please empty your brain below
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Does all this rely on the sundial having been installed on one of these four days?
dg writes: No.
We had a sundial in our garden when I was growing up - it wasn't firmly attached, so we'd occasionally reset it if it looked like it wasn't showing the right time.
martin | 15.04.20 - 7:09 a.m. | #
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Now isn’t that amazing. The number of times I’ve consulted equations of time by sundials, usually in the summer for obvious reasons, and not realised that Christmas Day was the only day a sundial is correct! Is this just an astonishing coincidence, or is there something behind it?
Thank you DG for another fascinating nugget.
RogerB | 15.04.20 - 7:32 a.m. | #
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For those of you living in Forest Hill (and for the rest of us in the future) visit Hornimans Gardens to follow their Sundial Trail. I believe it is the largest collection in Britain.
Alan S | 15.04.20 - 8:47 a.m. | #
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Fascinating. I didn't know that. Although my sundial is currently 300 miles away.
Blue Witch | Homepage | 15.04.20 - 9:08 a.m. | #
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Fascinating read DG. Didn't know that a sundial only tells the correct time 4 times a year.
Even a stopped clock gives the right time twice a day.
Richard | 15.04.20 - 9:12 a.m. | #
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Fascinating
DavidC | 15.04.20 - 9:20 a.m. | #
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Is this the same all over the world?
dg writes: Yes.
anonymous | 15.04.20 - 9:24 a.m. | #
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Sundials work best in the northern hemisphere because the equation of time never exceeds 6 minutes during spring and summer.
In the southern hemisphere variation during spring and summer is much greater.
diamond geezer | Homepage | 15.04.20 - 9:31 a.m. | #
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Good stuff, but most people will struggle to read a sundial to an accuracy of a minute (0.25°). The sun itself is 0.5° across, which means that shadows don’t have really sharp edges. Even four minutes (1°) will be tricky without a thin gnomon and a protractor. And solar time depends on longitude, so for example Bristol at 2.5°W is about 10 minutes behind London. Within 5 or 10 minutes should be fine for most practical purposes. Before running scheduled trains, anyway.
Andrew | 15.04.20 - 9:53 a.m. | #
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All the above discussion assumes that the sundial is installed "correctly". In such a position that its average display over the whole year is accurate. The installer could have, either deliberately or accidentally, installed it such that it is correct on a different number of days per year, but it will never be more than 4.
Malcolm | 15.04.20 - 10:07 a.m. | #
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fascinating
Caz | 15.04.20 - 12:56 p.m. | #
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Super interesting, brand new info to me, thanks DG.
Kevin | 15.04.20 - 4:54 p.m. | #
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My brain has fallen out!
Chris | 15.04.20 - 6:08 p.m. | #
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The blog states 'Clocks tell mean solar time, a constant based on the rotation of the Earth.' this has not been true since 1972 when UTC which is based on the constant ticking of atomic clocks replaced times such as GMT which were based on the Earth's uneven rotation. GMT is no longer kept which is interesting as it is the time used in UK legislation. However the difference between UTC & 'GMT' is at most a few seconds. This may change in the future if the proposed change to the definition of UTC goes ahead which removes leap seconds from UTC.
In terms of a sundial only showing the same time as a clock 4 times a year there is a sundial at The Royal Observatory at Greenwich that has the Gnomon made from the tails of two dolphins. This is arranged that it shows clock time on the scale as the sun's elevation also changes at the same time as the equation of time.
JohnM | 17.04.20 - 2:43 p.m. | #
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