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Our earth based time system is quite well defined and it makes perfect sense to say that SN1054 took place at a point in space time D light years away and D+954 years in the past. The real problem lies in the fact that D [~ 6300] is a little hard to define precisely with current technology.History of the USNO Time Ball
http://www.usno.navy.mil/millennium/tball_hist.shtml
<<The U.S. Naval Observatory, located in Washington, D.C., is the oldest scientific institution in the U. S. Navy, and one of the oldest in the country. Established in 1830 as the Depot of Charts and Instruments, its primary mission was to care for the U.S. Navy's chronometers, charts and other navigational equipment. In 1844, as its mission evolved and expanded, the Depot was reestablished as the U.S. Naval Observatory and was located on the hill north of where the Lincoln Memorial now stands, in the area known as Foggy Bottom.
As an institution that owed its founding to the necessity for accurate time in the form of the marine chronometer, time as determined by astronomical observation was at the very heart of its mission. Time was important not only for "rating" chronometers, but also for determining longitude, for measuring star positions, and, of course, for daily use by ordinary citizens. But how to disseminate time to the public? This problem was addressed only when the Naval Observatory moved to its new site in 1844. Secretary of the Navy John Y. Mason set events in motion when he wrote the Observatory's Superintendent Matthew Fontaine Maury on December 10, 1844:
"You will be pleased to devise some signal by which the mean time may be made known every day to the inhabitants of the city of Washington. When you are prepared to put your signal into operation, you will give notice of the kind you have adopted in the city newspapers, & at the same time inform the Department."
Faced with this task, Maury devised a time signal in the form of a falling ball, a method tested at Portsmouth, England as early as 1829 and in use since 1833 at the Greenwich Observatory. He may have known of this method through John Quincy Adams, who learned of the Greenwich time ball through correspondence with British Astronomer Royal George B. Airy, or by looking at the Greenwich Observation volumes for 1836 or 1840, which mention the "signal ball." The idea was simply that the ball would drop at a precisely known instant, 12 noon in Washington, from the dome of the Observatory. A number of balls and a variety of methods for dropping, including controlled lowering, dropping onto the dome, and throwing by hand were probably used in the early period, until an electromagnetic relay released the raised ball. The result was the same: anyone within visual sight of the ball could set their timepiece to 12 noon, and ships anchored on the Potomac River could calibrate their chronometers.
Although the first volume of Washington Observations (1846) made no mention of the time ball, and although later sources attribute its origin to the 1850s, the engraving (seen above) on the title page of that first volume shows one mounted on a flagstaff atop the 9.6 inch telescope dome. It is therefore probable that it was erected prior to the publication of that volume in 1846, a probability supported by the statement in Bohn's Hand-book of Washington (Washington, 1852) that
"John Quincy Adams, who was a devoted friend of the Observatory, and who used to visit it frequently in the last days of his life [died 23 February, 1848], has been known to walk all the way up to the Observatory from his lodgings, to see the ball fall."
Through early 1865, the USNO time service was limited to dropping this ball at noon from the flagstaff of the Observatory dome. During that year, however, more extensive dissemination of time became possible when the Observatory was connected by telegraph to the Washington Fire Alarm Office. The fire bells were struck daily at 7 am, noon and 6 pm by telegraphic signal from the Observatory. The Western Union Telegraph Company also tapped into this system, and once available to Western Union, time could be telegraphed to any location its lines traversed. By 1867 time was telegraphed "at noon, by the different lines of wires, to the northward, eastward, and westward, and as far southward as Texas." By 1869 Western Union distributed signals "which serve to regulate the clocks of nearly all the railroads in the southern States."
Once the time signal was disseminated to a particular city, the problem was to disseminate it within the city. In Washington, government clocks were telegraphically controlled from the Naval Observatory, beginning with the Navy Department in 1868, the Army Signal Office in 1871 and the Treasury Department in 1873. By 1884 20 public buildings, including the Executive Mansion and the Senate wing of the Capitol, had clocks controlled by the Observatory. By 1885 the number of controlled clocks in various public offices had increased to 84, by 1886 to 200 and by 1888 to 347.
The same system could be used in any city once it had the "correct" time, which was defined as local time until 1883. Such local time might be obtained from a local observatory, which in fact made money from selling time. It might also be telegraphed from Washington, appropriately corrected by the differences in longitude, which were accurately known for a number of large cities. In New York City, for example, a time ball was dropped by telegraphic signal from the Naval Observatory beginning Sept. 17, 1877.
