[dave543 (OP)]

Hi,

Our Moon has 8 distinct 'phases' e.g. Waxing Crescent Moon, Waxing Half Moon, Waxing Gibbous Moon and then Full Moon then the same 4 phases but waning.  .  It takes about 29 and a half days to do a full cycle from New Moon (unseen) to New Moon.  The 8 phases then are about  3 and a bit days long.

The 4 Moons of Jupiter do not orbit Jupiter in 29 days but rather 42 hours (IO) , 85 hours (EUROPA) , 172 hours (GANNYMEDE) and 17 days (CALLISTO) respectively. 

Let's take the shortest orbit i..e IO's orbit.  Wouldn't we only need to track IO's phases as I assume IO goes from New Moon to Full Moon in a mere 21 hours?  We could therefore take each of the 8 phases of IO as 11 hour intervals?   Is that all we have to go on?  11 hour intervals?  What use would that have been to a pre Harrison's- Clock armed sailor in the 17th century in the middle of the ocean?

 Am I simplifying this?  How do we tell the time by observing all of the 4 Moons of Jupiter and their respective phases? 

Thanks.

[Halc]

Let's take the shortest orbit i..e IO's orbit.  Wouldn't we only need to track IO's phases as I assume IO goes from New Moon to Full Moon in a mere 21 hours?

Only if A) you were on Jupiter, and B) you could see the moons.
Moon phases are only seen from the point of view of the primary. From the sun, all moons are permanently full, and Earth is close to the sun in comparison to Jupiter.

[dave543 (OP)]

Let's take the shortest orbit i..e IO's orbit.  Wouldn't we only need to track IO's phases as I assume IO goes from New Moon to Full Moon in a mere 21 hours?

Only if A) you were on Jupiter, and B) you could see the moons.
Moon phases are only seen from the point of view of the primary. From the sun, all moons are permanently full, and Earth is close to the sun in comparison to Jupiter.

I see so how did we use those Moons as a clock?  Was it the Cartesian- coordinate positions or an estimate of the positions?

[Halc]

I see so how did we use those Moons as a clock?

They make great clocks because of the precise times that they either pass behind the planet or especially throw a shadow onto it.

The first measurement of light speed was done using the moons as the remote clock and one of the recently invented pendulum clocks as the local one. The method wouldn't have worked without the remote Jovian clock, and the sundials used before then lacked sufficient precision.

[Bored chemist]

There was a suggestion of using them at sea to measure longitude.
https://www.rmg.co.uk/stories/blog/solving-longitude-jupiters-moons
Not entirely practical...

[dave543 (OP)]

I see so how did we use those Moons as a clock?

They make great clocks because of the precise times that they either pass behind the planet or especially throw a shadow onto it.

Do you know how exactly?   Again, was it a matter of noting the Cartesian-coordinate positions to determine the passing of a certain number of hours?

[Bored chemist]

I assumed it was a simple matter of having a calendar which said
25 Aug 2021 Io disappears behind Jupiter at 21 17 GMT
or some such.

(beware of made-up data)  :-)

[evan_au]

I see so how did we use those Moons as a clock?

Kepler's laws of motion suggested that elliptical orbits should be periodic ("like clockwork", to use a modern phrase).

Ole Rømer timed the passage of Io behind Jupiter over many months, and found that they weren't periodic.
- Sometimes these events were up to 22 minutes later than you would have predicted based on measurements at other times of the year (today we know the correct answer is closer to 16 minutes).
- He recognised that these timing differences were related to the relative positions of Earth & Jupiter on their orbits - these event were delayed when Jupiter & Earth were on opposite sides of the Solar system, compared to when they were at their closest points.
- This meant it took 22 minutes for light to cross the diameter of Earth's orbit, and gave the first fairly accurate estimate of the speed of light.

Once you had corrected for the speed of light, you could produce tables of the times that these occultations would occur, years in advance, in some standard timezone (eg GMT).
- Sailors could take these tables with them, and by watching for these events (on a clear night), they could adjust their clocks so they stayed locked to GMT.
- By comparing the local time with GMT, they could calculate their longitude.

