true 13,000 years ago?)
posted by clawsoon at 5:26 AM on March 6, 2019
(And does it mean that the same was true 13,000 years ago?)
posted by clawsoon at 5:26 AM on March 6, 2019
I was at the Hoover Dam a few weeks ago and spent a bunch of time lingering around this monument, it is so strange and so cool! What makes it strange is the voice recording playing on a loop over a speaker behind it the whole time, the voice and the text of the recording is very dated compared to the 26,000 year nature of the monument.
There are other plaques and commemorating the workers who died during the making of the dam, and some of the people who were most responsible for the construction nearby. A few paces away from these giant winged figures, there's a slab of cement on the ground. A small plaque above it reveals that a favorite dog who hung around the construction site is buried under the slab.
Hoover Dam tip: If you park on the Arizona side, its free, you just have to walk a little further.
posted by ProtoStar at 5:39 AM on March 6, 2019
There are other plaques and commemorating the workers who died during the making of the dam, and some of the people who were most responsible for the construction nearby. A few paces away from these giant winged figures, there's a slab of cement on the ground. A small plaque above it reveals that a favorite dog who hung around the construction site is buried under the slab.
Hoover Dam tip: If you park on the Arizona side, its free, you just have to walk a little further.
posted by ProtoStar at 5:39 AM on March 6, 2019
and in old literature the date of the spring equinox is known as the "first point in Aries."
"Whan Zephirus eek with his sweete breethposted by octobersurprise at 8:49 AM on March 6, 2019 [1 favorite]
Inspired hath in euery holt and heeth
The tendre croppes, and the yonge sonne
Hath in the Ram his half cours yronne"
Not really any Moors in ancient Rome, though, at least if you go with the definition of "Moor" as (I'm cribbing myself here, from the OED) "[a] member of a north-western African Muslim people of mixed Berber and Arab descent"--or if we're sticking with Wikipedia, "[Moor] refers primarily to the Muslim inhabitants of the Maghreb, the Iberian Peninsula, Sicily, and Malta during the Middle Ages."
While it's true that's how it's been used for at least the last several centuries, the word "Moor" derives from Strabo describing the Berbers of North Africa as "Mauroi" well before the rise of Islam, so I don't think it's out-of-bounds to refer to ancient/pre-Islamic Berbers as Moors, as that's precisely who the term originally referred to.
posted by Copronymus at 10:11 AM on March 6, 2019 [1 favorite]
While it's true that's how it's been used for at least the last several centuries, the word "Moor" derives from Strabo describing the Berbers of North Africa as "Mauroi" well before the rise of Islam, so I don't think it's out-of-bounds to refer to ancient/pre-Islamic Berbers as Moors, as that's precisely who the term originally referred to.
posted by Copronymus at 10:11 AM on March 6, 2019 [1 favorite]
We're currently closest to the sun (at perihelion) when the north pole is tilted furthest away from the sun. Does this mean that in 13,000 years we'll be closest to the sun when the north pole is tilted toward the sun?
Approximately, yes. The location of the perihelion drifts a little, due mostly to gravitational perturbations from Jupiter. Without looking it up, I would guess that the perihelion precession over 10k years is much smaller than the axis precession --- small enough that your statement is totally right. These kinds of orbital changes are known collectively as "Milankovich cycles" and play a role in historical climate analysis.
posted by fantabulous timewaster at 9:42 AM on March 7, 2019 [3 favorites]
Approximately, yes. The location of the perihelion drifts a little, due mostly to gravitational perturbations from Jupiter. Without looking it up, I would guess that the perihelion precession over 10k years is much smaller than the axis precession --- small enough that your statement is totally right. These kinds of orbital changes are known collectively as "Milankovich cycles" and play a role in historical climate analysis.
posted by fantabulous timewaster at 9:42 AM on March 7, 2019 [3 favorites]
fantabulous timewaster, your location is the North Pole? THE PLOT THICKENS.
posted by clawsoon at 4:36 PM on March 7, 2019
posted by clawsoon at 4:36 PM on March 7, 2019
The plot thins, as the spring ice melts.
posted by fantabulous timewaster at 4:50 AM on March 8, 2019 [1 favorite]
posted by fantabulous timewaster at 4:50 AM on March 8, 2019 [1 favorite]
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