Geometric positions and velocities

Geometric locations are 3D positions and velocities relative to an observer location. More...

Classes
class	supernovas::AstrometricPosition
	The 3D geometric equatorial position of an object, relative to a reference Solar-system location and a specific instant of time, in the coordinate system of choice. More...
class	supernovas::Geometric
	The geometric (3D) position and velocity of a source relative to an observer location. More...
class	supernovas::Position
	A 3D physical position vector in space. More...
class	supernovas::Velocity
	A 3D physical velocity vector in space. More...

Functions
Geometric	supernovas::Planet::approx_geometric_in (const Frame &frame) const
	Returns approximate eometric posiitions and velocities for a major planet, Sun, Moon, Earth-Moon Barycenter (EMB) – typically to arcmin level accuracy – using Keplerian orbital elements.
AstrometricPosition	supernovas::Apparent::astrometric_position () const
	Returns the projected 3D position, relative to the SSB or other Solar-System place, which gave rise to these apparent place on sky.
Geometric	supernovas::SolarSystemSource::barycentric_at (const Time &time, enum novas_accuracy accuracy=NOVAS_FULL_ACCURACY) const
	Returns the Solar-system barycentric position and velocity of this Solar-system source, or else an undefined geometry if the source's place cannot be determined at the requested time or accuracy.
Geometric	supernovas::Source::geometric_in (const Frame &frame, enum novas_reference_system system=NOVAS_TOD) const
	Returns the geometric position of a source (if possible) relative to the observer, or else an invalid position if the source could not be positioned in the observing frame.
Geometric	supernovas::Frame::geometric_moon_elp2000 (double limit_term=0.0) const
	Returns the Moon's geometric position using the ELP/MPP02 model by Chapront & Francou (2003), down to the specified limiting term amplitude.

Detailed Description

Geometric locations are 3D positions and velocities relative to an observer location.

They are corrected for light travel time to the observer, so they reflect the position and motion of a source when the observed light originated from the source.

Function Documentation

approx_geometric_in()

Geometric supernovas::Planet::approx_geometric_in

(

const Frame &

frame

)

const

Returns approximate eometric posiitions and velocities for a major planet, Sun, Moon, Earth-Moon Barycenter (EMB) – typically to arcmin level accuracy – using Keplerian orbital elements.

The orbitals can provide planet positions to arcmin-level precision for the rocky inner planets, and to a fraction of a degree precision for the gas and ice giants and Pluto. The accuracies for Uranus, Neptune, and Pluto are significantly improved (to the arcmin level) if used in the time range of 1800 AD to 2050 AD. For a more detailed summary of the typical accuracies, see either of the top two references below.

For accurate positions, you should use planetary ephemerides (such as the JPL ephemerides via the CALCEPH or CSPICE plugins) and novas_sky_pos() instead.

While this function is generally similar to creating an orbital object with an orbit initialized with novas_make_planet_orbit() or novas_make_moon_orbit(), and then calling novas_geom_posvel(), with an important difference:

• This function calculates Earth and Moon positions about the Keplerian orbital position of the Earth-Moon Barycenter (EMB). In contrast, novas_make_planet_orbit() does not provide orbitals for the Earth directly, and make_moot_orbit() references the Moon's orbital to the Earth position returned by the currently configured planet calculator function (see set_planet_provider()).

REFERENCES:

1. E.M. Standish and J.G. Williams 1992.
2. https://ssd.jpl.nasa.gov/planets/approx_pos.html
3. Chapront, J. et al., 2002, A&A 387, 700–709
4. Chapront-Touze, M., and Chapront, J. 1983, Astronomy and Astrophysics (ISSN 0004-6361), vol. 124, no. 1, July 1983, p. 50-62.

Parameters

Observing frame

the observing frame for which to calculate geometric positions.

Returns

approximate, orbital model based, geometric positions and velocities of this planet in the specified observing frame. It may be invalid either if this planet or the frame argument is invalid.

Since

1.6

See also

approx_apparent_in()

References supernovas::Frame::_novas_frame(), supernovas::Unit::AU, supernovas::Unit::AU_per_day, supernovas::Validating::is_valid(), supernovas::Frame::jd(), novas_approx_heliocentric(), novas_id(), NOVAS_TDB, novas_frame::sun_pos, and novas_frame::sun_vel.

astrometric_position()

AstrometricPosition supernovas::Apparent::astrometric_position

(

)

const

Returns the projected 3D position, relative to the SSB or other Solar-System place, which gave rise to these apparent place on sky.

