The Astrogators' Guide to

Sirius

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    Also known as Alpha Canis Majoris, Canicula, Aschere.

Location in Space

Radial Distance:

    Parallax = 0.379 arc-seconds, which leads to;

        1. 8.60 lightyears 0.04 lightyear (2.6364 0.01226 parsecs).

        2. 543,873 AU 2530 AU.

Equatorial Coordinates: (epoch 2000.0)

    Right Ascension; 6 hr, 45 min, 8.871 sec - 3.64t sec

    Declination; -16 deg, 42 min, 57.99 sec -122.314t arc-sec.

        (t=time in centuries after Jan 2000)

Imagine standing on the north side of a plane in which Earth's equator lies and look toward the First Point in Aries (the point where the Ecliptic crosses the celestial Equator, which point now lies in Pisces, slightly southeast of the Circlet). Slide your gaze eastward a little over 101-1/4 degrees along the celestial Equator, reaching a point south of where the Ecliptic passes through Gemini. Then tilt your view south by 16 degrees and almost 43 minutes. Sirius is the brightest star in your field of view.

Ecliptic Coordinates: (epoch 2000.0)

    Ecliptic Latitude; -39.6052 deg - 127.2t arc-sec.

    Ecliptic Longitude; 104.0814 deg - 41.96t arc-sec.

        [t 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 104 degrees, and then shift your gaze southward by a little over 39-1/2 degrees along a line perpendicular to the Ecliptic.

Galactic Coordinates:

    Galactic Latitude; -8.8903 deg - 83.32t arc-sec.

    Galactic Longitude; 227.2303 deg - 104.88t arc-sec.

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

The galactic latitude defines a conical surface whose vertex touches Sol and whose component lines, radiating from Sol, tilt almost 8.9 degrees south of the galactic plane. The galactic longitude defines a flat plane oriented due galactic north-south and, with one edge on Sol and turned in the manner of an opening door, turned 227.23 degrees east of the line extending from Sol to the galactic center. Imagine standing on the north side of the galactic plane. Look across the Orion-Sagittarius Gap toward the Sagittarius-Carina Arm and at a point about six degrees south of the Ecliptic on the west side of Sagittarius and two degrees south of X Sagittarii (the point of the arrow in the Archer's bow). Move your point of view 227.23 degrees left along the plane of the Milky Way and then 8.9 degrees south of that plane (at a right angle to the plane of the Milky Way). You will then be looking almost directly at Sirius.

Annual Proper Motion

    in Right Ascension = -0.54605 arc-sec/yr (1.4398 AU/yr = 6.825 km/sec).

    in Declination = -1.22314 arc-sec/yr (3.2251 AU/yr = 15.2887 km/sec).

        Total Proper Motion = 1.3394932 arc-sec/yr (3.532 AU/yr = 16.743 km/sec) in a direction 204.06 degrees counterclockwise from due celestial north, 198.26 degrees counterclockwise from due Ecliptic north, and 231.53 degrees counterclockwise from due galactic north.

    in Radial Motion = -9.4 km/sec = 1.9829 AU/yr.

        Total motion = 4.05 AU per year = 19.2 km/sec.

 

From the present; in 109,021 years Sirius will become an eclipsing binary for about 123 years as its orbital plane passes over the Sol-Sirius line and in about 65,705 years Sirius will reach its perihelion 7.5 lightyears (474,248 AU) from Sol in the southeast part of what is now the constellation of Columba after crossing 29.3 degrees of sky.

Orientation in Space

    Orbit size: 19.78 AU semi-major axis (e=0.592); 8.069 -- 31.49 AU.

    Inclination; the angle between the plane of the stars= orbits and the plane of the sky.

i=136.53 deg

Because this is greater than 90 degrees and less than 270 degrees, we see the stars appearing to revolve in the clockwise sense about their common center of mass.

    Position angle of the secondary's ascending node; the angle between the Ecliptic north vector and the line of nodes (the line where the plane of the orbit crosses the plane of the sky), measured counterclockwise toward the nodal point between 0 degrees and 180 degrees (the nodal point at which the secondary moves slightly toward the viewer).

Ω= 44.57 deg

    Longitude of Periastron; the angle between the line of nodes and the orbit's major axis (line of apsides), measured 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= 147.27 deg

    On a piece of stiff paper draw an ellipse of eccentricity e=0.59 and draw an arrow indicating the direction of the star's motion on the orbit that the ellipse represents. Look toward Sirius 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. Rotate the paper clockwise about the line of apsides by 45.43 degrees. Turn the paper 57.27 degrees in the prograde direction about the orbit's north vector. And then tilt the paper 46.53 degrees about the line of nodes, turning the part closer to you Ecliptic northeast (toward your upper left). With the paper in that position you can interpret the ellipse in two ways:

    1. if you see the apastron closer to you than the periastron, then the ellipse represents the orbit of Sirius A about the system's center of mass.

    2. if you see the apastron farther from you than the periastron, then the ellipse represents the orbit of Sirius B about the system's center of mass.

    Orbital Period: 50.09 yr.

        Time of Periastron passage:

            1. 1894.13

            2. 1944.22 (interpolated)

            3. 1994.31 (AD 1994 Apr 23)

            4. 2044.40 (AD 2044 May 26)

            5. 2094.49 (AD 2094 Jun 28)

            6. 2144.58 (AD 2144 Jul 31)

            7. 2194.67 (AD 2194 Sep 02)

            8. 2244.76 (AD 2244 Oct 04)

            9. 2294.85 (AD 2294 Nov 06)

The Stars Themselves

    Sirius A:

        Diameter; 2,339,000 km (1.68 Sol)

        Harvard Class; A0 V or A1 V (9600 K)

        Age; 300 Myr

        Mass; 2.14 Sol

        Brightness; 26.1 Sol

        Habitable zone: 4.6 AU, 6.8 yr

        Surface composition: hydrogen 71.5%, helium 25.8%, other 2.74% (Sol = hydrogen 73.7%, helium 24.5%, other 1.81%)

 

    Sirius B:

        Diameter; 11,700 km (0.0084 Sol)

        Harvard Class; White Dwarf (DA2-5 or A2-5 VII) (25,000 - 32,000 K)

        Age; 300 Myr

        Mass; 1.034 0.026 Sol

        Brightness; 0.00028 Sol

        Original Star: B5 of 6 B 7 Solar Masses.

        Surface composition: hydrogen 69.4%, helium 27.7%, other 2.89%

 

Planetary system properties:

    This system is unlikely to have planets, due to the evolution and gravitational influence of Sirius B.

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