Generating an Ephemeris

Terry N. Trees shows us how to create an ephemeris, a table showing a celestial object's calculated positions during a period of time, using JPL HORIZONS data.

Observing minor moons requires accurate position predictions, and it’s possible that your preferred planetarium software doesn’t include enough data for some of these more challenging objects. As Sky & Telescope author Terry Trees explains in our February 2015 issue, you might want to generate an ephemeris (a table of the coordinates of a given object at a given time) for your target object to supplement or double-check your software. This example shows how to use the JPL HORIZONS Web-Interface to generate an ephemeris for Himalia (Jupiter VI).

The default screen is shown in Figure 1. Each field under Current Settings is customizable, and you will probably need to adjust all but the first and last fields. (In other words, “Ephemeris Type” and “Display/Output” can be left as they are.)

Fig. 1. Default entry screen for JPL HORIZONS Web-Interface.

Figure 1. Default entry screen for JPL HORIZONS Web-Interface.

To customize the settings for Himalia, click [change] in the “Target Body” line. See Figure 2 for an example of the typical “Target Body” screen.

Figure 2. Target body field in NASA JPL Horizons Web-Interface.

Figure 2. Target body field in JPL Horizons Web-Interface.

Type “Himalia” in the Lookup the specified body: box and click the Search button. The Target Body field will be populated automatically by Himalia (JVI), as shown in Figure 3.

Figure 3. Populated Target Body screen for NASA JPL HORIZONS Web-Interface.

Figure 3. Populated Target Body screen for JPL HORIZONS Web-Interface.

You may also choose to enter an accurate observing site longitude and latitude in the Observer Location setting, or set the starting time/stopping time/frequency of predicted position outputs in the Time Span field. For observing faint moons, you should also select “1”, “2”, and “9” as Table Settings outputs (both astronomic and apparent Right Ascension and Declination and also visual magnitude).

Click the Generate Ephemeris button and the results appear almost instantly (Figure 4).

Figure 4. Generated ephemeris for Himalia (Jupiter VI).

Figure 4. Generated ephemeris for Himalia (Jupiter VI).

NOTE: If you are entering ephemeris data into a planetarium program you need to know if it requires astronomic (J2000) or apparent (JNow) coordinates. JNow coordinates are those adjusted to the actual date and time of your observing session. Note that since they are so distant, the difference produced by these two coordinate systems for deep sky objects is insignificant. However, solar system moons are relatively close to Earth and are orbiting around orbiting planets. Using the wrong coordinates with them will produce inaccurate results. Your software’s support team can provide information about which coordinate system you should use.

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