Alyssa Goodman edited introduction_1.tex  almost 11 years ago

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Determining the structure of the Milky Way, from our vantage point within, is a perpetual challenge for astronomers. We know the Galaxy has spiral arms, but it remains unclear exactly how many, cf. \citep{Vallee2008a}. Recent observations of maser proper motions give unprecedented accuracy in determining the three-dimensional position of the Galaxy's center and rotation speed \citep{Reid2009,Brunthaler2011}. But, to date, we still do not have a definitive picture of the Milky Way's three dimensional structure.  The analysis offered in this paper suggests that some infrared dark clouds\footnote Infrared Dark Clouds\footnote  {The term ``Infrared Dark Cloud" or ``IRDC" typically refers to any cloud which is opaque in the mid-infrared.}--in particular very long, very dark, clouds--appear to delineate the major features of our Galaxy as would be seen from outside of it. In particular, we study a $>3^{\circ}$-long cloud associated with the IRDC called ``Nessie" \citep{Jackson2010} \citep{Jackson2010},  and we show that it appears to lie parallel to, and no more than just few pc from, the true Galactic Plane. Our analysis uses diverse data sets, but it hinges on combining those data sets with a modern understanding of the meaning of Galactic coordinates. When, in 1959, the IAU established the current system of Galactic $(l,b)$ coordinates \citep{Blaauw1959}, the positions of the Sun with respect to the ``true" Galactic disk, and of the Galactic Center, were not as well determined as they are now. As a result, the Galactic Plane is typically \textbf{not} at $b=0$, as  projected onto the sky. The exact offset from $b=0$ depends on distance, as we explain in \S \ref{lookingdown}. Taking these offsets into account, one can