# Sidereal Angle Clock

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Not sure if this is the right section for this.

I had a nice day out in Greenwich yesterday and saw this Sidereal Angle Clock designed by John Flamsteed in the Royal Observatory. I'm not having much luck with google so can anyone explain how to read the Clock and explain what it's telling me. I understand sidereal time but I'm not sure what they mean by "the earths angular rotation". It was about 2.15pm when I took the pics and I was originally curious about whether the the Clock was correct. It looks to me as if it's reading 9 hours 8.5 minutes. I've no idea what the window with 11,12,13 means. Anyway, can anyone throw any light on this

Edited by Scooot
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If I understand it right, the angle of rotation is the angle between meridian and the distant object.  So say the angle is zero at midday, then at 10mins past mid day the angle between the meridian and to the distant object will be 10mins of angle. Regarding 11,12, and 13. Can you remember what date you took the picture?

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38 minutes ago, elliot said:

If I understand it right, the angle of rotation is the angle between meridian and the distant object.  So say the angle is zero at midday, then at 10mins past mid day the angle between the meridian and to the distant object will be 10mins of angle. Regarding 11,12, and 13. Can you remember what date you took the picture?

Thanks Elliot, so what's the distant object, the Sun?

I took the pics yesterday at 14:15. At this time the correct local sidereal time was 06:15.

The Clock, unless I'm misreading it, only shows a maximum of 10 hours.  I'm guessing it's not correct, although it was working.

Edited by Scooot
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It seems apparent to me that the dial to the right, showing 11, 12 and 13 has sufficient space for 36 numbers, presumably 0 to 35, and is showing tens of degrees.  The hands, therefore show degrees and minutes of arc.   It is unclear to me which hand is which.  The puzzle is, in my opinion, why the "Minute of Arc" hand has numerals around the periphery of its dial.  Unless these are decorative, it suggests that the clock has a dual function, that is presently not clear to me.

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13 minutes ago, Thomas said:

It seems apparent to me that the dial to the right, showing 11, 12 and 13 has sufficient space for 36 numbers, presumably 0 to 35, and is showing tens of degrees.  The hands, therefore show degrees and minutes of arc.   It is unclear to me which hand is which.  The puzzle is, in my opinion, why the "Minute of Arc" hand has numerals around the periphery of its dial.  Unless these are decorative, it suggests that the clock has a dual function, that is presently not clear to me.

Ah, so the clock is reading 120 degrees something? From memory the smaller hand was turning quickly, presumably then impacting on the larger dial. I wasn't there long enough to take note of the bigger hand moving much.

Edited by Scooot
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3 minutes ago, Scooot said:

Ah, so the clock is reading 12 degrees something?

Or maybe 120 deg + something (if the 11 12 13 window is showing tens of degrees)

I make it displaying 129deg 47minutes 35seconds (of arc) which I cant get to equate to 13:15 nor 6:15

however I may have the two hands assigned wrong, in which case it would be 127deg 39minutes 40seconds,,, still neither !  So maybe it isnt set to the correct time or I am barking mad

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Just now, SilverAstro said:

Or maybe 120 deg + something (if the 11 12 13 window is showing tens of degrees)

I make it displaying 129deg 47minutes 35seconds (of arc) which I cant get to equate to 13:15 nor 6:15

however I may have the two hands assigned wrong, in which case it would be 127deg 39minutes 40seconds,,, still neither !  So maybe it isnt set to the correct time or I am barking mad

Thank you, I'm sure your not going mad  I doubt they reset it regularly.

Still not quite sure how to read it correctly. The 12 shows 120 degrees, the big hand pointing just past the 9 showing the extra 9 degrees? Is that correct? So how do I read 47 minutes 35 seconds from the smaller dial. It Looks to me is if the little hand is pointing just under 3/4 round. Sorry for being a bit slow

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The little hand looks like it is in the 7th interval at just past the 3rd marker giving 7 and 30+ ( I interpreted +5)

The big hand ( sounds like we are back in primary school or Jackanory !!) is in the 9th interval on the 4th sub  =9 and  40

maybe

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Assuming that the small hand is the "Minutes of Arc" hand and that it makes one rotation for every ten minutes of arc, (explaining the  numerals) then the angle displayed is, in my opinion, 119 degrees and 7 minutes.   This is not accurate for 1:15 pm GMT on June 5th. The angle at that time should be very close to 90.

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1 minute ago, SilverAstro said:

giving 7 and 30+ ( I interpreted +5)

=9 and  40

ie. 7 and 35 out of sixty

likewise  9 and 40 of 60

4 minutes ago, Thomas said:

119 degrees

/

The angle at that time should be very close to 90.

or 129 ? looks to me that the index is just past the 12 towards the 13 ??

/

yes that was my problem also !

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1 hour ago, Scooot said:

14:15.

/

At this time the correct local sidereal time was 06:15.

that was BST ?

/

nearer 06:11 I think

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1 minute ago, SilverAstro said:

that was BST ?

/

nearer 06:11 I think

Yes 14:15 BST. I didn't change my location to Greenwich and used Sky Safari to go back in time to look up the local sidereal time so my 6:15 is only approximate.

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Now I have a better grasp I wonder whether it's reading 119 degrees 10 minutes 42 seconds

On the big dial a complete resolution is 10 degrees, each little interval being 1/6 degree or 10 minutes. The hand is clearly 1 notch past the 9, so 10 minutes past the  9. The marker in the window showing 11,12,13 is almost on the 12, travelling between the 11 & 12 making it 110 degrees plus.

The little hand showing arc seconds, the dial marked in ten 6 second intervals, the hand being on the 7 giving 42 seconds.

How does this sound?

Edited by Scooot
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26 minutes ago, Scooot said:

The hand is clearly 1 notch past the 9,

How does this sound?

That isnt how I read it, we are used to seeing the numerals 'on the line' but I think in this clock it is showing them as marking an interval, look at the little arrow heads each side of the lines marking the extent of the "9" range and the hand is just a gnats past  the 4th sub interval giving 40+ out of 60 in that interval  :-

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21 minutes ago, SilverAstro said:

That isnt how I read it, we are used to seeing the numerals 'on the line' but I think in this clock it is showing them as marking an interval, look at the little arrow heads each side of the lines marking the extent of the "9" range and the hand is just a gnats past  the 4th sub interval giving 40+ out of 60 in that interval  :-

Ah got it  so 119 deg 47 mins 35 secs

got there in the end.

Thanks for everyone's input. Mr Flamsteed was a clever old soul. It's a fascinating timepiece considering it was made in 1691.

Edited by Scooot
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14 minutes ago, Scooot said:

so 119 deg

Or maybe 129 ? the index being on, above,  the 13 side of 12 ?? (either way it has been an interesting topic, thank you,  way better than watching the telly ! )

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Or maybe not ! I'm not sure about the significance of the dots inbetween !

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4 minutes ago, SilverAstro said:

Or maybe not ! I'm not sure about the significance of the dots inbetween !

I thought if it was 129 the marker would have been much near the 13.

I agree the TV doesn't compare. Thanks again for your help.

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Good point ! Time I opened a bottle

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Imagine if you created a whole new type of clock, how would it work? Find Flamsteed's brilliant (but doomed) attempt to make a perfect astronomers' clock.

One of only two angle clocks ever made. This clock was designed by John Flamsteed, first Astronomer Royal, to show the time in degrees, minutes and seconds of arc.

In astronomy it is often more convenient to express time in this way. The earth turns through 360°on its axis every 24 sidereal hours. 24 hours = 360° 1 hour = 15° 4 minutes of time = 1° 4 seconds of time = 1' of arc 1 second of time = 15" of arc.

The clock reads as follows: small window right of centre – tens of degrees; 'minute' hand – degrees, with 10-minute of arc subdivisions; 'second' hand – minutes of arc with 10-second of arc subdivisions.

To convert the clock to read normal hours a special pendulum of 39.1 inches can be fitted and the small dial behind the aperture reversed to show I to XII hours. The clock then shows hours - and decimal parts of an hour.

In practice this system was not very useful for astronomers and it is believed that only two 'angle clocks' were ever made: this one, and one by John Arnold, also in the Royal Observatory collection

hope that helps

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30 minutes ago, Uhwaz said:

Imagine if you created a whole new type of clock, how would it work? Find Flamsteed's brilliant (but doomed) attempt to make a perfect astronomers' clock.

One of only two angle clocks ever made. This clock was designed by John Flamsteed, first Astronomer Royal, to show the time in degrees, minutes and seconds of arc.

In astronomy it is often more convenient to express time in this way. The earth turns through 360°on its axis every 24 sidereal hours. 24 hours = 360° 1 hour = 15° 4 minutes of time = 1° 4 seconds of time = 1' of arc 1 second of time = 15" of arc.

The clock reads as follows: small window right of centre – tens of degrees; 'minute' hand – degrees, with 10-minute of arc subdivisions; 'second' hand – minutes of arc with 10-second of arc subdivisions.

To convert the clock to read normal hours a special pendulum of 39.1 inches can be fitted and the small dial behind the aperture reversed to show I to XII hours. The clock then shows hours - and decimal parts of an hour.

In practice this system was not very useful for astronomers and it is believed that only two 'angle clocks' were ever made: this one, and one by John Arnold, also in the Royal Observatory collection

hope that helps

Yes it's a great help thanks . strangely enough I read it yesterday after another attempt to find some information from google. Don't know why I couldn't find it on google before I started the thread but I do know how to read the Clock now.

Edited by Scooot

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