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Getting An Ephemeris:

An Ephemeris (ee-FEM-ur-is) for an object in the sky is a listing of celestial coordinates pinpointing the object's location at a particular time as it moves in the sky. The first step in finding out where a planet will be at any given time is to find its ephemeris. At the NASA Jet Propulsion Lab (JPL) web site, you can use an Ephemeris generator program. But before you go there, you should write down the information you will need to be able to provide:

  1. Which planet you want to locate.
  2. From where on Earth you want to be observing. The program has a data base with the latitudes and longitudes of many cities and observatories around the world. Pick your own city or one not far from yours.
  3. In what time period you would like to observe. This includes: 

    a) what month/days you want to observe. 

    b) the interval between coordinate listings (days, hours, or minutes) on those days. You will use the listing to plot the path of the planet across the sky.

For planets that move fast (like Venus or Mars) you probably want a shorter interval like every week or even every day, but for the slower moving planets (Jupiter or Saturn) you may want to set an interval of more than 1 week or even each month.

Once you are on the Ephemeris Generator page, there are many features you can request the Ephemeris Generator to list using the "Select New Output Quantities" button. But for now, keep it simple--request only:

  • #2 Apparent RA and Dec. (Right Ascension and Declination) and possibly
  • # 29 Constellation ID

You may want to go to the bottom of the form and click the "Reset Form" button first. Note the box at the bottom of the page that gives the output format for the RA and Dec., then click "Use Selected Settings"

Once you get the ephemeris, copy it and store it in a file on your computer, or print it out on paper. Then go to the next step:

Plot Planet's Path

Find That Planet!

Celestial Coordinates


Mail questions or comments: 
updated October 9, 2001
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