Electrical Question in 'Pub Quiz'

[EFLImpudence]

What's the problem then? The length of the shadow (up the side of the partial 'soup bowl') will then surely always indicate the correct time, won't it -

No, you would then need most of the soup bowl.

after all, at the equator, there is no difference (in sun position) between January and June, is there? What am I missing?

Err - yes. Why are your days longer in June and shortest in December (I shouldn't have said January either but almost the same).

 

[EFLImpudence]

What complicated lines would you need for this?

 

[Harry Bloomfield]

So do sun dials move anti clock wise in southern hemisphere? That would mean they would not work on in the tropics as they were reverse direction summer and winter, but I think they do work. Like the direction of water down a plug hole, it does not really change.

It goes anti-clockwise north of the Equator, clockwise to the south of the Equator. On the very exact and precise equator, which varies depending upon time of the year due to the earth's wobble - it goes straight down.

 

[EFLImpudence]

It goes anti-clockwise north of the Equator, clockwise to the south of the Equator.

I think we are referring to the shadow on the dial which goes the opposite way.

 

[JohnW2]

No, you would then need most of the soup bowl.

You were talking about 'a vertical pin at the equator'. The shadow then moves in a straight line, so why would you need 'most of the soup bowl'?

Err - yes. Why are your days longer in June and shortest in December (I shouldn't have said January either but almost the same).

Again, you were talking about 'at the equator' where, when I last looked, days were of the same length in June, December and January.

Kind Regards, John

 

[DetlefSchmitz]

It goes anti-clockwise north of the Equator, clockwise to the south of the Equator.

No it doesn't.

 

[JohnW2]

What complicated lines would you need for this?

I'm afraid that I don't really understand your diagram!

Kind Regards, John

 

[Harry Bloomfield]

What's the problem then? The length of the shadow (up the side of the partial 'soup bowl') will then surely always indicate the correct time, won't it - after all, at the equator, there is no difference (in sun position) between January and June, is there? What am I missing?

The earth's wobble! If the pin is perfectly vertical and on the equator, the shadow will be long and to the north of the pin in January, then to the south in June - our shortest and longest days. The shadow though, will always be at its shortest of each day, precisely at midday.

 

[EFLImpudence]

I'm afraid that I don't really understand your diagram!

Oh, come on John.

At the equator,
the sun rises (and sets) on the equator on March 21 and September 21
on December 21 it rises at 23.5° South
on June 21 it rises at 23.5° North
and continually varies between.

Therefore, with a vertical gnomon the shadow would move between the extremes on my diagram

 

[winston1]

What's the problem then? The length of the shadow (up the side of the partial 'soup bowl') will then surely always indicate the correct time, won't it - after all, at the equator, there is no difference (in sun position) between January and June, is there? What am I missing?

Kind Regards, John

In June the sun is further away than in January as the earths orbit is not truly circular.

 

[JohnW2]

The earth's wobble!

Yep, it's the tilt I have been overlooking, but ..

If the pin is perfectly vertical and on the equator, the shadow will be long and to the north of the pin in January, then to the south in June - our shortest and longest days.

I'm not quite sure what you mean by 'the shortest and longest days' at the equator!

Kind Regards, John

 

[JohnW2]

In June the sun is further away than in January as the earths orbit is not truly circular.

That is irrelevant to what we are discussing.

 

[EFLImpudence]

...and it's 'farther'.

 

[JohnW2]

Therefore, with a vertical gnomon the shadow would move between the extremes on my diagram

Yep, I'm getting there ... but what tilting of the gnomon would alter what you're saying?

Kind Regards, John

 

[Harry Bloomfield]

I'm not quite sure what you mean by 'the shortest and longest days' at the equator!

By 'our' I meant our days in the UK, sorry I didn't make that clear. The day length would vary slightly at the equator, but I'm not sure by how much.