Catalogs, Ephemerides and Finding Charts

There are many, many different ways that astronomers refer to objects in the sky. You might read any of the following sentences in a technical paper:

• "We measured the parallax of asteroid 1998WT on March 4, 2005."
• "The Rosetta spacecraft will visit Comet 67P/Churyumov-Gerasimenko."
• "Photometry of HD 209458 shows a transit of depth 0.015 magnitudes."
• "We monitored the violent variable star 1RXS J170849.0-400910 for five hours on UT 2004 June 5."
• "Dave Roberts reports a new supernova in NGC 4588."

What do all these abbreviations mean? Where can you go to look up the positions and other properties of these sources? Back in the old days, we had to know the meaning of all the acroynms, so that we could walk to the proper shelf in the library and pull the correct catalog off the shelf. These days, however, there are several web sites that make it much easier.

• Ephemerides for solar system objects
• Making finding charts and accessing catalogs with Aladin

Ephemerides for solar system objects

Locating objects in the solar system -- planets, asteroids, comets -- can be tricky. Although we can predict the motions of the major planets and asteroids accurately, orbits of the smaller bodies may change significantly over a small number of years: they may be perturbed by the gravitational forces of other bodies, or additional observations may improve their orbital elements. Comets are subject to large non-gravitational forces as surface material sublimates and shoots off their surfaces, sometimes in jets:

Moreover, new asteroids and comets are being discovered all the time; any planetarium program will have an old list which doesn't contain the latest discoveries.

I recommend using the JPL Solar System Dynamics web site as a good starting point for all things in the solar system. The tool I use the most is

• JPL Horizons Ephemeris Generator

When you connect to the site, you'll see a page with several choices:

You should click on each one in turn to prepare to generate a list of positions for the object of interest.

• Target Body Select the comet or asteroid of interest; the easiest way is to give its numerical designation.
• Observer Location You can type the name of a location on Earth, or, for observatories which are registered with the Minor Planet Center, the number. The RIT Observatory is number 920.
• Time Span Choose the date(s) and time(s) over which you want the positions. Note that the default time system is Universal Time (UT), not Eastern time. I usually set items 1 (Time Span) and item 2 (Output Interval), leaving 3 and 4 alone. A reasonable Output Interval is anywhere between 1 and 4 hours, or 1 to 4 days if you are interested in a very long time interval.
• Table Settings I usually don't click on this button, since the defaults include everything I need to know. However, some people might want to use this choice to select the options:
  • Suppress output during daylight
  • Suppress output when body below horizon.
• Display/Output The default (HTML) is usually the best choice, but if you wanted to parse the output with a computer, or save it to disk, you could choose a different option.

At this point, I submit the form. The JPL site returns an "ephemeris", which is simply a list of positions (and other properties) at a set of times. The default output runs across many columns; the important quantities are

• Date and Time
• RA and Dec. The default provides two versions, the second of which (a-apparent) takes into account topocentric corrections and atmospheric effects. For almost all purposes, they are identical.
• Elevation, in degrees above the horizon.
• APmag, which is an estimate of the magnitude in B or V; don't expect it to be accurate to better than 0.3 mag for most bodies

Curious people may find a few additional columns interesting.

• r is the distance of the object from the Sun, in Astronomical Units (AU)
• delta is the distance of the object from the Earth, again in AU

Once you have a list of dates, times and positions, you can use a planetarium program to figure out where to look ... or you can use one of the several on-line tools to make finding charts.

Exercise:

1. Look up the (RA, Dec) position of asteroid 1998WT for the date March 4, 2005. Use Yerkes Observatory as the observer's location. Write down the asteroid's position at UT 02:45:00 and 02:50:00.

The answers

Here's a picture of the asteroid 1998WT at UT March 4, 2005, 02:45:00, as seen from Yerkes. Can you identify the asteroid?

So now we know the position of the asteroid -- but what about the stars around it?

Making finding charts with Aladin

There are two big players in the on-line astronomical database game:

• SIMBAD
• NED (the NASA Extragalactic Database)

They both are great resources, with slightly different points of view: SIMBAD concentrates on stellar data, whereas NED specializes in information on extragalactic objects. There is quite a bit of overlap between their databases and their capabilities. I'll discuss SIMBAD here, since we can't do much extragalactic work at RIT.

If you just want some information on a star, you can go to the SIMBAD "basic search" home page and enter a query by identifier or coordinates. For example, a search on 61 Cygni returns a page with a bunch of basic information:

If that's not enough, further down on the returned page are

• other names for this object
• a tool to make plots and images around the object
• links to references to the object in journal articles
• links to additional measurements of the object

If you want to make a finding chart for an object, I recommend the

• Aladin tool within SIMBAD

It will create charts which are well-matched to the scale of the RIT Observatory's CCD camera, and provide a wealth of additional information. For example, let's consider the asteroid 1998 WT.

1. Start Aladin, and wait for the applet to load in your browser.
2. Choose File -> Load Astronomical Image -> Aladin Image Server
3. Type the coordinates of 1998WT you found above into the Target box, in a format like this:
   

   then press Submit

4. You will be given a list of images -- pick the "POSS I E-DSS1" item with field 14.2 x 14.2 arcminutes. This choice creates a chart which is roughly the same size as a typical CCD image taken by a telescope. Again, press Submit. If you see a little window pop up with information about this image, press the LOAD button on it.
5. Now wait for a copy of the sky survey image to be transferred to your computer -- the little green light on the right-hand side of the main window will blink if the transfer is active
   

6. At the center of the image, the crosshairs will indicate the position of the object you provided. If you move your cursor around in the image, the current coordinates at any moment will be displayed in the box just above the image.
7. If you choose the "Dist" item on the right-hand edge of the image, then left-click and drag, you can measure the distance and position angle between any locations in the image.
8. Now, choose again File -> Load Astronomical Image -> Aladin Image Server
9. Choose the Surveys item on the right-hand side.
10. You will be presented with a list of catalogs. The seventh one listed, "USNO-A2", is often a good starting point. Select it and press Submit
11. Eventually, a colored overlay should appear over the image. If you move the cursor over a colored item, a message appears below the bottom of the image. Click the mouse and a line(s) will appear in the small grey box below the image:
    

12. This gives (for the USNO2 catalog)
    • catalog entry name
    • RA, in decimal degrees
    • Dec, in decimal degrees
    • B-band magnitude
    • R-band magnitude
    • epoch of the photographic plate from which measurements taken
13. If you click on the Vizier column for an object, a new window pops up with the full description of this object in the catalog.

    

Exercise:

2. What are the (RA, Dec) coordinates of star A in the Yerkes image below? Can you provide them both in decimal degrees, and in HH:MM:SS.s format?
3. What is the distance between star A and the asteroid? Express this distance in arcseconds.
4. The brightness of stars (as we will see later) is often expressed as a "magnitude". What is the R-band magnitude of star A?
5. In the Aladin image, which way is North? (Hint: Declination increases as one goes to the North)
6. In the Aladin image, which way is East? (Hint: Right Ascension increases as one goes to the East)

The answers

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.