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#21
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Lat/Long and timekeeping system for Mars
On Mon, 3 Dec 2018 16:40:46 -0800 (PST), Quadibloc
wrote: On Monday, December 3, 2018 at 5:23:32 PM UTC-7, Chris L Peterson wrote: In all technical matters, time on Mars will be the same as time on the Earth, a count of SI seconds. It certainly is true that SI seconds would be used for measures of _interval_ for such things as assigning radio frequencies or specifying the speed of microprocessors. SI seconds, however, don't work very well for _epoch_ as there's a rather odd number of them in a Martian day, so the jury is still out there. Right. That's what I meant about using sols for conventional time keeping. But I expect most technical things will continue to us UT, since the connection to local solar time is unlikely to be important in most cases. |
#22
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Lat/Long and timekeeping system for Mars
On Mon, 3 Dec 2018 10:56:51 -0800 (PST), Quadibloc
wrote: . Earth's day (or rotational period) is exactly 23.9345 hours (or, 23 hours= , 56 minutes, 4.2 seconds). No. A sidereal day is 23h 56m 4.09054s and even this value is not exact but only approximate. |
#23
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Lat/Long and timekeeping system for Mars
On Mon, 03 Dec 2018 09:28:32 -0700, Chris L Peterson
wrote: On Mon, 03 Dec 2018 16:19:39 +0100, Paul Schlyter wrote: On Sun, 2 Dec 2018 18:00:00 -0800 (PST), Quadibloc wrote: Earth's day is 24 hours. Period. Full stop. Correct. The solar day is 24 solar hours, and the sidereal day is 24 sidereal hours. Except there's no formal definition of a "sidereal hour". Like there was no formal definition of "planet" before 2006. And according to the 2006 formal definition of "planet", exoplanets do not exist since they don't revolve around our Sun. Yet the sidereal hour is a useful quantity, how else could we build a sidereal clock? And the RA of your local meridian increases by one hour every sidereal hour. A sidereal day is not defined in terms of sidereal hours, but in terms of seconds. Isn't the sidereal day defined by the Earth's rotation rate? The only widely recognized, semi-formal definition of "solar day" is 86,400 seconds; the idea of a solar day being the length of time for the Sun to return to the meridian is colloquial. And yet we add leap seconds to UTC because the solar day isn't exactly 86400 SI seconds. A sidereal or stellar day is taken as the time for a given fixed point on the celestial sphere to return to the same longitude, or for the Earth to make one complete revolution. The mean period for that can be expressed in seconds, but not in "sidereal hours". Then I'll make what you consider impossible: One sidereal day is 24 sidereal hours or 86400 sidereal seconds. See? That was easy... Note that the analogy to sidereal time in solar time is UT1 rather than UT. Sidereal time is slightly irregular, just like the Earth's rotational rate. There is even a quantity called GMST - Greenwich Mean Sidereal Time. It is defined as the Hour Angle of the First Point of Aries as seen from the Greenwich meridian. And then we have GMST0 which is GMST at 0 hours GMT. GMST0 differs from the Sun's mean longitude by almost exactly 180 degrees - the discrepancy is equal to the constant of aberration of 20.46 arc seconds. |
#24
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Lat/Long and timekeeping system for Mars
In order to keep all objections in this thread, here are a few adult pointers.
