FIGURE 13. Calculating the distance to Lalande 21185

Figure 13 is a screen grab from the running applet. To try this virtual experiment, select the ATLAS icon to start the applet.

See the list of other virtual experiments.

VIRTUAL EXPERIMENT 5
Calculating the Distance to Lalande 21185 using the Parallax Timer

• Calculate the distance to Lalande 21185 using virtual parallax measurements made over a period of many simulated months.
• Understand how the location of a star in its celestial hemisphere effects the direction of the parallax path.

• Set the selected star to Lalande 21185.
• Set the field of view to 2 arc seconds.
• Set the operation mode to parallax with timer
• Repeatedly click the clock icon in the upper left-hand corner of the display window to denote the passage of virtual months.
• When the clock reads Dec, use two mouse clicks to measure the Lalande 21185 parallax angle. To do this, click the mouse once in the centre of the ellipse, and once at the end of the major axis.
• Use the measured parallax angle and Equation 3 to calculate the distance to Lalande 21185.
• Note the direction of the parallax path that Lalande 21185 appears to sweep out as the months progress. Compare this with the direction for Tau Ceti.
• In the applet data window, note the declination for both Lalande 21885 and Tau Ceti. Use this to explain your observation.

• Lalande 21185 has a declination of 35.9 deg 38.465 m
• Tau Ceti has a declination of -16.0 deg 12.0 m
• Tau Ceti and Lalande 21185 appear to move in different directions. This observation is consistent with the fact that they reside in different celestial hemispheres.

In Figure 13, the Lalande 21185 parallax angle was measured to be 0.39 arc seconds. The distance to Tau Ceti can be calculated using this measurement and Equation 3 according to:

Appendix 4 of Kaufmann and Freedman (1999) reports a known distance to Lalande 21185 of 8.32 ly. In this case, the virtual experiment produced an error of only 0.4%.