Ephemerides and Finding Charts

• Ephemerides for solar system objects
• Making finding charts 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

   
           http://ssd.jpl.nasa.gov/cgi-bin/eph 
     

When you connect to the site, you'll see a page with four buttons:

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

1. Target Body Select the comet or asteroid of interest; the easiest way is to give its numerical designation.
2. 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.
3. 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.
4. Output Quantities I usually don't click on this button, since the defaults include everything I need to know.

   Item 2 on the main page, Select Desired Options, contains two choices I usually select:

   • Suppress output during daylight
   • Suppress output when body below horizon.

   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.

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   Making finding charts with Aladin

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

   • SIMBAD

           http://simbad.u-strasbg.fr/cgi-bin/WSimbad.pl   (true home site in France)
           http://simbad.harvard.edu/cgi-bin/WSimbad.pl    (US mirror at Harvard)
       

   • NED (the NASA Extragalactic Database)

           http://nedwww.ipac.caltech.edu/ 
       

   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 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 (which can also be accessed from SIMBAD home page)

           http://vizier.hia.nrc.ca/viz-bin/nph-aladin.pl?-rm=14.1&-server=Aladin 
       

   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 star WZ Sge, an interacting binary which very rarely increases in brightness by over a factor of 100.

   1. Start Aladin, and wait for the applet to load in your browser.
   2. Choose Load
   3. Type "WZ Sge" into the Target box, and press Submit
   4. You will be given a list of images -- pick the "DSS1/STSci E (Red)" item. The "DSS2" items with fields of 12.9x12.9 arcminutes are also good choices. Again, press Submit
   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 catalogued position of the object. If you move your cursor around in the image, its 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, click again on the Load button.
   9. Choose the Catalogs in Vizier item.
   10. You will be presented with a list of catalogs. The first one listed, "USNO2", 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 catalog name, a new window pops up with the full description of this object in the catalog.

       

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   Exercises:

   1. Look up the position of the following object for last night (Sep 16, 2003 EST = Sep 17, 2003 UT). Be sure to request position each hour.
      • asteroid Renate
   2. Make a finding chart for your object. Mark by hand the position of the object at 9:00 PM, 11:00 PM, and 1:00 AM local time.
   3. Find two stars in the field which have approximately the same magnitude as your object. Mark them by hand on your chart.
   4. The RIT 12-inch telescope and CCD camera (without focal reducer) yield a field of view about 16 by 12 arcminutes. How does your chart compare to this field?
   5. Compare your chart to the image of Renate taken last night by Professor Davis. Can you figure out the orientation of the image? Can you find the asteroid Renate? Perhaps it might help to blink.

      

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