[HN Gopher] Fifteen years of radar reveal Venus's most basic facts ___________________________________________________________________ Fifteen years of radar reveal Venus's most basic facts Author : sohkamyung Score : 85 points Date : 2021-06-02 06:14 UTC (16 hours ago) (HTM) web link (eos.org) (TXT) w3m dump (eos.org) | oaiey wrote: | What about the protomolecule? | ceejayoz wrote: | Still on Phoebe at this point in the timeline. | anticristi wrote: | With such a huge transfer of momentum -- a day on Venus is +/- 3 | minutes the previous one -- the winds on the ground must be | crazy! | qayxc wrote: | The winds on Venus' surface are actually pretty tame ("light | breeze", 1 Beaufort, 0.5 to 1 m/s) [1]. Problem is, that even | this very slight wind packs a considerable punch at 92 bars of | pressure and is comparable to a category 4 hurricane on Earth | in terms of power (i.e. it would roughly translate to about 90 | m/s; 325 km/h; 200 mph on Earth). | | [1] | https://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.ht... | munk-a wrote: | Yea actually, prior to reading this article I always thought of | the atmosphere as a mostly static shell that just sorta floats | around planets - it's interesting to consider how, when the | wind pushes on a mountain the mountain pushes back (not | literally of course) and that can cause subtle variations in | day cycle on earth that could be influenced by spikes in | externally acquired heat in differing parts of the atmosphere - | but it's crazy to think how much more dramatic the effect is on | Venus. | mrfusion wrote: | Why does the atmosphere rotate faster than the planet? | qayxc wrote: | Because unlike Earth, Venus receives a whole bunch of solar | radiation on one hemisphere for months on end. | | This drives a substantial temperature differential that causes | strong winds. Since a day on Venus is longer than a year on | Venus, it doesn't take much for the atmosphere to rotate | faster, either. | science4sail wrote: | Having each day be 3 minutes longer/shorter than the previous one | is pretty crazy, even if it's mitigated by the fact that 1 | Venusian day is ~243 Earth days. | | That really makes me wonder how timekeeping would work for | civilizations that evolved on planets with variable day length. | Earth has variable periods of _sunlight_ depending on latitude, | but Earth 's day length itself is relatively constant. | harywilke wrote: | Interesting. Noon is when the sun is highest. I had fun | explaining time zones to my son the other day. I think he got | lost in my train analogy: If the train travels around the earth | at the same speed the earth turns, then it's always noon on the | train. | cdumler wrote: | Earth already has this problem. Years aren't consistent both | because the number of rotations don't evenly go into a single | revolution, but also because Earth's rotation time is variable. | For instance, above normal fresh water accumulation on land | will measurably change the length of a day. | | Generally, there are two solutions. One is to track absolute | time by increasing the clock as necessary to realign the day to | the year. There are Earth days with where the last minute of | the day is 61 seconds long. Daylight Saving Time makes days | with 23 hours or 25 hours. And, of course, leap years add a | day. Unfortunately, a lot of software/hardware is built by | people with misconceptions about time, like a minute can have | only 60 seconds or a day can have only 24 hours. | | So, the second solution is to update the system periodically. | Computers often poll internet time to update the clock and will | jump to the new time. Alternatively, some systems skew the | system clock with additional milliseconds per second until it | catches up with the time change. As long as the values are | valid typical ranges, most software doesn't care about sudden | changes. | swiley wrote: | Yeah, a lot of people don't realize you can't compare | intervals of UTC seconds without an almanac generated by a | random natural process. | anticristi wrote: | Very well summerized. For those who care about the longer | version: | | https://en.m.wikipedia.org/wiki/International_Atomic_Time | | https://en.m.wikipedia.org/wiki/Universal_Time | | https://en.m.wikipedia.org/wiki/Coordinated_Universal_Time | benpbenp wrote: | > Very well summerized. | | Actually, in Sumer, they used an intercalary month every | three years or so, much like the leap year in the Gregorian | Calendar. | | https://en.wikipedia.org/wiki/Sumer | dotancohen wrote: | > Years aren't consistent both because the number of | rotations > don't evenly go into a single revolution, | but also because > Earth's rotation time is variable. | | Lots of civilizations have found novel solutions to the fact | that the number of days in a year is not an integer. Adding | an occasional leap month (Hebrew calendar) or leap day | (Gregorian calendar), or just letting the calendar get out of | sync with the seasons (Islamic calendar). | | However, it should be noted that although the Earth's | rotation time is measurably variable, such measurements | require precision instruments that have only been available | for a bit over a century. Human senses could not notice such | small changes over the course of a lifetime or even over the | lifespan of an empire. | dfsegoat wrote: | > Human senses could not notice such small changes over the | course of a lifetime or even over the lifespan of an | empire. | | I don't disagree in this case. But I am continually amazed | at how resourceful and technical past civilizations could | be. | | For instance early polynesian navigators ("wayfinders") | could look at patterns in wave refraction across the ocean | surface, to locate islands over the horizon [1]. | | They also memorized the setting and rising positions of | hundreds of stars. [1] | | Unfortunately, much of this knowledge was lost, as the | polynesians did not have a written system of language. | | 1 - https://manoa.hawaii.edu/exploringourfluidearth/physica | l/nav... | wahern wrote: | > Unfortunately, much of this knowledge was lost, as the | polynesians did not have a written system of language. | | Having a writing system might have been sufficient to | prevent the loss, but not necessary. I'd argue the | immediate cause was the disappearance of folk songs and | similar rituals that encoded this knowledge in the | culture. | | > In Oceania's oral culture, narrative was the primary | tool to memorize and transmit complex accounts of | interconnected voyaging routes through the sea of | islands. These accounts would have been replete with | their respective star (and sun) courses, with bearings, | instructions for seasons for travel, the expected quality | of swell, winds, sea marks and other indispensable | information for reckoning and island finding. In other | words, Oceanic geography was, like Oceanic history, | genealogy and all other matters of education, a narrative | art, taught and memorized at specialized marae primarily | through the recitation of chants. | | https://www.tandfonline.com/doi/full/10.1080/00223344.201 | 8.1... (Lars Eckstein and Anja Schwarz, The Making of | Tupaia's Map: A Story of the Extent and Mastery of | Polynesian Navigation, Competing Systems of Wayfinding on | James Cook's Endeavour, and the Invention of an Ingenious | Cartographic System) | | Unfortunately it seems that these particular oral | traditions lost their fidelity before the age of audio | recording. The introduction of writing systems has | preserved much knowledge, but it has also destroyed much | knowledge by supplanting other modes of preservation. I'm | not even sure which is greater. Most of the utility in | the adoption of writing systems is prospective, I think. | | Interestingly, modernity may be its own worst enemy in | this regard. A friend recently pointed me to this | experiment that pitted Memory Palaces against an | Australian Aboriginal memorization technique[1] by | assigning incoming medical students to three different | groups: https://journals.plos.org/plosone/article?id=10.1 | 371/journal... The authors' conclusion was that the | aboriginal technique offered substantially superior | recall, _but_ _they_ _refrained_ _from_ _detailing_ _the_ | _technique_ out of fear of cultural insensitivity! _sigh_ | Maybe it would have been insensitive, and maybe they did | the right thing, but the irony is astounding. | | [1] Apropos the navigation theme, the technique seems to | be one way Aborigines teach their star charts. In fact, | the technique itself seems to encode some cultural | knowledge, which is presumably why even providing the | details might seem culturally invasive--because such | knowledge isn't intended for outsiders, except unless | taught by a member. | vikiomega9 wrote: | > for instance, above normal fresh water accumulation on land | will measurably change the length of a day | | Is there a name for this that I can google for? | echelon wrote: | I was curious: | | https://en.wikipedia.org/wiki/Day_length_fluctuations | HPsquared wrote: | The general physics concept is moment of inertia. If water | is moved uphill, this redistributes some of the Earth's | mass further away from the axis of rotation, which | increases its moment of inertia. Since the total angular | momentum of the Earth is not changed by doing this, the | result is a reduction of the angular velocity, in other | words lengthening the day. | | NASA calculated, for instance, that filling the Three | Gorges Dam reservoir would lengthen the day by 0.06 | microseconds. | BitwiseFool wrote: | I'm imagining a system where you have two different lengths of | time units. | | You can have a "scientific second" which is defined by distance | and the speed of light. Just like we have now in SI. When the | rotation of the planet changes slightly you need to compensate | by adding leap seconds every few years for things to sync up. | Eventually they will need to add a leap second every day as the | earth's rotation slows down. Some planet with high variation | would be adding and removing these frequently. | | But, you can define a second as 1/86400th of a "day". For most | civil purposes you just want to know when an event happened | relative to the day/night cycle and the year. In which case the | order of events is more important than the precise amount of | time between the events. If precision is needed, you can always | measure in scientific seconds and convert accordingly. I figure | the "civil second" is fine for such a purpose because it's not | like living beings can consistently and accurately keep track | of time in their heads. | | Now for a planet with a rotation as slow as Venus the concept | of a "day" is probably meaningless. It would be more like a | season of light and darkness. So I suppose you'd just only use | scientific seconds and count upwards? | sandworm101 wrote: | Start the clock at zero and just start counting seconds. The | problem only occurs when you are trying to calculate | sunrise/set times. Your solar power system doesn't care about | leap seconds/days/years. It knows to expect sunrise at | 353478515 seconds without regard for human concepts of a | particular day or month. | munk-a wrote: | Well I assume the drift based on atmospheric anomalies | doesn't tend toward a total delta of +/- 0 over time - or at | least we're not accurately accounting for it yet. As such | we'd face the same january in summer problem the romans faced | right? Slowly midnight will move further and further from the | time when you're furthest from the sun. | tintor wrote: | Water clocks and sand clocks could still work, or some other | periodic natural phenomena (like moon phases). | BurningFrog wrote: | Obviously, the planet as a whole rotates at a very constant | speed. But the "hard" planet and the atmosphere can move in very | different ways. | | The 3 minute difference comes from not seeing the atmosphere as | part of the planet. ___________________________________________________________________ (page generated 2021-06-02 23:00 UTC)