Why did I miss the occultation?

[Socco]

Hi there

Tonight I setup to see Io get occulted by Jupiter in my 5" f6. Astronomy 2009 said it would occur at 21:01.

Just after dark I could see Io close-ish to Jupe and I went away and did family things, and came back about 10 minutes before the gazetted time. After lining up and looking it became clear that Io had already disappeared at around 20:54.

Can someone please help to explain why it disappeared earlier than forecast?

Now I'm just outside Brisbane at 27.1962 S, 152.8243 E and we use EST up here so daylight saving wasn't the reason I missed it. The yearbook does go into how to adjust rise/set times on p135 but how do I use these if Jupiter is neither rising nor setting? ... ... ...

...

[thinking, calculating]

...

NEW QUESTION:
Are my calcs correct? My 1st attempt at deciphering p135 follows:

Step 1
Dayboro (home) is 1.57 deg east of Sydney = 6.28 minutes earlier. hmm that's most of my measured time delta already.

Step 2
Jupe is at dec -16 today (p116) so table 2 gives 10 minutes (+10 for rise and -10 for set). Given that Jupe was closer to setting I am guessing that I should have subtracted another say 3 to 5 minutes? which means I lucked out by just a few minutes - about as long as I was giving the missus a foot massage after putting kids in bed - d'oh!

So are my step2 calcs correct? Do we guess what percentage between rise and set the object is and do a simple ratio/interpolation between + and - minutes?

thanks in advance for helping a newb.

Socco

[CoombellKid]

My advice is, keep yer mind on the job at hand. Either massage the
missus feet or watch your timings on accultations. It is easy to miss
some things if your not on the job. I've done it myself on a number
of occasions.

Cheers,CS

[Socco]

Yeah, a fair enough point - except that I WAS being aware of the time, just not the time delta from the printed time. I think I learnt how to calculate that in the process of writing the post so it was quite an educational night!

My question shrinks down to the last one in original post - regarding step 2:
"Do we guess what percentage between rise and set the object is and do a simple ratio/interpolation between + and - minutes?"

Is this logic correct?

[Wavytone]

No it is not correct. The time of an occultation of one of the Jovian moons and Jupiter is not significantly influenced by the difference in longitude of the place of observation and the place assumed for the ephemeris calculation.'

The ephemeris calculation will be a geocentric one (ie for the centre of the earth) because from the distance of Jupiter, the parallax angle subtended by the earth is miniscule, ie it matters not one jot where you are on earth - all observers would see the occultation at the same Universal time.

(This provides a means to determine longitude, by the way, much as James Cook did with the transit of Venus to check the accuracy of Harrison's clock).

The real problem is more likely that the prediction is in ephemeris time and did not correctly allow for the light-time between Jupiter and Earth - which is 40 +/- 8 minutes depending on where the earth is in its orbit around the sun. Almost certainly the ephemeris applied a correction of 40 minutes as this is the light-time between Jupiter and the Sun (as a first estimate) rather than work out the distance Earth-Jupiter and compute it properly. This alone would easily account for an error of 8 minutes.

In the past the correction for the Earth's position was often not applied because this allowed amateur observers to do two things:

(a) it permits an easy experiment to measure the speed of light as a function of the diameter of the earths orbit by measuring the difference between the observed time and the ephemeris time at different times of the year.

This method provided one of the earliest reliable measurements of the speed of light. http://www.physlink.com/Education/AskExperts/ae22.cfm?CFID=22660023&CFTOKEN=43547e87da260a14-FF2F0658-15C5-EE01-B98793E43D2B5D82

(b) you could use simple arithmetic to extrapolate the timings of the occultations of the satellites for a month either way, assuming they have circular orbits without bothering to correct for the earth's position each time. Made life simpler for those without a computer.

Cheers...

[AstroJunk]

Socco, I've checked the prediction and it seems ok. The only thing that may have happened is that Io was so close to the disk of Jupiter that you didn't (or couldn't) notice it when you looked at it with a few mins to go.

This friday at 22:22:38 Dayboro time (same as Samford time where I am!), Ganymede will occult Io. If you watch the event starting about 10 or 15 mins earlier, make a note of what time the two points become as one - I guess it will be a few mins before the actual event. Should be an ineresting experiment if it's clear.