This tool can be used at any time during moving target visualisation, but is particularly useful when you want to determine the approximate angular separation of a moving target and a nearby fixed target, or two moving targets on a given date. To activate it, go to the “Overlays” menu on the HSpot main menu bar and select "Distance Tool." Now use the left mouse button to select a point and drag to another point. A coloured line should appear, denoting the distance that you dragged the cursor, and the distance in arcseconds will be displayed above the line (Figure 19.7, “Using the distance tool”).

Figure 19.7.  The Distance Tool in action. The blue arc indicates the distance between the two selected points. The distance, in arcseconds, is displayed on the image. Detailed information can be brought up in the “Distance” dialogue box.

Note that estimating the distance between two moving targets using this tool is only valid between points on the two trajectories that correspond to the same date. We have exact date point matching for two moving bodies in HSpot. With this hand drawn tool, it is possible to estimate separation distances to an accuracy of a fraction of an arcsecond, when compared with the calculated ephemeris positions. However, if your observation crucially depends on separation estimates to higher accuracy, we strongly recommend that you calculate these separations using JPL's Horizons software. For instructions on how to use Horizons to assist with Herschel observation planning, please see the Herschel website at http://www.rssd.esa.int/Herschel/.