The Astrogators' Guide to

Stellar Location and Orientation

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    Alternative names of the target star and other data suggesting its importance to interstellar exploration.

Location in Space

Radial Distance:

    Parallax is measured in arc-seconds on a base 2 AU wide, which leads to;

        1. distance in lightyears possible error in lightyear. One arcsecond equals 3.262 lightyears.

        2. distance in AU possible error in AU. One arcsecond equals 206,265 AU.

Equatorial Coordinates:

    Right Ascension; measured in hours, minutes, and seconds + proper motion multiplied by time elapsed since AD 2000 Jan 01.

    Declination; measured in degrees, arcminutes, and arcseconds + proper motion multiplied by time elapsed since AD 2000 Jan 01. Declination is measured north (+) or south (-) of the celestial equator.

        [elapsed time measured in centuries, Jan 2000 is t = 0]

    Imagine standing on the north side of a plane in which Earth's equator lies with the north celestial pole directly overhead and look toward the First Point of Aries (now in Pisces just southeast of the Circlet). Then look eastward (to the left) by the right ascension and then north or south (up or down) by the declination.

Ecliptic Coordinates:

    Ecliptic Latitude; measured in degrees, minutes, and seconds (or degrees and fractional degrees) east along the Ecliptic from the First Point in Aries.

    Ecliptic Longitude; measured in degrees, minutes, and seconds (or degrees and fractional degrees) north (+) or south (-) from the Ecliptic.

    Look toward the First Point of Aries (the point on the sky that the sun occupies on the first day of spring), shift your gaze eastward along the Ecliptic (the line that the sun traces through the Zodiac in the course of a year) by the longitude, and then shift your gaze northward (+) or southward (-) by the latitude along a line perpendicular to the Ecliptic.

Galactic Coordinates:

    Galactic Latitude; measured in degrees, minutes, and seconds north (+) or south (-) of the galactic plane.

    Galactic Longitude; measured in degrees, minutes, and seconds eastward along the galactic plane from the center of the galaxy.

    Look toward the radio source Sagittarius-A, move your gaze in an easterly direction along the plane of the Milky Way by the longitude, and then tilt your gaze by the latitude in either a northerly direction or a southerly direction at right angles to the plane of the Milky Way.

Annual Proper Motion

    in Right Ascension = in temporal seconds (in AU per yr)

    in Declination = in arcseconds (in AU per yr)

    in Parallax = in arcsec (in AU per yr)

        Total motion = in AU per year = in kilometers per second. One AU per year =4.74047 kilometers per second.

    in Ecliptic latitude = in fractional degree per year.

    in Ecliptic longitude = in fractional degree per year.

From the present; in X years target star will become an eclipsing binary (for about Y centuries) as its orbital plane passes over the Sol-target star line and in Z years the target star will reach its perihelion W lightyears (__ AU) from Sol in the *** part of the constellation of ###.

Orientation in Space

    We generally only get this information for binary stars.

    Orbit size: the general orbit's semi-major axis, the sum of the semi-major axes of the two stars' orbits, in AU (e=the orbit's eccentricity); minimum separation between the stars at periastron - maximum separation between the stars at apastron, given in AU.

    We refer the following to the plane of the sky, which we define as the plane comprising all straight lines that cross our line of sight through the barycenter of the system under study and that pass through that barycenter. One of those lines coincides with the system's line of nodes, the line where the system's orbital plane crosses the plane of the sky. As a matter of definition, astronomers refer to the node where the system's secondary crosses the plane of the sky moving at least partly away from Earth as the ascending node.

    Inclination; the angle between the plane of the stars' orbits and the plane of the sky. Imagine laying the orbit flat on our line of sight to the target star system. In that orientation the stars would appear to us to move from side to side on a straight line. We define that inclination as 90˚ and we measure the actual inclination of the system by an angle between 0˚ and 180˚. If the system has an inclination less than 90˚, the stars appear to revolve counterclockwise about our line of sight (which we imagine always passing through the system's barycenter) and we call the orbit prograde. If the system has an inclination greater than 90˚, the stars appear to revolve clockwise about our line of sight and we call the orbit retrograde.

i=inclination in degrees.

    Position angle of the secondary's ascending node; the angle between the Ecliptic north vector and the line of nodes, measured counterclockwise from the line pointing Ecliptic north toward the system's ascending node.

Ω=position angle of the ascending node in degrees.

    Longitude of Periastron (or Argument of Periastron); the angle between the line of nodes and the orbit's major axis (line of apsides), measured from 0˚ to 360˚ in the prograde direction (the direction of the secondary's motion) in the plane of the true orbit, from the secondary's ascending node to the secondary's periastron

ω=longitude of periastron in degrees.

    On a piece of stiff paper draw an ellipse of eccentricity equal to that of the orbit and draw an arrow indicating the direction of the star's motion on the orbit that the ellipse represents. Imagine standing on the Ecliptic plane with the Ecliptic north pole directly overhead. Look toward the target star and so hold the paper that the line of apsides coincides with your line of sight and the north vector of the orbit (defined by the right-hand rule: when your right thumb, extended in a thumbs-up gesture, points north, the fingers of that hand curl in the same way that the body moves on its orbit) points Ecliptic north. Designate the farther focus of the ellipse as the barycenter, the system's center of mass. In that orientation your ellipse has a position angle of +270 degrees, an inclination of 90 degrees, and a longitude of periastron of 90 degrees. Rotate the paper counterclockwise about the line of apsides by Ω-90 degrees (clockwise if you get a negative number). Turn the paper ω-270 degrees in the prograde direction about the orbit's north vector (retrograde if you get a negative number). And then tilt the paper i-90 degrees about the line of nodes, turning the paper by the right-hand rule if you get a positive number and by the left-hand rule if you get a negative number, the appropriate rule stating that when the thumb of the appropriate hand points from the orbit's ascending node to its descending node, the fingers curl in the direction you must turn the paper. With the paper in that position you have a picture of the orbit of the system's secondary star. The ellipse represents the orbit of the system's primary star if you change the position of the barycenter from one focus of the ellipse to the other.

    Orbital Period: given in Earth years.

        Time of Periastron passage: calculated out to AD 2300 from Twentieth Century. The presentation format is:

            1. 1900.00 (AD 1900 Jan 01)

The Stars Themselves

Comments on the system as whole, such as age.

    Star A:

        Diameter; measured in kilometers (or as a multiple of Sol's diameter, 1,391,900 km).

        Harvard Class; one of OBAFGKM + one of 1-10 (photospheric temperature in degrees Kelvin).

        Mass; given as a multiple of Sol's mass.

        Brightness; given as a multiple of Sol's brightness.

        Habitable zone: minimum AU - maximum AU (median AU, orbital period in years)

        Surface composition: hydrogen X%, helium Y%, other Z% (Sol = hydrogen 73.7%, helium 24.5%, other 1.81%)

    Star B:

        Diameter; measured in kilometers (or as a multiple of Sol's diameter, 1,391,900 km).

        Harvard Class; one of OBAFGKM + one of 1-10 (photospheric temperature in degrees Kelvin).

        Mass; given as a multiple of Sol's mass.

        Brightness; given as a multiple of Sol's brightness.

        Habitable zone: minimum AU - maximum AU (median AU, orbital period in years)

        Surface composition: hydrogen X%, helium Y%, other Z% (Sol = hydrogen 73.7%, helium 24.5%, other 1.81%).

Planetary system properties:

    stable planetary orbits lie within 1/5 of closest approach of components, __ AU in this case.

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