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"Nothing in the universe can travel at the speed of light, they say, forgetful of the shadow's speed." -Howard Nemerov I know many of you are still mad at the night sky because of the full Moon preventing you from seeing the recent, close-by supernova in the Pinwheel Galaxy, at least until the end of the week. ....
By Patrick Dennis (not verified) on 27 Jan 2015 #permalink @Patrick Dennis #1: Parallax? I guess you mean effective parallax for observers on opposite sides of the Earth; clearly single-observer parallax is infinitesimal. At opposition, Jupiter is a minimum of 4 AU (6e+8 km) from Earth, which is where any parallax would be maximized. Using a baseline of 12,800 km, the parallax is 0.213 mrad or 0.73 arcminutes. At opposition, Jupiter s angular diameter is 50 arcseconds, or 0.83 arcminutes, comparable to the parallax. What is more significant is the speed of light! The difference in distance to Jupiter between opposition (4 AU) and conjunction (12 AU) is 1.2e+9 km. That corresponds to a light travel-time of about 4000 seconds, or more than an hour! If you used ephmerides derived at opposition, you could (near conjunction) be off by an hour in your calculation of time, or as much as 1000 km in distance! ....