Expert: Courtney Seligman - 3/13/2011

Question
Hi!,
Read a lot of information on the recent earthquakes and shift of earths axis etc. I wonder if these shifts will effect the computation of longitudes of planets, since we consider the Obliquity & eccentricity. Can throw some light on this?
Thanks

Answer
It will have no effect on planetary longitudes. The orbital motions are calculated in comparison with our orbit, not our rotation, and that can't be affected by any event occurring on the Earth itself. So the planetary longitudes (and latitudes) will remain exactly the same as before.

Now, if you had asked about their right ascensions and declinations, which are measured relative to the axis of the Earth's rotation (and which would include the obliquity of the Ecliptic in their calculation), the answer would be very slightly different. But the obliquity would be affected (even by the unusually large effects of the recent earthquake) by considerably less than a hundredth of a second of arc, which is beyond the accuracy of any calculations I've ever seen. It is also beyond the measurement capability of any object on the surface of the Earth, because even things such as atmospheric refraction, which changes the apparent position of things in the sky, vary by more than a hundredth of a second of arc during the course of a night.

Of course, orbiting spacecraft sometimes measure positions accurately enough to notice such small effects; but since they are not on the Earth, but out in space, they are not affected by any change in the rotation of the Earth itself.

So I can't think of any theoretical or practical calculation which would be in any way affected by the earthquake; and although it is interesting to note that there were (presumably) relatively significant changes in our rotation as a result of the crustal displacement, they just aren't big enough to make any practical difference.

Given your interest in the topic, however, you might want to look up the Chandler wobble, which regularly changes the position of the Pole by about 30 or 40 feet, over a period of about two years. That does produce observable effects, which observatories fixed on the surface of the Earth have to take into account if they want to produce extraordinarily accurate astronomical measurements.