[HN Gopher] Fifteen years of radar reveal Venus's most basic facts
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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.
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