SiderealTime

SiderealTime[]

gives the right ascension of the local meridian for the current date and location.

SiderealTime[date]

gives the right ascension of the local meridian for the specified date.

SiderealTime[loc]

gives the right ascension of the local meridian for the specified location.

SiderealTime[loc,date]

gives the right ascension of the local meridian for the specified date and location.

SiderealTime[{{loc1,date1},{loc2,date2},…}]

gives the right ascensions of the local meridians for all specified locations on the specified dates.

SiderealTime[loc,date,func]

uses func to determine what to return for extended locations.

SiderealTime["MeanTime",loc,date,func]

gives the mean sidereal time for the specified date, location and aggregation function.

Details

  • Sidereal time is typically used to locate celestial objects in the night sky and to decide when and where to point a telescope for optimal observation.
  • SiderealTime returns a Quantity angle expressed in mixed units of hours, minutes and seconds of right ascension, as is traditionally done with angles measured along the celestial equator.
  • SiderealTime[loc,date], equivalent to SiderealTime["ApparentTime",loc,date], computes the local apparent or true sidereal time, based on the apparent equator and equinox of date, hence including the effects of both precession and nutation.
  • SiderealTime["MeanTime",loc,date] computes the local mean sidereal time based on the mean equator and equinox of date, including precession but averaging over nutation.
  • SiderealTime[] makes use of $GeoLocation and $TimeZone to determine your location and time zone.
  • Locations can be specified as GeoPosition objects, {lat,lon} pairs in degrees, Entity geo locations or GeoGraphics primitives.
  • datespec can be a DateObject expression, a TimeObject expression, a date string or a {y,m,d,h,m,s} date list.
  • datespec is assumed to be in $TimeZone, unless it is a DateObject or TimeObject expression with an explicit TimeZone option value.
  • loc and date can be either individual items or lists of them.
  • If datespec is a list of dates, then the results will contain TimeSeries objects.
  • datespec can be specified as {start,end,increment} for compatibility with DateRange specifications.
  • SiderealTime[…,func] is used to specify the format of output when extended locations are specified.
  • Possible settings for func include:
  • Automaticreturns intervals for extended locations only
    Intervalreturns intervals for all specified locations
    Meanreturns mean value for extended locations
    Minreturns minimum values for extended locations
    Maxreturns maximum values for extended locations
    StandardDeviationreturns standard deviation for extended locations

Examples

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Basic Examples  (5)

Compute the sidereal time for your current location:

Compute the sidereal time for a specified date:

Compute the sidereal time for a specified location:

Compute the sidereal time for a specified latitude/longitude and date:

Compute the sidereal time for a specified city and date:

Scope  (8)

Dates  (3)

Dates can be specified as a DateObject:

Dates can be specified as a date string:

Generate the sidereal time for a range of dates:

Locations  (5)

Locations can be latitude/longitude pairs:

Cities are treated as single, specific locations:

Results for extended locations are intervals, by default:

The form of the results for extended locations can be overridden:

Generate the sidereal time for multiple locations:

Applications  (2)

Plot the equation of time by finding the difference between the Sun's right ascension at noon and the sidereal time at noon:

The difference between apparent and mean sidereal times is called the equation of the equinoxes:

Display the equation of the equinoxes over a year:

The same computation over 20 years, which shows the main nutation cycle of 18.6 years:

The primary period corresponds to an oscillation of 17.2 arc seconds of nutation in longitude:

Properties & Relations  (3)

The output of SiderealTime is an angle, not a time:

SiderealTime tracks Earth's rotation with respect to the fixed stars, with a full rotation taking one sidereal day:

Earth rotates with respect to the fixed stars in less than one day, and it needs to rotate a bit more to complete a full (solar) day:

The effect accumulates during one year and hence Earth rotates 366.242 times with respect to the fixed stars in 365.242 days:

Equivalently:

Possible Issues  (1)

Although it looks like a time-based concept, SiderealTime is actually an angle:

Neat Examples  (1)

Choose a location and a date:

This is the sidereal time of that location at that time:

Take the right ascensions of the Sun, the Moon and the planets:

Construct text labels for them:

Plot the directions of the Sun, the Moon and the planets as viewed from the South Pole, with respect to the fixed stars:

Wolfram Research (2014), SiderealTime, Wolfram Language function, https://reference.wolfram.com/language/ref/SiderealTime.html (updated 2021).

Text

Wolfram Research (2014), SiderealTime, Wolfram Language function, https://reference.wolfram.com/language/ref/SiderealTime.html (updated 2021).

CMS

Wolfram Language. 2014. "SiderealTime." Wolfram Language & System Documentation Center. Wolfram Research. Last Modified 2021. https://reference.wolfram.com/language/ref/SiderealTime.html.

APA

Wolfram Language. (2014). SiderealTime. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/SiderealTime.html

BibTeX

@misc{reference.wolfram_2024_siderealtime, author="Wolfram Research", title="{SiderealTime}", year="2021", howpublished="\url{https://reference.wolfram.com/language/ref/SiderealTime.html}", note=[Accessed: 22-December-2024 ]}

BibLaTeX

@online{reference.wolfram_2024_siderealtime, organization={Wolfram Research}, title={SiderealTime}, year={2021}, url={https://reference.wolfram.com/language/ref/SiderealTime.html}, note=[Accessed: 22-December-2024 ]}