SUMMARY / RELATED TOPICS

Ephemeris time

The term ephemeris time can in principle refer to time in connection with any astronomical ephemeris. In practice it has been used more to refer to: a former standard astronomical time scale adopted in 1952 by the IAU, superseded in the 1970s; this time scale was proposed in 1948, to overcome the drawbacks of irregularly fluctuating mean solar time. The intent was to define a uniform time based on Newtonian theory. Ephemeris time was a first application of the concept of a dynamical time scale, in which the time and time scale are defined implicitly, inferred from the observed position of an astronomical object via the dynamical theory of its motion. A modern relativistic coordinate time scale, implemented by the JPL ephemeris time argument Teph, in a series of numerically integrated Development Ephemerides. Among them is the DE405 ephemeris in widespread current use; the time scale represented by Teph is related to, but distinct from, the TCB time scale adopted as a standard by the IAU. Most of the following sections relate to the ephemeris time of the 1952 standard.

An impression has sometimes arisen that ephemeris time was in use from 1900: this arose because ET, though proposed and adopted in the period 1948–1952, was defined in detail using formulae that made retrospective use of the epoch date of 1900 January 0 and of Newcomb's Tables of the Sun. The ephemeris time of the 1952 standard leaves a continuing legacy, through its ephemeris second which became duplicated in the length of the current standard SI second. Ephemeris time, adopted as standard in 1952, was designed as an approach to a uniform time scale, to be freed from the effects of irregularity in the rotation of the earth, "for the convenience of astronomers and other scientists", for example for use in ephemerides of the Sun, the Moon, the planets, it was proposed in 1948 by G M Clemence. From the time of John Flamsteed it had been believed, but in the nineteenth and early twentieth centuries, with increasing precision of astronomical measurements, it began to be suspected, was established, that the rotation of the Earth showed irregularities on short time scales, was slowing down on longer time scales.

The evidence was compiled by W de Sitter who wrote "If we accept this hypothesis the'astronomical time', given by the earth's rotation, used in all practical astronomical computations, differs from the'uniform' or'Newtonian' time, defined as the independent variable of the equations of celestial mechanics". De Sitter offered a correction to be applied to the mean solar time given by the Earth's rotation to get uniform time. Other astronomers of the period made suggestions for obtaining uniform time, including A Danjon, who suggested in effect that observed positions of the Moon and planets, when compared with their well-established gravitational ephemerides, could better and more uniformly define and determine time, thus the aim developed, to provide a new time scale for astronomical and scientific purposes, to avoid the unpredictable irregularities of the mean solar time scale, to replace for these purposes Universal Time and any other time scale based on the rotation of the Earth around its axis, such as sidereal time.

The American astronomer G M Clemence made a detailed proposal of this type based on the results of the English Astronomer Royal H Spencer Jones. Clemence made it clear that his proposal was intended "for the convenience of astronomers and other scientists only" and that it was "logical to continue the use of mean solar time for civil purposes". De Sitter and Clemence both referred to the proposal as'Newtonian' or'uniform' time. D Brouwer suggested the name'ephemeris time'. Following this, an astronomical conference held in Paris in 1950 recommended "that in all cases where the mean solar second is unsatisfactory as a unit of time by reason of its variability, the unit adopted should be the sidereal year at 1900.0, that the time reckoned in this unit be designated ephemeris time", gave Clemence's formula for translating mean solar time to ephemeris time. The International Astronomical Union approved this recommendation at its 1952 general assembly. Practical introduction took some time. During the currency of ephemeris time as a standard, the details were revised a little.

The unit was redefined in terms of the tropical year at 1900.0 instead of the sidereal year. Although ET is no longer directly in use, it leaves a continuing legacy, its successor time scales, such as TDT, as well as the atomic time scale IAT, were designed with a relationship that "provides continuity with ephemeris time". ET was used for the calibration of atomic clocks in the 1950s. Close equality between the ET second with the SI second has been verified to within 1 part in 1010. In this way, decisions made by the original designers of ephemeris time influenced the length of today's standard SI s

James Guthrie (conductor)

James Kelley Guthrie was an American symphony conductor and newspaper executive. At the age of 15, he founded the San Bernardino Community Orchestra, today the San Bernardino Symphony. In 1936, after Guthrie conducted the first performance of the Hollywood Grand Opera Association, Time magazine called him the "youngest full-fledged symphony conductor in the U. S."He was the owner and publisher of the San Bernardino Sun newspaper from 1964 until 1979. In 1974, he established the Guthrie Music Rental Library, which rents "scores and orchestra music to thousands of schools and orchestras at affordable prices in order to encourage music performance." From 1964 until 1973, he was the conductor of the Riverside Symphony Orchestra in Riverside, California known as the Inland Empire Symphony

Pacific sand sole

The Pacific sand sole known as sand sole, is a flatfish species inhabiting the northeastern Pacific waters where it lives on sandy bottoms. The only species in the genus, Psettichthys, it ranges from the Bering Sea to Northern California; the Pacific sand sole, belongs to the order Pleuronectiformes, the family Pleuronectidae. They are considered a flatfish due to their body shape, just like Hippoglossus and Platichthys; this fish can get over 62.95 cm in length and on average this fish can weigh 2.484 kg. Their dorsal side, which houses both of their eyes, can come in a variety of colors like gray, green, or brown, with blotches of dark brown or black, their underside or ventral side is white. By obtaining these colors, the pacific sand sole fish can have a better advantage of camouflaging into their surroundings; when they are first born, their eyes are on both sides of their body like other fish. However, when the pacific sand sole starts to mature, one of their eyes starts to migrate to either the left or the right side of their body, just like a flounder when their eyes migrate.

From that point, their eyes will stay that way. They will live on the bottom of the ocean, on the sand, lying on one side of their body, while both of their eyes are on top of their body; these fish can be found in and around the Bering Sea, all the way down to Southern California, in the North Pacific Ocean. These fish will stay on the bottom of the ocean at an average depth of 155 m, they live on the sandy bottoms of the ocean. They feed on other fish, worms and molluscs. Media related to Psettichthys melanostictus at Wikimedia Commons]