By the time of Cook's voyages (around 1777), clock technology had improved enough that a mechanical clock could keep good time at sea, and they weren't so dependent on seeing Jupiter to calibrate to GMT.
- They were still dependent on seeing the Sun or Moon to find their local time. Local time is also vital for navigation.

See: https://en.wikipedia.org/wiki/R%C3%B8mer%27s_determination_of_the_speed_of_light

[Bored chemist]

and they weren't so dependent on seeing Jupiter to calibrate to GMT.

I'm not sure that many sailors ever used this technique.

[dave543 (OP)]

By the time of Cook's voyages (around 1777), clock technology had improved enough that a mechanical clock could keep good time at sea, and they weren't so dependent on seeing Jupiter to calibrate to GMT.
- They were still dependent on seeing the Sun or Moon to find their local time. Local time is also vital for navigation.

Yes every 15 degrees west of Greenwich (360 degree turn of planet/ 24= 15) meant if e.g. you knew it was noon in Greenwich and you knew it was 1400 hours where you were at sea then you would consult the grids on your map (you'd be 2 grids west of Greenwich in that example).

How would you know what time it was at sea?  Position of Sun from east to west?  How about at night?   

[alancalverd]

Longitude was something of a guess until the 1740s when Harrison made the first chronometer that could keep adequately accurate time at sea. Progress from then on was very rapid. Local time was always best established by the noon sighting of the sun - an art in itself on a small ship, and frequently mentioned in historic logs as "poor" or "not visible". Rising stars are some help if you know your latitude, which is fairly easy in the northern hemisphere from the altitude of Polaris but more of a problem Down Under.

Every time I've attempted an astronavigation course, I've taken my hat off to the sailors and pioneer air navigators who did it for a living with books of tables, in a storm or under fire, and just hope that HM Government finds some way of contributing to EGNOS and GALILEO again!

[Bored chemist]

just hope that HM Government finds some way of contributing to EGNOS and GALILEO again!

Well, we had a system, but for some reason we haven't any more.

No longer having access is called  "a considerable upside".

[alancalverd]

I'm a bit  baffled by this one. Neither is an EU project, non-European nations are members, and EU-registered aircraft presumably would benefit if EGNOS allowed them to land in the UK. Apparently the cost of UK membership was increased by some ludicrous factor on leaving the EU but on the presumption that everyone previously contributed either according to need or ability, and neither changed on leaving, one can only assume that the intention was punitive - not the best way to run an international aviation safety system. 

Meanwhile the box continues to give me heights and headings, but tells me not to rely on them if they are from EGNOS!

[Bored chemist]

Neither is an EU project

Quite; it's a European commission project.
"The European Geostationary Navigation Overlay Service (EGNOS) is a satellite based augmentation system (SBAS) developed by the European Space Agency and EUROCONTROL on behalf of the European Commission. "

from
https://en.wikipedia.org/wiki/European_Geostationary_Navigation_Overlay_Service

The EC is, of course, the law making bit of the EU.

[alancalverd]

Not all the EU  states were founding members of ESA, and Switzerland, which was never a member of the EU, is a founding member.

Eurocontrol was founded in 1960 with the UK as a member, long before we joined the accursed Union, and is not run by the EU (not that it works much better, but it's less corrupt).

[Bored chemist]

When it comes down to it, you re still saying "We have left the club, and won't be paying our subs, but we still want to be on the club committee and we want to use its facilities."

Feel free to edit the wiki page if you think it's wrong.

Eurocontrol was founded in 1960

GPS wasn't.

[Bored chemist]

Not all the EU  states were founding members of ESA

Remind us where anyone said they were?

Perhaps Boris should go back to telling the time by astronomy.
It's difficult so we can safely assume it would take  up lots of BJ's time.
And, of course, it wouldn't matter if he got it wrong.