The Astrogators' Guide to

Gamma Virginis

Location in Space

Radial Distance:

Parallax = 0.08453" 0.00118 arcseconds, which leads to

1. 38.59 lightyears ± 0.539 lightyear (11.83 ± 0.1654 parsecs).

2. 2,440,140 AU ± 34070 AU.

Equatorial Coordinates:

Right Ascension; 12 hr, 41 min, 39.6423 sec -3.7667t sec

Declination; -01 deg, 26 min, 57.75 sec +1.2t sec.

[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 190 degrees and a little less than half a degree and then look south (down) by a little less than one and a half degrees.

Ecliptic Coordinates:

Ecliptic Latitude; +2.7902896 deg -21.026t arc-sec.

Ecliptic Longitude; 190.1414577 deg - 52.46t arc-sec.

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

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 slightly more than 190 degrees, and then shift your gaze northward (+) by a little over two and three-quarters degrees along a line perpendicular to the Ecliptic.

Galactic Coordinates:

Galactic Latitude; +61.3256 deg + 3.48t arc-sec.

Galactic Longitude; 297.8343 deg - 55.97t arc-sec.

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

Look toward the radio source Sagittarius-A by casting your gaze across the Orion-Sagittarius Gap toward the Sagittarius-Carina Arm of the galaxy and at a point about six degrees south of the Ecliptic on the west (right) side of Sagittarius and two degrees south of X Sagittarii (the point of the arrow in the Archer's bow), move your gaze in an easterly direction along the plane of the Milky Way by almost 298 degrees, and then tilt your gaze by sixty-one and one-third degrees in a northerly direction, more or less upward and to your left, at right angles to the plane of the Milky Way.

Annual Proper Motion

in Right Ascension = -0.565 arcsecond per year (6.685 AU/yr = 31.6897 km/sec).

in Declination = +0.012 arcsecond per year (0.142 AU/yr = 0.673 km/sec).

Total Proper Motion = 0.56513 arc-sec/yr in the direction 271.22 degrees counterclockwise from due celestial north, 248.16 degrees counterclockwise from due Ecliptic north, and 273.56 degrees counterclockwise from due galactic north.

in Radial Distance = -19.8± 0.9 km/sec (4.1768 AU/yr).

Total motion = 7.883 AU/yr = 37.369 km/sec.

From the present; in 1,078,311 years Gamma Virginis will become an eclipsing binary (for about 4 centuries) as its orbital plane passes over the Sol-Gamma Virginis line and in 163,943 years Gamma Virginis will reach its perihelion 32.733 lightyears (2,069,795 AU) from Sol in the middle part of the constellation of Sextans after crossing 31.98 degrees of sky.

Orientation in Space

Orbit size: the general orbit's semi-major axis (42.36 AU), the sum of the semi-major axes of the two stars' orbits, in AU (e=0.8781± 0.0025); varies between 5.16 AU and 79.56 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=150.38± 0.35 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=32.88± 0.18 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=252.62± 0.16 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 -57.12 degrees (clockwise if you get a negative number). Turn the paper -17.38 degrees in the prograde direction about the orbit's north vector (retrograde if you get a negative number). And then tilt the paper +60.38 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: 168.93" 0.43 Earth years.

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

1. 1836.45" 0.19 (AD 1836 Jun 14)

2. 2005.38 (AD 2005 May 20)

3. 2174.31 (AD 2174 Apr 24)

4. 2343.24 (AD 2343 Mar 30)

The Stars Themselves

Star A:

Diameter; 1.3379 Sol (1,862,260 km).

Harvard Class; F0-V (7200± 500 degrees Kelvin).

Mass; 1.3225 Sol.

Brightness; 4.325 Sol.

Habitable zone: 1.976 AU - 2.849 AU (2.08 AU, 2.609 years)

Star B:

Diameter; 1.3257 Sol (1,845,220 km).

Harvard Class; F0-V (7200± 500 degrees Kelvin).

Mass; 1.3165 Sol

Brightness; 4.2462 Sol.

Habitable zone: 1.957 AU - 2.822 AU (2.06 AU, 2.578 years)

Planetary system properties:

Stable planetary orbits lie within 1/5 of closest approach of components, 1.032 AU in this case. This system has no habitable planets, either revolving about one or the other of the stars or revolving about both stars.

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