The SETI League, Inc., a membership-supported, non-profit {501(c)(3)}, educational and scientific organization Searching for Extra-Terrestrial Intelligence

Ask Dr. SETI ®

Chapter 1: Astrophysics

Pointing at a Moving Target

Dear Dr. SETI:
One thing that's never mentioned in the SETI literature: when you point a dish at a star. it's not fixed in space, but rather moves in time. Stars targeted for SETI, looking for a signal, will have moved in the 30, 100, 500 years for any signal to arrive here. Hence, when you look for techno signatures, do you point the antenna where the target star X light years from us, actually was X years ago, or toward were the star appears to be today?
A related question becomes, is it impossible to create a long distance communications link by radio waves, because all stars move in time? Is it futile to look for a signal from ET by pointing the receiver at the position the star is today? Do you need to point the dish at the location where the star was decades, hundreds or thousands of years ago, depending on the target?
Erich, Germany

The Doctor Responds:
The short answer, Erich, is that we point where the star appears to be today. The reason is that's where the star was, when the signal was transmitted. Yes, we do indeed see starlight coming from a different point than where the star is now. But, if both the SETI signal and the starlight move at the same speed, and came from the same location, if we can see the star "over there," that's where we'll also find the signal. After all, the starlight seen now, and the SETI signal received now, both originated in the same place, both traveled the same distance to reach us, and both were moving at exactly the same speed.

But, since we don't know exactly how far away the star is, we can't know precisely where it is today. (Not that it matters!) We are, after all, just searching for existence proof of other technological civilizations. Once we receive something, if we decide to respond, we have a different problem to solve - where to aim our transmit antenna. Clearly, we don't want to send the signal to where the star appears to be in our sky today. Neither do we want to send it to where the star actually was when their transmission left their planet. Rather, we want to aim our response to where the star (assumed to be X light years away, and still moving) will be X years from when we transmit our response. So, in the context of two-way communications, you are entirely right - it gets complicated.

Getting back to SETI detections, this whole discussion underscores the importance of conducting not just targeted searches of known stars, but also complementary all-sky surveys that point in no particular direction (or, even better, in all possible directions). After all, the famous Ohio State University "Wow!" signal did not emanate from any particular star known to us, and was detected by a drift-scan survey covering the entire sky that can be seen from Ohio.

Click to email the Webmaster
| Home | General | Memb Svcs | Publications | Press | Technical | Internet | Index |
entire website copyright © The SETI League, Inc.; Maintained by Microcomm
this page last updated 25 September 2021
Click for top of page
Top of Page