by Sarah Scoles

Extend your arm and hold up one finger. Now close one eye and open the other one. Now open that eye and close the other one. You have either just completed the world's most awkward wink or demonstrated the concept of parallax.

Why is parallax important and/or cool?

It can make you famous, and also help you help us make sense of our place in the universe. Parallax just earned Dr. Mark Reid of the Harvard-Smithsonian Center for Astrophysics the Jansky Prize, given once a year to a person who has made significant contributions to the advancement of Even the President winks. radio astronomy. Using parallax, Reid has changed our perception of the galaxy, placing us in a Milky Way 50% larger and rotating 161,000 km/hr faster than we thought. And he has done so, as well as produced many other results highlighted below, by measuring parallax.

Let's go back to your finger.

Your finger's position relative to whatever (don't want to know) was behind it in your living room changed depending on which eye was open. That's because your eyes are on different sides of your head, so they have slightly different views. That makes a big difference for your finger's apparent position, but less of a difference for objects behind your finger (farther away). If you were doing this in view a bookshelf in front of a window, the trees outside would have shifted even less than the books on your shelves, which would have shifted less than your finger. But from all of those shifts, you can calculate how far away the trees (and the books and your finger) are, as long as you know how far apart your eyes are.

This concept doesn't just have to apply to you and your living room. It can apply to distant astronomical objects. And it should be applied! Determining distances in space is hard. After all, we can't fly a spaceship there at a given acceleration and use Physics 101 equations to calculate out how far we traveled in the elapsed time. That would take way too long and people would die.

For nearby objects, you can get parallax by taking measurements at different places on the Earth. For more distant objects, you have to wait for a the Earth to go around the Sun so that it itself has a different viewpoint (source. AstroBob).

For a rundown on the ways that astronomers find distances, see this post about the cosmic distance This guy would have preferred to just measure X (source: iamhilarious.com). ladder. The problem with this ladder is that a lot of the rungs rest on assumptions and/or require extrapolation. They are not direct measurements. They are calculations. Which is okay; we like calculations. But each step or assumption introduces more error and makes the distance less certain.

So what great things did Dr. Reid and his collaborators do with parallax?

Reid likes to use the Very Long Baseline Array to measure parallaxes. This is a set of 10 telescopes spread across 5351 miles of the Earth. When used together, they simulate a telescope with a diameter of 5351 miles (aka "big"). The bigger the telescope, the better its resolution. A telescope this big can see tiny structures...or, in the case of parallax, tiny changes in an object's apparent position over the course of the year.

By the way, doesn't the VLBA sound awesome? Doesn't it sound like there's no other telescope that could do this work? Why, then, would the National Science Foundation want to stop funding this telescope? Consider writing your representatives to tell them that parallaxes are a priority (Source: NRAO).

Using the VLBA, Reid and collaborators have

- Determined distances to masers (like lasers but for longer wavelengths of light that you can't see) in other galaxies (more than 100 million light-years away). With these distances, and information about how fast the galaxies are moving away from us, Reid and collaborators made the first direct measurement of how fast the universe is expanding.

M. J. Reid, J. A. Braatz, J. J. Condon, K. Y. Lo, C. Y. Kuo, C. M. V. Impellizzeri, & C. Henkel (2012). The Megamaser Cosmology Project: IV. A Direct Measurement of the Hubble Constant from UGC 3789 ApJ arXiv: 1207.7292v1

- Shown that Cygnus X-1 really is a black hole like everybody thought, by showing that it is far enough away that it has to be massive enough to be a black hole. I wrote about this a while ago here, and all three associated papers are linked from that post!

- Observed the motions of and distances to Milky Way masers, which are located where stars are being formed, which is also where spiral arms are. These measurements led to the realization that the Milky Way is faster and more massive than we'd thought, and showed where the Solar System (precisely) is in all that mess.

Reid, M., Menten, K., Zheng, X., Brunthaler, A., Moscadelli, L., Xu, Y., Zhang, B., Sato, M., Honma, M., Hirota, T., Hachisuka, K., Choi, Y., Moellenbrock, G., & Bartkiewicz, A. (2009). TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. VI. GALACTIC STRUCTURE, FUNDAMENTAL PARAMETERS, AND NONCIRCULAR MOTIONS The Astrophysical Journal, 700 (1), 137-148 DOI: 10.1088/0004-637X/700/1/137

Distances not only put us in our place, but also tell us what that place is. And we are small. But bigger than we thought. But stuff is far away. And lots of it is getting farther away all the time. And it's time we knew all about that. Exactly. Or close as we can get. Wink