TheSkySearchers

Recently I have been watching the orbit of Jupiter's moons. I've been timing ingress and egress of transits and occultations and noticed something. My times are slightly off from the posted times. I usually chock it up to poor eyesight, poor telescope resolution, or the difficulty of seeing exactly when something starts or stops. But last night I was off by nine minutes. Before I started I checked my clock with the atomic clock in Colorado. I was watching Ganymede go into eclipse. Sky & Telescope's interactive tools listed eclipse at 12:48 AM. I lost sight of Ganymede at 12:57 AM. Seeing was exceptional last night. Is it possible that it took nine minutes from the start of eclipse to full eclipse? Or is there another explanation?

BTW, I was using a C11 at 280X.

http://www.alpo-astronomy.org/jupiter/GaliInstr.pdf

Don’t expect your observed time to agree with the predicted time; you are timing the beginning or the ending of an event, while the prediction is for the middle. Thus, Io and Europa typically reappear about 1-2 min- utes “early” and disappear the same amount “late.” Ganymede’s time difference is more like 4-8 min- utes, and can be up to about 15 minutes for Callisto.

THANK YOU! That is exactly the article I needed to see.

If you are using software like Stellarium, make sure it is set to compensate for light travel time. By default, it shows events around the planets as they happen, not as we see them happen, which can be up to an hour later for Jupiter.

KathyNS wrote: ↑Fri Jul 29, 2022 10:06 pm If you are using software like Stellarium, make sure it is set to compensate for light travel time. By default, it shows events around the planets as they happen, not as we see them happen, which can be up to an hour later for Jupiter.

Yes, that too. That’s how the finite speed of light was discovered.