- 21 Mar '06 19:27The seconds in a year are usually this: 86,400 (seconds in one day)

times 365.26 and that gives 31558464 seconds in a year. But is that the actual #? Can you get it down to milli or microseconds? I think it has been proven the earth speeds down and slows up a bit but how many days in a year? I can't believe its known only to 5 digit accuracy. - 21 Mar '06 19:54

31,536,000,000 milliseconds in a year.*Originally posted by sonhouse***The seconds in a year are usually this: 86,400 (seconds in one day)**

times 365.26 and that gives 31558464 seconds in a year. But is that the actual #? Can you get it down to milli or microseconds? I think it has been proven the earth speeds down and slows up a bit but how many days in a year? I can't believe its known only to 5 digit accuracy.

No idea what you're driving at. - 21 Mar '06 20:49

It's amazing what google can do.*Originally posted by sonhouse***The seconds in a year are usually this: 86,400 (seconds in one day)**

times 365.26 and that gives 31558464 seconds in a year. But is that the actual #? Can you get it down to milli or microseconds? I think it has been proven the earth speeds down and slows up a bit but how many days in a year? I can't believe its known only to 5 digit accuracy.

1 year = 365.242199 days

Also:

1 year = 3.1556926 × 10^10 milliseconds - 21 Mar '06 22:27 / 4 edits

Actually we have three separate answers. I got 365.26 from an article on accurate clocks in the special issue of Scientific American on Time.*Originally posted by SJ247***Odd, your answer means that mine is wrong.**

I used an online unit of time conversion calculator. Maybe it used 365 days.

Thats pretty much what I was asking, what is the best number to cite for the number of seconds or milliseconds in a year. The article on clocks was interesting, now closing in on clocks accurate to one part in 10^18, an accuracy that would be able to time the entire life of the universe within a few milliseconds. One problem with such a clock however is how to transfer that timing accuracy to another clock of the same type, that is to say, how to synchronize the two. I used to work on atomic clocks at NASA, cesium beam, rhubidium beam and quartz standards. Now there is an atomic clock smaller than a grain of rice accurate to one part in 10^11!

If Xanth's # is correct, then there are 31,556,925,990 milliseconds in a year. Of course the spin of the earth isn't that accurate so that may be just an average of ten years or something.

I was trying to convert this into a more accurate measure of the light year, since C is fixed but everywhere you look you get a differant # for the length of the year. I even see differant #'s for C, 186282 MPS * 1.6=298051.2 KM/S, and others.

Wikopia gives C as EXACTLY = 299,792,458 meters/second and then says its Approx. = 186,282.397 MILES/SEC. Dividing those two makes

the mile = 1.609344 Km. So how accurate is THAT #? - 22 Mar '06 02:46

That figure for the length of a mile in kilometres is the same as the one you get by typing "1 mile in kilometres" into Google. This makes sense as when you ask it for the number of miles in a light year it is likely converting to miles from metres. When you divided your answers you are just reversing this (at the same accuracy) and you will get the same answer.*Originally posted by sonhouse***Actually we have three separate answers. I got 365.26 from an article on accurate clocks in the special issue of Scientific American on Time.**

Thats pretty much what I was asking, what is the best number to cite for the number of seconds or milliseconds in a year. The article on clocks was interesting, now closing in on clocks accurate to one part in 10^18, ...[text shortened]... .397 MILES/SEC. Dividing those two makes

the mile = 1.609344 Km. So how accurate is THAT #? - 22 Mar '06 05:05 / 1 edit353, 354 or 355 days — the lengths of common years in some lunisolar calendars

354.37 days — 12 lunar months; the average length of a year in lunar calendars

365 days — a common year in many solar calendars

365.24219 days — a mean tropical year near the year 2000

365.2424 days — a vernal equinox year.

365.2425 days — the average length of a year in the Gregorian calendar

365.25 days — the average length of a year in the Julian calendar;**the light year is based on it**is 31,557,600 seconds

365.2564 days — a sidereal year

366 days — a leap year in many solar calendars

383, 384 or 385 days — the lengths of leap years in some lunisolar calendars

383.9 days — 13 lunar months; a leap year in some lunisolar calendars

see: http://en.wikipedia.org/wiki/Year#Variation_in_the_length_of_the_year_and_the_day

and:

http://en.wikipedia.org/wiki/Light_year - 22 Mar '06 17:09 / 2 edits

That makes it 1,609,344 millimeters per mile. Divide by 63360 (# of inches per mile) and you arrive at the well known 25.4 mm/inch which makes that one self consistent anyway.*Originally posted by XanthosNZ***That figure for the length of a mile in kilometres is the same as the one you get by typing "1 mile in kilometres" into Google. This makes sense as when you ask it for the number of miles in a light year it is likely converting to miles from metres. When you divided your answers you are just reversing this (at the same accuracy) and you will get the same answer.**

Using those numbers then, C=299,792,458,000 MM/sec. Divide by 1609344 gives C in MPS as 186282.3971 MPS.

