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Thread: Closest Bright Star to the Origin of the Celestial Spherical Surface

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    Default Closest Bright Star to the Origin of the Celestial Spherical Surface



    I've got a celestial sphere that has all the constellations printed on it, with the constituent stars. The globe doesn't have any lines of right ascension and declination drawn on it. Although not marked, I CAN tell fairly closely where the celestial equator is located (the ecliptic plane), but I have no way of telling where the zero point of right ascension is on that equator. I.e., I can't tell where the origin of the coordinants of the celestial spherical surface is. Can anyone tell me what the name of the star is (and/or in which constellation it resides) which is closest to the point of zero right ascension and zero declination?

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    Default Re: Closest Bright Star to the Origin of the Celestial Spherical Surface

    Mike
    Pisces is the constellation. Pisces is a nondescript constellation of faint stars.
    There are no bright named stars within 10 degrees of the 0,0 point.

    Formalhaut in Pisces Austrinaus (mag 1) is at approx RA 23hrs DEC -30
    Mirach in Andromeda (mag 2) is at approx RA 1hr Dec +35

    The point roughly half way between them is close to the celestial origin as you call it.

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    Default Re: Closest Bright Star to the Origin of the Celestial Spherical Surface

    I would suggest you download Stellarium (link below) and take a look for yourself. That way you can compare it directly to your celestial globe and get a better understanding of what it represents in the sky. Stellarium can be set for your location so you can see exactly what your sky looks like at any date/time. Enjoy the learning curve.

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    Default Re: Closest Bright Star to the Origin of the Celestial Spherical Surface

    Quote Originally Posted by ozeclipse View Post
    Mike
    Pisces is the constellation. Pisces is a nondescript constellation of faint stars.
    There are no bright named stars within 10 degrees of the 0,0 point.

    Formalhaut in Pisces Austrinaus (mag 1) is at approx RA 23hrs DEC -30
    Mirach in Andromeda (mag 2) is at approx RA 1hr Dec +35

    The point roughly half way between them is close to the celestial origin as you call it.

    Joe
    Many thanks for your reply. I have found the Pisces constellation and the Andromeda constellation on my globe, but the stars in the constellation are not identified by names, but rather by Greek letters (accompanied by ordinary numbers, and colored circles of different sizes. So I'm having trouble locating the particular stars in the constellation that you subscribe. I HAVE seen what MIGHT locate the 0,0 point though: there is a thin yellow line that crosses the great circle on the celestial globe corresponding to the plane of the earth's equator. On that great circle, the longitude is shown, and the point marked zero longitude is where the yellow line crosses. That yellow line is not a great circle: it goes upward to the right, then curves downward, crossing the great circle again (at 180 degrees) on the opposite side of the globe, continues along below the great circle, and then curves upward until it intersects the great circle (from the left) at the zero point again. But the SLOPE of the yellow line at the intersection appears to me to correspond to what I've called the celestial equator. (I.e., if I draw a great circle perpendicular to that yellow line, it will pass through the north and south poles of the celestial globe). I SUSPECT that intersection is the origin of the coordinate system I'm seeking. It seems to roughly correspond to your description relative to the two constellations.
    Last edited by Mike_Fontenot; 02-21-2019 at 10:47 PM.

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    Default Re: Closest Bright Star to the Origin of the Celestial Spherical Surface

    [QUOTE=KT4HX;1058734726]I would suggest you download Stellarium (link below) and take a look for yourself. That way you can compare it directly to your celestial globe and get a better understanding of what it represents in the sky. Stellarium can be set for your location so you can see exactly what your sky looks like at any date/time. Enjoy the learning curve.



    Thanks for that recommendation. I've found the webpage, and I will probably download it if and when I ever figure out the earth-independent coordinates I'm seeking ... I want to be able to do that first, before worrying about how to find constellations in my own night sky on a particular date and at a particular time of night.

