Among the other reasons listed in the thread, we can take lots and lots and lots of photographs of it from our southern hemisphere, and stitch the photos together into a composite, and compare such composites with the many many many distant galaxies we can see in the sky, many of which are disk like and presented at various degrees of edge-on-ness versus face-on-ness. Although we cannot be totally certain, the Milky Way is consistent with being a barred spiral galaxy, as it looks very much like what you get when you tilt a not-quite-edge-on view of a distant barred spiral galaxy into a completely edge-on view.
Here's one of my favourite composites. Check out the 4000 x 1212 pixel version.
Long exposure times, carefully stabilized equipment, and even moving the equipment outside the atmosphere have been a boon in producing these types of images.
Correct me if I'm wrong, but I asked my astronomer brother in law some time ago:
We can't look at the Milky Way but we can look at similar galaxies. Our current understanding of what our own galaxy looks like is based on measuring the shape and direction of (groups of) stars. Stars contain(ed) Hydrogen and Hydrogen emits a radiation with a specific wavelength, that we can use to detect it. With Doppler Shift we can measure direction, if the emitting atoms are moving away, the wavelength is slightly larger. And they'll be slightly less if they're moving towards us.
Stars orbit the center of our galaxy and if we can get their orbit radius we can calculate how fast they are going. With enough measurements in all directions and using a lot of trigonometry, we can get to a map.
As far as I understand, the Doppler shift can't be used to measure the distances of the stars in the Milky Way, but it is used to estimate the distance of the farthest galaxies, billions of light years away from the Milky Way. My other post here has the links about the parallax, the method that measures the actual distances of the stars around us. And then there's the Cosmic distance ladder:
The starting principle is simple, then the scientists build a lot of clever stuff, the telescopes on Earth, the telescopes on the satellites and the remote controlled
radio telescopes...
"the same math that enables you to hold your thumb at arm’s length, close one eye and then switch eyes and watch your thumb appear to shift, allowed us to measure the distances to the stars."
"Known as parallax, the fact that our planet’s orbit is some 300 million kilometers in diameter around the Sun means that if we view the stars today versus six months from now, we’ll see the closest stars appear to shift position in the sky relative to the other, more distant stars."
"For thousands of years, astronomers from the ancient Babylonians to Tycho Brahe had preoccupied themselves with noting the stars’ precise locations, a crucial foundation to understanding the cosmos. But the field sputtered in the 1960s, when scientists reached the smallest parallaxes that Earth-based telescopes could measure, stymied by interference from our rippling atmosphere.
It wasn’t until the 1980s and 1990s that the ESA satellite Hipparcos took astrometry to space, where it ultimately measured the precise distances of more than 100,000 stars. Gaia is even better: Hipparcos’s gaze reached only as far as 1,600 light-years away, barely leaving our celestial backyard, but Gaia is able to spy on stars up to 30,000 light-years away."
Remote controlled radio telescopes across the Earth, as in the article we comment to:
"The Very Long Baseline Array (VLBA) is an interferometer consisting of 10 identical antennas on transcontinental baselines up to 8000 km (Mauna Kea, Hawaii to St. Croix, Virgin Islands). The VLBA is controlled remotely from the Science Operations Center in Socorro, New Mexico. Each VLBA station consists of a 25 m antenna and an adjacent control building. The received signals are amplified, digitized, and recorded on fast, high capacity recorders. The recorded data are sent from the individual VLBA stations to the correlator in Socorro."
"Precision astrometry is a VLBA science centerpiece. The relative astrometric accuracy of ~ 10 micro-arcseconds achievable with the VLBA is better than what the Gaia satellite is designed to achieve for most stars in its catalog (scheduled for release after 2015"
"In 2010, the VLBA will begin a long-term program to determine the complete 3D structure of the Milky Way by measuring parallaxes with 10 mas accuracy or better to ~ 400 high-mass star-formation regions."
In order to achieve the stated accuracy it is is necessary to use together the radio telescopes which are as distant as possible.
The clearer formulation would probably be: the same effect that we see by closing one then another eye is seen from the Earth when we look at the stars months apart, and the formulas that could be derived using the distance of your eyes and your arm can also be applied to the cosmic distances.