The position is antedated to the time the observed light originated from the observed body,

Returns

The position referenced to the given Solar-system body or place, and antedated for light travel time for this apparent position.

Since

1.6

References astrometric_position(), supernovas::Unit::au, novas_app_to_geom(), NOVAS_TOD, and supernovas::Position::undefined().

Referenced by astrometric_position().

barycentric_at()

Geometric supernovas::SolarSystemSource::barycentric_at	(	const Time &	time,
		enum novas_accuracy	accuracy = NOVAS_FULL_ACCURACY ) const

Returns the Solar-system barycentric position and velocity of this Solar-system source, or else an undefined geometry if the source's place cannot be determined at the requested time or accuracy.

Parameters

Time of observation and astronomical timescales	The astrometric time
accuracy	(optional) NOVAS_FULL_ACCURACY (default), or NOVAS_REDUCED_ACCURACY.

Returns

The Solar-system barycentric geometric position and velocity of this source at the specified time, as obtained from ephemeris lookup; or else an undefined (invalid) object if the place ould not be determined.

Since

1.6

References supernovas::Source::_novas_object(), supernovas::Observer::at_ssb(), supernovas::Unit::AU, supernovas::Unit::AU_per_day, ephemeris(), supernovas::Time::jd(), NOVAS_BARYCENTER, NOVAS_ICRS, NOVAS_TDB, and supernovas::Geometric::undefined().

geometric_in()

Geometric supernovas::Source::geometric_in	(	const Frame &	frame,
		enum novas_reference_system	system = NOVAS_TOD ) const

Returns the geometric position of a source (if possible) relative to the observer, or else an invalid position if the source could not be positioned in the observing frame.

After the return, you should probably check for validity:

Geometric geom = my_source.geometric(frame);
if(!geom.is_valid()) {
  // We could not obtain geometric positions for some reason.
  ...
}

There are multiple reasons why we might not be able to calculate valid geometric positions, such as:

• the frame itself may be invalid.
• the system parameter may be outside of the enum range
• for Solar system sources:
  • SuperNOVAS may not have a planet or ephemeris provider function configured for the given accuracy.
  • the planet or ephemeris provider function does not have data for the source, or planet at the orbital center (for orbital sources), for the requested time of observation.

A geometric position is the 3D location, relative to the observer location, where the light originated from the source before being detected by the observer at the time of observation. As such, geometric positions are necessarily antedated for light travel time (for Solar-system sources) or corrected for the differential light-travel between the Solar-system barycenter and the observer location (for sidereal sources).

In other words, geometric positions are not the same as ephemeris positions for the equivalent time for Solar-system bodies. Rather, geometric positions match the ephemeris positions for an earlier time, when the observed light originated from the source.

Parameters

Observing frame	observer frame, which defines the observer location and the time of observation, as well as the accuracy requirement.
system	coordinate reference system in which to return positions and velocities.

Returns

the geometric (3D) position and velocity of the source, which may be invalid if the position cannot be determined.

Since

1.6

See also

apparent_in()

References supernovas::Frame::_novas_frame(), _object, supernovas::Unit::au, supernovas::Unit::AU_per_day, novas_geom_posvel(), NOVAS_TOD, and supernovas::Geometric::undefined().

geometric_moon_elp2000()

Geometric supernovas::Frame::geometric_moon_elp2000

(

double

limit_term = 0.0

)

const

Returns the Moon's geometric position using the ELP/MPP02 model by Chapront & Francou (2003), down to the specified limiting term amplitude.

NOTES:

• The initial implementation (in v1.6) truncates the full series, keeping only terms with amplitudes larger than 1 mas (around 3400 harmonic terms in total), resulting in a limiting accuracy below 1 km level (and less than 100 meter error typically for 1900 – 2100).

REFERENCES:

1. Chapront-Touze, M., & Chapront, J., A&A, 190, 342 (1988)
2. Chapront, J., Francou G., 2003, A&A, 404, 735
3. Chapront, J., & Francou, G., "LUNAR SOLUTION ELP version ELP/MPP02", (October 2002), https://cyrano-se.obspm.fr/pub/2_lunar_solutions/2_elpmpp02/

Parameters

limit_term

(optional) [arcsec|km] Sum only the harmonic terms with amplitudes larger than this limit.

Returns

The calculated geometric geocentric position and velocity of the Moon in the ICRS.

Since

1.6

See also

apparent_moon_elp2000()

References _novas_frame(), supernovas::Unit::AU, supernovas::Unit::AU_per_day, supernovas::Validating::is_valid(), NOVAS_ICRS, novas_moon_elp_posvel_fp(), and supernovas::Geometric::undefined().