Angular coordinates are stored using degrees, and distances are stored using astronomical units:

Specify an angular position on the celestial sphere, in the CIRS frame, assuming the current date:

Find the positions of a list of astro entities:

Provide coordinates using a QuantityArray object:

Specify named directions in space:

AstroPosition uses by default the horizontal frame whose - plane is tangent to the Earth at your geo location:

Use the galactic frame, whose space axes are oriented along the galactic plane and center and that is centered at the solar system barycenter:

Use the GCRS (Geocentric Coordinate Reference System) frame, whose space axes are oriented along the ICRS and that is centered at the geo center:

Use the MEME (mean equatorial, mean equinox) frame of a given epoch:

Use the J2000 frame, the mean equatorial, mean equinox frame of date J2000, very close to the ICRS frame:

Use the B1950 frame, the mean equatorial, mean equinox frame of date B1950:

Use the TETE (true equatorial, true equinox) frame of a given epoch:

Use the mean ecliptic frame of a given epoch:

Use the true ecliptic frame of a given epoch:

Use the CIRS (Celestial Intermediate Reference System) frame:

Use the TIRS (Terrestrial Intermediate Reference System) frame:

Use the ITRS (International Terrestrial Reference System) frame:

Use the horizontal frame at a given geo location and date:

Specify the observation date as a DateObject expression:

Dates are taken in any time system and converted into the "TT" time system:

Date strings are assumed to be in the local time zone, the default smeared UTC time system, and are converted into "TT", in time zone 0:

Some especially important dates can be given as named strings:

Specify the geo location where the - plane of the horizontal frame is tangential to the Earth's surface:

Observe the star Vega on this date from your location, using the MEME frame of date:

Perform the same observation, on the same date and from the same location, but now using the MEME frame of 100 years ago, which is different from the current frame due to precession:

Position of Mars at the current instant in the main BCRS coordinate system of the solar system:

Position of Mars when the light was emitted that arrived at Earth's center at that instant:

The difference is on the order of tens of arcseconds:

Position of Mars when receiving light emitted from Earth's center at that instant:

The difference is similar:

Eddington observed this star during a solar eclipse on May 29, 1919:

The Sun and Moon were essentially at the same position:

This was the observed position, including light deflection:

This would have been its position with no light deflection:

The difference is about one arcsecond:

The Shapiro effect introduces a small delay in light propagation for light rays that pass near massive bodies like the Sun, which also affects positions. Observe Venus on this date:

Differences are very small, in the order of microarcseconds:

Observed positions change due to the motion of the observer, for example, the orbital motion of the Earth around the Sun:

This is the current observed position of Polaris:

This would be its position with no correction for aberration:

Six months later, the aberration correction will have the opposite sign:

The instantaneous rotation axis of the Earth wobbles around the geographical polar axis of the Earth, introducing small corrections in the positions of objects with respect to the "ITRS" and "Horizon" frames:

The angular differences can be obtained from GeoOrientationData:

By default, the "ITRS" and "Horizon" frames use measured values of polar motion:

This would be the position of Sirius without taking into account polar motion:

Specify your own values of polar motion:

Compute the time of sunset on this date:

That is the time when the upper limb of the Sun touches the horizon on its way down, so the Sun's center already has negative altitude at that time:

By default, "Horizon" computations on Earth use a refraction model. Without it, this would have been the position of the Sun at that instant:

The difference is approximately 36 arcminutes for standard atmospheric conditions. Specify your own conditions of pressure, temperature and relative humidity, as well as the light wavelength:

Coordinates are given by default in spherical form, as longitude, latitude pairs:

Convert to Cartesian coordinates, still in the same equatorial frame:

Convert to cylindrical coordinates:

Express in a given cartographic projection:

Compute spherical horizontal coordinates for the Moon:

Compute SEU Cartesian coordinates:

Use Cartesian coordinates with a different orientation:

Use cylindrical coordinates using hour angle:

Extract properties from an AstroPosition object:

Extract coordinate properties:

Angles are given as numbers of angular degrees:

Distance from the frame origin is given in astronomical units:

Extract frame properties:

Date of observation:

Location of the observation, and frame origin:

Extract the coordinate system or projection being used: