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    Re: Orion North Arrow
    From: Frank Reed
    Date: 2022 Jan 13, 11:17 -0800

    Davis Pike, you wrote:
    "Achieving north or south on the horizon from Orion’s sword is very latitude dependent. On the Equator, it’s tail end always points to south on the horizon. The farther north you go, the more it only works when the sword is close to vertical, so it doesn’t work well all night. "

    No, that is not so --if you do it properly! You have probably not tried the Orion North Arrow under the real sky. You have convinced yourself in your armchair that it's impossible so why bother stepping outside to try it? Ah, well, the reason you must experiment outside is because three dimensions is different from the two-dimensional flatland of your computer screen. Even the 3-dim illusion of a planetarium dome is not up to the task. Give it a go. You'll see that it works beautifully.

    In the past I have described the Orion North Arrow using one's own hand as a pointer. I think this confuses some people because it's too damn easy. So I'll make it slightly harder by bringing a pencil or pen into the game (I'll call it a pencil in what follows).

    We are accustomed to seeing the constellation Orion as four stars (Betelgeuse, Bellatrix, Rigel, and Saiph) in a big box representing the body of Orion the Hunter. Smack in the middle of that box are the three stars that are known as Orion's Belt, and also known in many countries with Latin or Roman Catholic histories as the Three Marias. Hanging below the belt is a distinct stripe of luminosity consisting of a number of stars and also the famous Orion nebula which can be seen as a cloudy patch with minimal optical aid, like even a pair of binoculars. Of note, the star Mintaka which is the star among the three belt stars closest to Aldebaran and furthest from Sirius lies just half a degree below the celestial equator, which means it passes through the zenith for observers standing just south of the Earth's equator. A line drawn through the belt points very nicely toward brilliant Sirius on one side and roughly toward Aldebaran and the Pleiades on the other side, helping to identify those important stars and that famous star cluster.

    In traditional maritime culture in the Caribbean and some other parts of the world, Orion is viewed differently as a navigation tool: the Orion North Arrow. Ignore the four bright stars of the box. Focus on the belt and sword, and add one other moderately bright star to the picture: eta Orionis. This is a third magnitude star located 3° from Mintaka generally in the direction of Rigel. A line drawn from Mintaka to eta Orionis is almost exactly the same length as Orion's belt and almost exactly perpendicular to it. The belt plus this additional star, eta Orionis, were apparently formerly sometimes known as the "yard and ell". For us the three stars of the belt plus eta Orionis form a nice, square arrow-head, and what we saw formerly as the "sword" of Orion now represents the shaft of the arrow connected to the arrow-head at Mintaka. See the images in my post from four years ago.

    The Orion North Arrow is visible everywhere on the globe, and it's in the sky for at least a portion of the night for roughly nine months out of the year. With the exception of eta Orionis, it is also visible even under light-polluted skies or under the light of the Full Moon. To use the arrow, grab a pencil. Hold your arm out pointing right at the Orion North Arrow and hold the pencil in your fist so that it is perpendicular to your arm. The point of the pencil should sit right on Mintaka and the other end should align along the shaft of the arrow, with the eraser end of the pencil at the Orion nebula, if that helps to picture it. 

    A pencil or other pointer aligned with the Orion North Arrow is parallel to the Earth's axis. This means that the pencil points rather closely towards the north celestial pole. Its direction, projected on the ground, is the direction of true north. Its angular tilt, relative to the horizontal plane, is approximately equal to your latitude. If the pencil points below the horizontal plane, that's a negative angle implying a negative latitude. In other words, you're in South latitude.

    This time of year is an easy time to test out the Orion North Arrow since Orion is in the sky in convenient evening hours. A little trick if you want to make it more accurate: after spotting it quick visually for a rough orientation, you can improve things by slightly rotating your "pencil" about 7° counter-clockwise so that the shaft runs from the Orion nebula to Alnilam, the middle star of the belt, instead of Mintaka. The alignment is now correct within just a couple of degrees. The pencil points north. Its tilt is your latitude.

    Frank Reed

    PS: Pretty cool, right? In fact British astronaut Tim Peake says that when he was a kid he thought it the "coolest thing evah" which I mentioned in a post about a year ago. Listen to him describe it briefly for a recent BBC talk show:


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