Sidereal time

Time metering system

synonym Stellar time system (Astronomical term) generally referring to stellar time

Sidereal time refers to the Rotation period Fiducial Time measurement System. take Vernal equinox Two times in succession Midheaven The time experienced is called Sidereal day , which is equal to 23:56:4.09 seconds Flat solar time And take the moment when the vernal equinox is in the sky above the ground as the starting point of this measurement system, that is, when the star time is zero, the vernal equinox is used Hour angle To measure.

For the convenience of measurement, the star day is divided into 24 Sidereal hour , 60 hours per star Sidereal minute , a star is divided into 60 minutes Sidereal second 。 All these units are collectively referred to as the stellar time unit for measuring time, which is called stellar time unit for short. The time measured by the above system is called stellar time in astronomy.^[1]

Chinese name: Sidereal time
Foreign name: Sidereal time

Nature: Timing unit
Foundation: Sidereal day
Direction: astronomy and Geodesy

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Sidereal time

Star time is astronomy and Geodesy indicative Celestial meridian circle Value, which is a Time system , based on the true rotation of the earth: fixed star The rising starts until the star rises again (23:56:4). consider earth rotation The uneven influence is True sidereal time , otherwise Flat sidereal time 。 Sidereal day And Solar day 。 A distant star (small red circle) and the sun Midheaven , local meridian On. Middle: There is only a distant star in the sky (a flat star day). Right: A few minutes later, the sun will finish a solar day in the middle of the sky.

with Vernal equinox by Reference point , by vernal equinox Diurnal visual movement The time determined is called stellar time. The vernal equinox passes through the local meridian circle twice in a row time interval It is a star day, including 24 Sidereal hour 。 So the stellar time is numerically equal to the vernal equinox relative to the local meridian circle Hour angle 。 Because the star time is calculated based on the time when the vernal equinox passes through the local meridian circle as the origin Survey station The stars of Local sidereal time 。

Because of the earth's nutation The vernal equinox is not fixed on the celestial sphere, but swings around the average vernal equinox in a cycle of 18.6 years. So the star time is divided True sidereal time and Flat sidereal time 。 True star time is through direct measurement The time angle between the meridian and the actual equinox is obtained, while the flat star time ignores the nutation of the Earth. The maximum difference between the true star time and the flat star time is about 0.4 seconds.

The local star time of a place is the same as greenwich observatory The difference between the stellar hours of is the longitude of this place (see Celestial navigation ）。 Therefore, the local longitude (if the star of Greenwich Observatory is known) or the time (if the local longitude is known) can be determined by observing the star.

Time angle of a star t , its right ascension α And local star time θ The relationship between t = θ - α
The local star is equal to zenith Right ascension of a star
The local star time is equal to Midheaven Right ascension of stars

Astronomical calculation can be simplified by determining stars. For example, it is convenient to calculate which stars are exactly in the horizon above.

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Star time: is the vernal equinox Meridional circle Time angle of. There is a kind of Local sidereal time 。 Star time is Universal Time At 0:00 (8:00 Beijing time) Greenwich star time 。

Starting point of star time

Now visible Herdsman Star a (14:16 in right ascension) is a little west of the meridian circle, scorpio Star a (right ascension 16:29) is in the east of the meridian circle. It is required that the local star time s at any time in a place can be expressed as follows s = m + S o + Mμ calculation. Where m by Local peacetime ， M For and m Corresponding Greenwich Mean Time ， S o Is the star time of 0h Universal Time, μ =1/365.2422。 For example, Nanjing（ longitude λ =7h55m04s) local star time at 19h35m Beijing time on March 31.

Find Nanjing from known Beijing time Local peacetime m =19h35m+（7h55m04s-8h）=19h30m04s； According to the popular astronomical almanac, we can know the star time of 0h Universal Time on March 31 S o =12h33m52s； from Local time M-corresponding Greenwich Mean Time M=m－ λ =11h35m, further calculate Mμ =1m54s。 So local star time s = m + S o + Mμ =8h05m50s。 Hour angle t =Local sidereal time s- Right ascension a

If you do not know the star at 0h Universal Time on a certain day, you can use s=right ascension a+hour angle to calculate. For example, the local star time at 19h35m Beijing time on March 31, 2019 in Nanjing. Nanjing at that time Arcturus The hour angle is - 6h12m12s (or 17h47m48s-24h), and the right ascension of the star is 14h15m38.4s, so s=14h15m38.4s-6h12m12s=8h03m26.2s

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