Telegraphic time was eventually replaced by radio time signals, pioneered by the Navy beginning in 1904. But time balls remained in use well into the twentieth century. The U. S. Naval Observatory time ball was removed from the dome at its Foggy Bottom site shortly before 11 July 1885. Then it, or a new model, was installed on the central pavilion (on the side facing the White House) of the building now known as the Old Executive Office Building. The Washington time ball remained in service until December 16, 1936.
Today, the Naval Observatory still maintains the Master Clock for the United States, and disseminates time via telephone, computer, two-way time transfer, and the Global Positioning System of satellites.>>
Likewise, the boundaries of many western U.S. states were defined according to the American Meridian [through the U.S. Naval Observatory] NOT the Greenwich Meridian primarily because land surveying was the only accurate technology at the time:
http://en.wikipedia.org/wiki/Washington_meridian
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In about 10,000 years the boreal summer solstice sun will be near perihelion [and the North Polar star will be Delta Cygni (Ruc)]. This will indeed make summers in the northern hemisphere hotter but not nearly so hot as they were just 10,000 years ago when the boreal summer solstice sun was also at perihelion [and the North Polar star was Delta Cygni (Ruc) Tau Herculis]. This is because both the obliquity and the eccentricity of the earth's orbit have been on the decline for the last 10,000 years and they should continue to decline over the next 10,000 years.interstellaryeller wrote:Right now Polaris is our north polar star. In 12,000 years Alpha lyrae will be, correct? How will this effect the summer in the northern hemisphere.
http://en.wikipedia.org/wiki/Precession_(astronomy)
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Memorable quotes for 10,000 BC (2008):
D'Leh: [to Tic'Tic] We need you.
Tick tock ... tick: Extra second added to 2008
By Jim Wolf Reuters
Monday, December 29, 2008; 10:00 AM
WASHINGTON (Reuters) - Those eager to put 2008 behind them will have to hold their good-byes for just a moment this New Year's Eve.
The world's official timekeepers have added a "leap second" to the last day of the year on Wednesday, to help match clocks to the Earth's slowing spin on its axis, which takes place at ever-changing rates affected by tides and other factors.
The U.S. Naval Observatory, keeper of the Pentagon's master clock, said it would add the extra second on Wednesday in coordination with the world's atomic clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time, or UTC. That corresponds to 6:59:59 p.m. EST (23:59:59 GMT), when an extra second will tick by -- the 24th to be added to UTC since 1972, when the practice began.
UTC is the time scale kept by highly precise atomic clocks around the world, accurate to about a billionth of a second per day, the Naval Observatory says. For those with a need for precision timing, it has replaced Greenwich Mean Time, or GMT.
The decision to add or remove a second is the responsibility of the International Earth Rotation and Reference Systems Service, based on its monitoring of the Earth's rotation. The goal is to make sure clocks vary from the Earth's rotational time by no more than 0.9 seconds before an adjustment. That keeps UTC in sync with the position of the sun above the Earth. Mechanisms such as the Internet-based Network Time Protocol and the satellite-based Global Positioning System depend on precision timing.
The first leap second was introduced into UTC on June 30, 1972. The last was added on December 31, 2005. They have been added at intervals ranging from six months to seven years, Daniel Gambis, head of the IERS Earth Orientation Center at the Observatoire de Paris, wrote in an explanatory piece this month (http://hpiers.obspm.fr/eop-pc/).
Among the reasons for Earth's slowing whirl on its axis are the braking action of tides, snow or the lack of it at the polar ice caps, solar wind, space dust and magnetic storms, according to the U.S. Commerce Department's National Institute of Standards and Technology, another timekeeper.
By contrast, a leap day, February 29, occurs once every four years because a complete turn around the sun -- our year with all its seasons -- takes about 365 days and six hours.
In 1970, an international agreement established two time scales: one based on the Earth's rotation and another on highly accurate atomic clocks.
The U.S. Naval Observatory's master clock is based on a system that now includes 50 atomic clocks, 36 based on the element cesium and 14 known as hydrogen masers.
With the Earth's rotation gradually slowing, the periodic insertion of a leap second into the atomic time scale is needed to keep the two systems within a second of each other.
