How so? At any point in time, we can define its position or its distance from any other object, including moving objects.Ron-Astro Pharmacist wrote:I'll butt in. A moving object is little hard to define at a point in time.
Of course, in today's caption, the distance is being compared at a specific moment, when the comet passes nearest to Mars. That distance is given as 139,500 km, from which we can unambiguously infer that the closest known comet flyby of Earth was about 1.4 million km (which would be C/1491 B1 in 1491; the closest in modern times was C/1983 H1 in 1983 at 4.7 million km, or 34 times farther than C/2013 A1 will pass Mars).