AstroReferenceFrame

is an option of AstroGraphics that specifies the reference frame and setup of the observation corresponding to the sky map returned.

Details

An AstroGraphics sky map uses spherical coordinates associated with a given reference frame, an oriented triple of orthogonal axes centered at a definite location, from which an observation is performed at a given time.
Possible inertial frames, not rotating with respect to distant stars and centered at the solar system barycenter, include:

	"ICRS" or "BCRS"	international celestial reference system
	"J2000"	mean equatorial, mean equinox of date J2000
	"B1950"	mean equatorial, mean equinox of date B1950
	"Galactic"	inertial frame oriented along the plane of the Milky Way

Non-inertial frames centered at Earth's center but not rotating with the Earth include:

	"GCRS"	geocentric celestial reference system
	"MEME"	frame oriented along Earth's mean equator and using the mean equinox as axis
	"TETE" or "Equatorial"	frame oriented along Earth's true equator and using the true equinox as axis
	"CIRS"	celestial intermediate reference system
	"MeanEcliptic"	frame oriented along the ecliptic and using the mean equinox as axis
	"TrueEcliptic" or "Ecliptic"	frame oriented along the ecliptic and using the true equinox as axis

	"TIRS"	terrestrial intermediate reference system, centered at Earth's center and sharing the same polar axis as "CIRS"
	"ITRS"	international terrestrial reference system, centered at Earth's center and corotating with the ellipsoidal Earth
	"Horizon"	frame oriented along the local horizontal plane at a given geo location and centered at that location

The orientation of an astro reference frame attached to a celestial body depends on a choice of orientation model for physical effects such as precession, nutation or polar motion. Other physical effects such as light aberration or light deflection also affect an astronomical observation. These effects are specified as "param"value modifiers of the astro reference frame.
Possible parameters for a frame of the form {"frame","param₁"val₁,"param₂"val₂,…} include:

	"LightTime"	whether to use geometric, retarded or advanced coordinates
	"LightDeflection"	whether to correct for deflection of light by massive bodies
	"GravitationalTimeDelay"	whether to correct for Shapiro time delay
	"Aberration"	whether to correct for aberration, due to observer motion
	"PrecessionNutationModel"	model of precession and nutation used
	"Date" or "ObservationDate"	date of observation
	"Location"	origin of the frame and location of observation
	"Epoch" or "FrameDefinitionEpoch"	date defining the orientation of rotating frames
	"PolarMotion"	coordinates {x',y'} of the ITRS axis in the TIRS frame
	"Refraction"	values of pressure, temperature, relative humidity and wavelength for the computation of atmospheric refraction

{"frame","Date"date,"Location"loc,otherparams} can also be given as {"frame",date,loc,otherparams}, with the first three elements given in any order or omitted and assuming default values.

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Construct a map of the sky around the star Rigel using the equatorial frame:

Observe the area around the galactic center using the galactic frame:

Show the conjunction of Jupiter and Saturn in December 2020:

Show the southern sky at Chicago on November 8, 2022, at 2:30 am, with Jupiter about to set in the west:

By default, AstroGraphics uses the "Horizon" frame, showing the current sky above your horizon, displayed in gray:

Show the whole sky using the "Equatorial" frame, with the celestial equator displayed in blue:

Show the whole sky using the "Ecliptic" frame, with the ecliptic displayed in yellow:

Show the whole sky using the "Galactic" frame, centered at the Earth, with the galactic plane displayed in a tone of red:

Center the "Equatorial" frame at your current location, also displaying your local horizon in gray:

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