Publications of the Astronomical Society of the Pacific, Vol. 111, No. 763 (September 1999), pp. 1163-1168 (6 pages) ABSTRACT Trigonometric parallaxes of three planetary nebulae (A21, A29, and He 2‐11 ...

Try this experiment: extend your thumb at arm’s length and close one eye at a time. Your thumb will seem to ‘jump’ between two positions as you switch the eye that is closed. That jump is known as ...