There will only be the Martian 24 hour day and the Earth 24 hour day, each day with their own hours, minutes and seconds overlaid on the planet's geometry as a Lat/Long system. All the comments I have seen so far can't manage to affirm the system which works so far on Earth. Mars has an Equatorial circumference of 21,344 km therefore 15 degrees of geographical separation will be 889.3 km at the Equator . By applying an integer of 1 Martian hour time difference for each 15 degrees of rotation and where noon, both natural and human timekeeping is symmetrical either side of Martian sunrise/sunset , it becomes possible to determine location on Mars using timekeeping in much the same way as the Lat/Long system works on Earth. All the lingo of a linear progression of 'sols' looks weak when the structure of timekeeping on Mars will include an integer of rotations/ Martian days to orbital circuits where a refined value can be presented for the proportion of rotations to one orbital circuit. Presently, in Earth days, the value is 687 days or the equivalent of 365 days for the Earth when a more refined value is a proportion of 365 1/4 rotations for one orbital circuit. At least the opportunity for a distinct timekeeping system for Mars has been spotted whether the continuing investigation from the surface of Mars is done by machines or humans in future. |
#25
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Lat/Long and timekeeping system for Mars
On Mon, 3 Dec 2018 11:12:58 -0800 (PST), Quadibloc wrote:
On Monday, December 3, 2018 at 8:19:44 AM UTC-7, Paul Schlyter wrote: Simple, isn't it? All you have to think about is to match your selected type of day to the appropriate type of hour. Perhaps. But I think that even though Oriel is a very special case, ordinary people might come away from an attempt to teach astronomy with such brutality - I'm thinking of the phrase "the brutality with which mathematics is... taught to the innocent" - muttering that those astronomers are nuts. In ordinary language, a day is 24 hours, and whatever a period of 23 hours and 56 minutes may be, it is definitely not a day, as it's four minutes short of a day. Whatever you do, don't begin by flatly contradicting the meaning of words in ordinary language - that comes across as making a statement that is *false*: _because statements in a given language are understood by taking the words in them, and replacing them by their meanings_. So it's almost as if "a day is 23 hours and 56 minutes" *becomes* "a solar day is 23 hours and 56 minutes" somewhere between one's eyes and the part of one's brain that actually is aware of reading the sentence, if one is not already an astronomer... I don't know if this makes sense, but it's as close as I can come to explaining this. (Or, perhaps more directly: people expect words in anything they read to mean what they *expect* them to mean - _new_ words, or even new meanings for old words, can be introduced, and used subsequently, but don't expect people to backtrack and put the new meanings in what they've already read.) Therefore, in statements on this matter addressed to the general public, you should begin by using the term "day" only in its ordinary sense of the solar day, and subsequently introduce the term "sidereal day" as a special technical term by astronomers. Failing to do this - failing to come out of one's ivory tower when talking to those outside - is just what leads to confusion like Oriel's, even if most of those so confused simply ignore astronomy from then on, instead of building a whole system of "corrected" astronomy and then advocating for it while all the time blindly refusing to examine one's own original mistaken assumptions and confusion as he does. So I think a careless legitimate astronomer is a little bit to blame for getting him started, although he is now responsible for his own failure to change course and understand what was explained to him wrong once, but which has now been explained to him correctly many times over - by me alone, as well as by others. John Savard Idk, I'm not an astronomer, but I could live happily with Paul's statement by simply choosing a definition of "day" that matches his context; but I have to agree that I'm certain to be confused if the context is ambiguous. -- Email address is a Spam trap. |
#26
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Lat/Long and timekeeping system for Mars
"The first motion, named nuchthemeron by the Greeks, as I said (I, 4), is the rotation which is the characteristic of a day plus a night." Copernicus, De Revolutionibus
https://en.wikipedia.org/wiki/Nychthemeron Unknown to Copernicus is a separate day/night cycle processed by the North/South Polar latitudes where sunrise/sunset happens on the Equinoxes and midnight/noon of the Solstices. He does refer to this in De Revolutionibus but as he uses the Ptolemaic framework of the motion of the Sun through the Zodiac instead of the transition of stars from left to right of the Sun, the explanation of Copernicus becomes garbled and flawed. Mars also will have two distinct surface rotations which will also surface as natural noon variations. The equalisation of a unique Martian 24 hour day along with equal hours,minutes and seconds must fit with a Lat/Long system where one Martian hour equates not only to 15 degrees of geographical separation but also a rotation rate of 15 degrees of rotation per Martian hour.. The mongrel system of applying Earth timekeeping to Mars is counter-productive and nothing like what is being sought in this thread. |
