Sundials and the Equation of Time

[@Cici_Dial]

I'm an American diallist who once had the pleasurable honor of designing a large analemmatic dial for an elementary school in England. I thought I knew a lot about sundials, especially the EOT, but i learned a few things from your excellent and thoroughly enjoyable video. The lovely lady assisting you impressed me by anticipating exactly when ypu needed the globe moved. She was impressive. Thanks to both of you for a thoroughly enjoyable video. Cheers!

[@jimmathy]

Glad you enjoyed it - and so is my wife!

[@puma7171]

Lovely video, very clear even if my brain needed some pauses here and there. I also learned that New Zealanders are truly standing upside down, when they stand, and that the whole antipode thing is actually their problem, not ours. ;)

[@jimmathy]

Thanks!

[@laurengrant7315]

Excellent! I really enjoyed this and definitely learned a lot. There were many, many pauses for the brain to catch up...

[@jimmathy]

Glad it was helpful!

[@clintonshiells3095]

Thank you for a very dense and precise explanation of the equation of time. My interest stems from wanting to understand watches with the equation of time complication and how clocks are (or at least were) used for navigation. Having watched a couple of videos whose explanation didn’t really make sense to me, thank you for making a video that makes the issues crystal clear, though still not easy. It’s also nice to see the concept of precession applied.

[@jimmathy]

Thanks.

[@fajn]

Thank you for explaining the ecliptic effect. And a nice ending.

[@DannyPRobinson]

That was a lovely clear explanation and I learned a lot from it For many years I've observed something but never been able to work out why it's happening. A vertical stick at right angles to the ground does not cast a shadow in the same direction every day at the same time. Twelve hours later, the afternoon shadow will not necessarily be at 180 degrees to the morning shadow. When the sun is below the horizon and thus unobservable, tables can still show the notional direction of the sun and show that both the direction and the time it takes to travel 180 degrees varies through the year. You didn't set out to answer that question, but nonetheless I think you have. Would you be able to address this question specifically in a future video?

[@sashachip]

So what's the answer to the question in the video about what time of day is it in New Zealand? The question was never answered.

[@jimmathy]

It is answered just before the end, at about 37:55. It's the morning.

[@sashachip]

@@jimmathy i just re-watched the video and at no point did you say it's morning. You only said that the sun is lowest in the sky toward the North. Anyhow- thanks for the answer. I'm still having a difficult time understanding how the solar day is shorter at the equinoxes than at the solstices. My understanding is that if the ecliptic is further from the Equator, as it is during the solstices, then the distance the apparent sun travels during the solstice in 4min does not matter because though the circumference of the ecliptic is smaller and therefore the speed of rotation is slower than at the equator- the distance in degrees of longitude remains the same. Four minutes of rotation is 1 degree of longitude no matter where you are on Earth.

[@jimmathy]

@@sashachip what W hear at the end is ".. so we must be looking sort of North-East, so that's the morning." But maybe I'm being influenced by know that that is what I meant to say. What do you hear? It is true that 4 minutes of rotation about the Earth's axis is 1 degree no matter where you are, but 4 minutes of rotation about the Sun drives the subsolar point past a varying number of degrees of latitude, because it always goes the distance along the ecliptic.

[@sashachip]

@@jimmathy it must be your accent that is throwing me off but i just turned on captions and it says 'mooring' which is close enough i guess lol. I'm still having a difficult time getting my head around the concept of the sub-solar point on Earth traveling a different distance in 4min from 360deg at differing latitudes. Here's how i see it- if the sub-solar point on Earth on the Summer Solstice is at 23.4N deg then the ecliptic of the sun on that day will follow a great circle around the Earth at 23.4N deg latitude all around. It will travel at a distance of 1 deg longitude/4min. Now if we move forward to the Autumnal Equinox then the sub-solar point is at 0 deg and the ecliptic of the sun on that day will follow a great circle around the Earth at 0 deg latitude all around and travel a distance of 1 deg longitude/4min. In either case- it takes exactly 4min to travel that extra 1 degree of longitude so how can you say that it takes longer to get back to the 360deg point? I can't get my head around that.

[@wcovey9405]

The equation of the analemma. The anaiemma came about because the hours became linear due a Greek water clock.

[@jimmathy]

Interesting. I'd wondered about that. You can tell there is a diffence between solar time and star time without a clock.

[@wcovey9405]

@@jimmathy Yes, it can and has been done by using a calendar to measure the difference in time over time.

[@KipIngram]

@@wcovey9405 Well, just notice that the constellations in the sky change season to season. If star time and sun time were the same, they wouldn't.

[@wcovey9405]

@@KipIngram What? The Earth tilt, rotation is the same for the Sun and Stars. It is seen from Earth that the Sun moves through the constellations at the same rate in linear time but using sundial time. Think about that in distance per time traveled.

[@colinubeh1180]

I always thought that Solar time = clock time. Maybe because I live near the equator where day = night throughout the year.

[@jimmathy]

On the equator, hours of daylight are always close to hours of darkness, though not exactly. But the equation of time and the analemma are the same the world over.

[@Chakrawat-Pakshii]

Ware the past scientists of Indian origin?

[@niktak4658]

I got lost

[@normanacree1635]

In a nut shell: Because of variances in the earth's rotation and revolution around the sun, not every day has exactly 24 hours from one solar noon to the next. There is a chart that shows the differences during the year. This phenomenon is called 'equation of time'. Four times a year there are exactly 24 hours from solar noon to solar noon. Depending on the day of the year, your watch may be as much as 16 minutes different from your sundial, if you have one set up correctly. When reading your sundial, it is good to have the 'equation of time' chart handy so you can match your sundial to your watch. You don't need to do this if you ignore the time showing on your watch. If you want to build and set up a sundial that is as accurate as possible, be sure to research 'declination'. Hope this helped.

[@deanemilne6275]

need to talk slower and more clearly

[@KipIngram]

Um, I don't think the problem is with the way he was talking.