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    Default Re: Closest Bright Star to the Origin of the Celestial Spherical Surface

    The yellow line is the line of the ecliptic, which is the path that the Sun takes (and which the planets more or less follow), being above the equator in the (Northern Hemisphere) summer months.
    This is different to the celestial equator, which is midway between our celestial poles. If you're not sure which is which, the Celestial Equator passes through Orion's Belt (with the ecliptic passing above it, through Taurus and Gemini)

    The ecliptic is also a great circle (or at least it should be if it's plotted accurately on your globe).
    As you have identified, where the ecliptic crosses the equator into the Northern Hemisphere is the origin of the Right Ascension system.
    Last edited by Gfamily; 02-21-2019 at 11:48 PM.
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    Default Re: Closest Bright Star to the Origin of the Celestial Spherical Surface

    Formalhaut in Pisces Austrinaus (mag 1) is at approx RA 23hrs DEC -30 is designated Alpha Pisces Austrinaus
    Mirach in Andromeda (mag 2) is at approx RA 1hr Dec +35 is designated Beta Andromedae

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    Default Re: Closest Bright Star to the Origin of the Celestial Spherical Surface

    Quote Originally Posted by ozeclipse View Post
    Formalhaut in Pisces Austrinaus (mag 1) is at approx RA 23hrs DEC -30 is designated Alpha Pisces Austrinaus
    Mirach in Andromeda (mag 2) is at approx RA 1hr Dec +35 is designated Beta Andromedae

    Joe
    YES! I found those two stars on my globe (using the Greek letters you provided). Formalhaut (alpha) is a large circle on my globe, with a blue color. The Mirach (beta) circle is a little smaller (although still fairly large), with a red color. And the great circle connecting those two stars DOES pass very close to the (0,0) point where the yellow line crosses the great circle corresponding to the earth's equatorial plane on the celestial globe. I'm sure now that the yellow line (which I now realize IS a great circle), where it crosses the (0,0) point, IS the celestial equator, and the great circle perpendicular to that yellow great circle, at the point (0,0), IS the celestial longitudinal line corresponding to zero right ascension. Thank you! You've given me EXACTLY the information I needed.
    Last edited by Mike_Fontenot; 02-22-2019 at 04:43 PM.

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    Default Re: Closest Bright Star to the Origin of the Celestial Spherical Surface

    Quote Originally Posted by Gfamily View Post
    [...]
    The ecliptic is also a great circle (or at least it should be if it's plotted accurately on your globe).
    As you have identified, where the ecliptic crosses the equator into the Northern Hemisphere is the origin of the Right Ascension system.
    Thanks. I just looked at the yellow line more closely, and it IS a great circle. I was mistaken when I said it was curved.
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    Default Re: Closest Bright Star to the Origin of the Celestial Spherical Surface

    Mike, the "zero" point has a name. It's called the "First point of Aries" (see: https://en.wikipedia.org/wiki/First_Point_of_Aries ) and, ironically, it isn't located in Aries (but it was long ago)

    It's defined as the point at which the Sun officially crosses the celestial equator at the Vernal (northern-hemisphere spring) equinox. In other words, each year at the Vernal Equinox, the Sun passes precisely through the RA 0h 0' 0" Dec 0° point -- but not because we're so good at astrophysics, but because we redefine the point each year based on the location of the event.

    It was defined by Hypparchus back in 130BCE but the precession of the Earth's poles has caused the point to slowly migrate westward. Today it's in Pisces but is approach Aquarius. It'll remain in Pisces through most of the 26th century. Looks like 2597 will still have the point in Pisces, but in 2598 it finally crosses the boundary into Aquarius.

    Annual precession which moves the First Point of Aries means the celestial coordinates of any object will change each year (never mind any actual "proper motion" of the object itself). It is also why, when using charts or software, you can view objects based on their "J2000" or "JNow" coordinate position. J2000 is used only because that was the most recent update still used by most printed charts and atlases. But since computer software can model the precession shift, they offer the more precise "JNow" option.
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