Why are actual times so far off the posted times?

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jrkirkham United States of America
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Why are actual times so far off the posted times?

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Post by jrkirkham »


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. :sci-fi-grayalien:
Rob
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Re: Why are actual times so far off the posted times?

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Post by notFritzArgelander »


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.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: Why are actual times so far off the posted times?

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Post by jrkirkham »


:text-thankyouyellow:

THANK YOU! That is exactly the article I needed to see.
Rob
Telescopes: 50mm refractor, ED80 triplet, 90mm makcass, 10" dob, 8"SCT, 11"SCT
Mounts: Celestron CGX, Orion Sirius + several camera tripods
Cameras: Canon 6D, Canon 80D, ZWO-ASI120MC
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Observatory: SkyShed POD XL3 + 8x12 warm room
AL Projects Completed: Lunar #645, Outreach #0280, Universe Sampler #93-T, Binocular Messier #871, Messier #2521, Messier Honorary #2521, Constellation Hunter Northern Skies #112, Planetary Transit Venus #1, Galileo #26, Outreach Stellar 0280, Meteor Regular #157, Solar System Telescopic #209-I, Observer Award #1
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KathyNS Canada
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Re: Why are actual times so far off the posted times?

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Post by KathyNS »


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.
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Re: Why are actual times so far off the posted times?

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Post by notFritzArgelander »


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.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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