Divide by 12, C= 1.180285268 E10 Inches per second. Ten digit accuracy. - 23 Mar '06 06:12

Sonhouse, I'm curious now, what work did you do with the atomic clocks at NASA?*Originally posted by sonhouse***That makes it 1,609,344 millimeters per mile. Divide by 63360 (# of inches per mile) and you arrive at the well known 25.4 mm/inch which makes that one self consistent anyway.**

Using those numbers then, C=299,792,458,000 MM/sec. Divide by 1609344 gives C in MPS as 186282.3971 MPS.

Divide by 12, C= 1.180285268 E10 Inches per second. Ten digit accuracy. - 31 Mar '06 02:29

yes, 3155846400 milliseconds in a year.*Originally posted by sonhouse*

times 365.26 and that gives 31558464 seconds in a year. But is that the actual #? Can you get it down to milli or microseconds? I think it has been proven the earth speeds down and slows up a bit but how many days in a year? I can't believe its known only to 5 digit accuracy. - 31 Mar '06 05:10

Sorry, I missed your post. I was on the Apollo project, at Goddard Space Flight Center in the years 70 and 71 working on Apollo timing and tracking, the timing part was the successful attempt to co-ordinate the timing of the reception of apollo (and subsequent space program projects) data to within 100 nanoseconds. That is to say, when a deep space probe of any kind is way out there and being tracked, say first by Goldstone, the time Goldstone gets to aquire the signal is limited to the time the probe is actually in line of sight. Once it goes over the horizon, another dish had better be ready to take up the slack and that slack window established by NASA was one hundred nanoseconds or one tenth of a microsecond. By now, it might be down to ten nanoseconds, not sure, but in order to do that NASA and all the downrange tracking stations needed to have all the timing co-ordinated to well under that one hundred nanosecond window. Thats where the timing part comes in. We had three kind of clocks, well four counting the mighty hydrogen clock, but at our humble tracking station we had three: a master whoopdidoo cesium beam atomic clock accurate to within (ATT) one second in 6,000 years and a Rhrubidium*Originally posted by leisurelysloth***Sonhouse, I'm curious now, what work did you do with the atomic clocks at NASA?**

atomic clock accurate to within one second in a few hundred years as the primary BACKUP and a back up back up, a quartz clock accurate to within one second in a couple three years. I never saw anything but the cesium beam clock run myself, it never crapped out. Anyway thats what the timing part was. My job was called timing and TRACKING.

the tracking part was, on the apollo (of course Skeeter denies all this), how do you figure out exactly how far away the lunar craft is on a line going away from earth? So they had this transponder, a signal, a very special signal, was transmitted to the craft and the transponers job was simply to sling the same signal back to earth like a mirror reflection. So a complicated code was embedded in the signal, I mean, REAL complicated!, and when the signal got back to Goddard, the exact timing of the signals, the transmitted one and the reflected one, was compared. Due to the complicated nature of the signal, there was only one distance that a particular signal/reflection would match so that was the distance to the apollo, kind of like extending a long arse ruler into space and bringing the end back to earth, you then knew by comparing the ends, what distance the ship had to be.

Sorry to be so long winded about it, its not easy to condense into a few hundred words. - 31 Mar '06 15:29

Actually in this case, they modified the definition of the 'meter'*Originally posted by Schumi**Exactly*? Nothing is exact.

so the speed of light, considered a constant by many, so they sync together now. You just define the meter as being a number that

measures out 299, 792,458 meters is how far light goes in one second and the second is known to something like 10E-18: 1 or so,

and thats only the beginning. In about 5 years or so, the second will be defined within 10E-21:1, enough to time the whole expansion of the universe to within the width of your finger. Now THATS accurate.

So yep, the speed of light is DEFINED as 299,792,458 meters per second. Now you have a good standard for time and distance. Thats what its all about. Don. - 04 Apr '06 13:16 / 1 edit
*Originally posted by sonhouse*

times 365.26 and that gives 31558464 seconds in a year. But is that the actual #? Can you get it down to milli or microseconds? I think it has been proven the earth speeds down and slows up a bit but how many days in a year? I can't believe its known only to 5 digit accuracy.*mean*solar day. Now, not every solar day is 86400 seconds long (they can be as short as 86378s or as long as 86429s), but the mean was calculated from nearly 150 years' data.

By 1956 it was recognised that the Earth's rotation was not a constant enough clock and so the second was re-defined in terms of the earth's rotation around the Sun:

1 second = 1/31,556,925.9747th of the tropical year for 1900 January 0 at 12 hours ephemeris time.

By 1967, however, the invention of the atomic clock meant that even this value was not accurate enough and the second was redfined to be:

1 second = the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

At this point the connection between time and astronomical events was broken. It is no longer meaningful to ask exactly how many seconds in a year, because each year will be different. There is no longer an exact ratio between seconds and days or seconds and years - the current mean solar day is 86,400.002 s for instance and increasing at 1.7 ms/century.

If you wanted to "time" a year say then the accuracy would not be limited by the clock used but rather the accuracy of your atronomical observation. If for instance you use "noon" as a datum; how accurately can you determine the time at which the Sun is at its highest elevation? I doubt that an accuracy better than 1/100th is possible.