The parallax to asteroid 1998WT

The Small Telescope Parallax Group (who are they?)
Mar 4, 2005
Mar 6, 2005
Mar 8, 2005

In 1998, astronomers discovered an asteroid which is not particularly interesting. It's not very large, or very bright. But it does have an orbit that's slightly unusual: instead of lying between the orbits of Mars and Jupiter, this particular asteroid -- called 1998WT -- flies in closer than Venus and out farther than Mars.

At times, the asteroid comes relatively close to the Earth. Don't worry, it won't hit the Earth any time soon. But it does approach closer than most asteroids ... as it did in early March 2005. Watch it move over several days.

We took advantage of this opportunity to try to measure the distance to this asteroid by a technique called parallax, which involves simultaneous observations from different locations on Earth.


Larry Marschall and Christy Zuidema at Gettysburg College took a series of exposures with a relatively large field of view. If you click on the image below, you will see a short "movie" showing a set of 4 images taken at five-minute intervals. (The "movie" will loop twice.) Can you find the asteroid?

Vivian Hoette at Yerkes Observatory acquired a sequence of images over the same period. Her telescope and camera together have a smaller field of view.

With fewer stars to check, it should be easier to find the asteroid. Once again, click on the image to see a short movie.

The field

These images show stars near the position (RA, Dec) = (07:26:43, +00:46:40). Here's a small finding chart created using the Aladin tool which shows a closeup (about 14 arcminutes on a side) of the stars in this area.

Let's label a few stars so that we can use them as references for position and brightness in our analysis.

The UCAC2 catalog and provides the following positions and magnitudes for these stars:

star              RA                              Dec             UCAC2 mag
       HH:MM:SS.sss    degrees   degrees
 A     07:26:44.867  111.6869450      +00:45:56.52  0.7657003      10.5
 B     07:26:35.094  111.6462248      +00:43:42.61  0.7285028      11.3

 C     07:26:28.154  111.6173065      +00:44:41.63  0.7448975      13.6
 D     07:26:37.618  111.6567406      +00:46:27.28  0.7742450      13.1

 P     07:27:04.918  111.7704927      +00:42:32.06  0.7089064      10.2
 Q     07:26:50.352  111.7097995      +00:51:24.35  0.8567634       9.7

Can you use these positions to figure out the distance -- in degrees, and in arcseconds -- between particular stars? Try to calculate

If you can measure the positions of the same stars on images from Gettysburg, in units of pixels, then you can figure out how many arcseconds correspond to one Gettysburg pixel. We call that the plate scale of the image. Can you figure out the plate scale for both the Gettysburg and Yerkes images?

If you are very ambitious, you can do something similar to measure the angle by which the Gettysburg images (and Yerkes images) may need to be rotated to bring them into perfect North-South-East-West alignment.

Grab the images

Here are copies of selected images from each site, in FITS format. I have translated the Gettysburg images from 32-bit floating point to 16-bit integer format.

For more information

The Small Telescope Parallax Group is a very loose collection of astronomers with small telescopes who enjoy occasional parallax campaigns; not to improve the orbits of asteroids, but just because it's fun to do.

Other parallax resources: