Anti-sidereal time and extended-sidereal time are artificial

time standard A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters ...

s used to analyze the daily variation in the number of

cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...

s received on

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

. Anti-sidereal time has about 364.25 days per year, one day less than the number of days in a year of solar time, 365.25. Thus each anti-sidereal day is longer than a solar day (24 hr) by about four minutes or 24 hr 4 min. Extended-sidereal time has about 367.25 days per year, one day more than the number of days in a year of

sidereal time Sidereal time (as a unit also sidereal day or sidereal rotation period) (sidereal ) is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coord ...

, 366.25. Thus each extended-sidereal day is shorter than a sidereal day (23 hr 56 min) by about four minutes or 23 hr 52 min. All years mentioned have the same length.

Amplitude modulation sidebands

Cosmic rays received on Earth exhibit daily variations in amplitude in solar time due to the distribution of cosmic rays in the inner heliosphere and to the Compton-Getting effect caused by Earth's

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

al velocity around the Sun. Other daily variations in amplitude in sidereal time are caused by the anisotropy in the direction from which cosmic rays are received relative to the plane of our

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

, the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

. Both are contaminated by an annual seasonal variation. The daily solar variation is

amplitude modulated Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to t ...

by the seasonal variation of , producing

sideband In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, that are the result of the modulation process. The sidebands carry the information transmitted by the radio signal. The sidebands co ...

s on either side of the solar frequency, about , of and . Similarly, an annual amplitude modulation of of the sidereal frequency, about , produces sidebands of and . The upper sideband of the solar frequency contaminates the amplitude of the sidereal frequency, while the lower sideband of the sidereal frequency contaminates the amplitude of the solar frequency. Because the magnitudes of the two sidebands produced by amplitude modulation of the solar frequency are the same and no known natural phenomenon recurs at , the spurious amplitude at the sidereal frequency can be corrected by subtracting any signal present at the anti-sidereal frequency. Similarly, the spurious amplitude in the solar frequency of per year can be corrected by subtracting any signal present at the extended-sidereal frequency of per year.K. Munakata et al,
Galactic anisotropy of ~10TeV cosmic-ray intensity observed by the Tibet air shower array
, ''28th International Cosmic Ray Conference'' (2003).

References

{{reflist Astroparticle physics Cosmic rays Time scales Time in astronomy

Amplitude modulation sidebands

See also

References