#27
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Lat/Long and timekeeping system for Mars
On Tuesday, December 4, 2018 at 12:23:32 AM UTC, Chris L Peterson wrote:
On Mon, 3 Dec 2018 11:56:16 -0800 (PST), Gerald Kelleher wrote: Here is a new planet with instrumentation sent by humans with the necessity to create a timekeeping system with its own features that are not arbitrary like a leap day correction within a Martian year, a Martian 24 hour day/night cycle anchored to noon with an integer of hours reflecting 15 degrees geography/geometry and all the other things that work so well on Earth. Martian time is likely to be based locally on martian solar time, with periodic leap second corrections to keep it synchronized with the variable rotation rate... just like on Earth. In all technical matters, time on Mars will be the same as time on the Earth, a count of SI seconds. Get back to me when you can manage to affirm the Earth's Lat/Long system determines an Equatorial speed of 1037.5 miles per hour constituting a 360 degree circumference of 24901 miles. |
#28
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Lat/Long and timekeeping system for Mars
Quadibloc:
Earth's day is 24 hours. Period. Full stop. True enough‹for civil purposes, and depending on how you measure it. The length of the day is defined in seconds, not hours. Paul Schlyter: Correct. The solar day is 24 solar hours, and the sidereal day is 24 sidereal hours. Chris L Peterson: Except there's no formal definition of a "sidereal hour". A sidereal day is not defined in terms of sidereal hours, but in terms of seconds. The only widely recognized, semi-formal definition of "solar day" is 86,400 seconds; the idea of a solar day being the length of time for the Sun to return to the meridian is colloquial. A sidereal or stellar day is taken as the time for a given fixed point on the celestial sphere to return to the same longitude, or for the Earth to make one complete revolution. The mean period for that can be expressed in seconds, but not in "sidereal hours". I must confess that I have never heard of a "sidereal hour;" no such interval exists in the Système international. An hour is an hour is an hour, 3600 seconds. It is the second that is the base unit of time in SI units, not the hour, which is too broad a brush. I also don't get why this is so complicated for an amateur astronomer‹why it is difficult to understand that one's measurement (in seconds) of the period of rotation of a body depends on one's frame of reference. If one is standing on another body with complex motions (the Sun, e.g.) the measured rate will be different from a measurement taken from a hypothetical fixed point at an infinite distance. For the amateur astronomer only two simple time schemes apply: my planetarium software tells me, in civil time (day is 24 hours), /when/ to go out to observe M42‹maybe 2300 EST today. Then sidereal time, in which the duration of the day is ~23h 59m 4.091s, tells me (more specifically, tells my telescope mount) /where/ to observe M42. Works every time, with the added bonus that the length of the sidereal day changes only slightly over long periods; it's not like one needs to replace the gears in one's mount every five years. Does anyone think that I fret over this, or give it a moment's thought when I go out to make a photo https://www.flickr.com/photos/primeval/45304511761? So where's the beef? -- I agree with almost everything that you have said and almost everything that you will say in your entire life. usenet *at* davidillig dawt cawm |
#29
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Lat/Long and timekeeping system for Mars
I am sure the ISIS branch of astronomy would find 'sidereal vs solar' convictions acceptable however the integrity of the Lat/Long system and the 24 hour system is as intimate as it is inviolate - the 'average' 24 hour day and its creation substitutes for 'constant' rotation at a rate of 15 degrees per hour.
Before the accumulative inspiration/genius of humanity and its long endeavor to create a system of timekeeping as principles and engineering, people should not behave like brutes even though they presently do. The mongrel system which is currently applied to Mars will not serve geostationary satellites for the planet and especially not a system like GPS which refers longitude coordinates to timekeeping. The equable hour,minute and second will be as individual to Mars as the maximum equatorial distance across 15 degrees of longitude. Clearly none of you are up to the necessary principles which will eventually go into an individual calendar framework with a leap day/rotation correction. |
#30
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Lat/Long and timekeeping system for Mars
On Tuesday, December 4, 2018 at 11:03:38 AM UTC-7, Davoud wrote:
I also don't get why this is so complicated for an amateur astronomer仇hy it is difficult to understand that one's measurement (in seconds) of the period of rotation of a body depends on one's frame of reference. I don't think this is complicated for an amateur astronomer to understand. But I'm talking about trying to be understood by people who are not astronomers. And to ordinary people, a "day" is a unit of time between successive periods of light or of dark, without rotation even entering into their thoughts. John Savard |
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