I just have trouble with believing that we can determine the position of a molecule on a star 20 million light years away, and yet we aren’t exactly sure what is right under our feet.
We can't determine the position of a single molecule on a distant star; what's being discussed in this article is the means by which changes in the amount of energy stored in the various vibrational and rotational modes of countless trillions of identical molecules surrounding the star can be determined.
Depending on their structure, individual molecules can absorb and emit light energy (photons) in the various bonds between the atoms that comprise the molecule.
Think of it like the strings of a guitar, which can be excited to vibrate by sound emitted from a second guitar, assuming both guitars are tuned accurately.
Energy can be stored in "vibrational" modes, in which the various atoms of the molecule move back and forth with respect to one another. Different bonds will in general resonate at different frequencies, although its possible for more than one bond in the same molecule to vibrate at the same frequency, depending on the molecule's degree of symmetry.
It's also possible for the molecule to absorb energy in rotation; that is, the entire molecule rotates end over end. Due to quantum mechanics, a molecule rotating in this way isn't free to rotate at any old speed; it has to rotate at a speed that causes the wave functions of its component atoms to interfere constructively.
Many (if not most) vibrational transitions correspond to frequency in the optical part of the spectrum. This is a big reason why we see so many colors, each of which corresponds to a particular molecular transition, either absorbtive (as is the case with the molecules that give plants their colors, and cause the sky to appear blue) or emissive, as is the case with things that glow by discharge, like mercury vapor lamps, LEDs, etc.
Rotational modes are much less energetic, meaning that they absorb and emit light at much lower frequencies, or longer wavelengths. Most rotational modes are in the microwave spectrum radio spectrum, although some are so far up in the microwave region that they actually qualify as long-wavelength infrared light, and can be focused by large telescope mirrors.
Feet are the windows to the sole.