THE EQUATION OF TIME
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- čas přidán 25. 06. 2024
- Converting sundial time to clock time. Part 2 - The Equation of Time. It is based upon the obliquity of the earth and the eccentricity of its orbit around the sun. How to apply the Equation of Time to sundials or clocks.
You have put in 10 minutes video what would need an entire book to explain. So greatful that I come across your videos. Thank you
Glad it was helpful!
Beautifully explained, thank you.
This is what I was looking for 😸 Thank you so much sir.
Good video
Hi Roger, I have been doing some research into timekeeping and sundials and came across your channel. You have put out some nice videos, they are well explained and easy to listen to, thank you. This one, on the equation of time, was particularly useful as I was stuck with times from a nautical almanac and wanted to know how they would appear on a sundial. Your reverse graph was great! 👍🏻
I have a question about an idea for a sundial, would it be ok to email or message you or could I put it on here?
Thanks again, Boqer
Hi Boqer, thanks for your positive feedback. I enjoy designing, making, and sharing what I know about sundials. Admittedly I am not the foremost expert on the subject. Would gladly discuss your idea with you. I have added an email address to my channel for contact through filters. Thanks, Roger
Go to the ABOUT tab on my channel and you will be able to access the email.
Thanks for this nice video. I am failing to understand only one thing - How come the obliquity graph is showing the value zero on the two Solistices? If it was zero on the two Equinoxes then it should be much and maximum different on the time when earth is on it maximum tilt on the two solistices?
Good question! This is hard for me to explain without drawing a diagram but I will try. The earth's axis is at a constant tilt throughout the year. There are 4 points in the year when the time adjustment for obliquity will equal zero. That is when the sun and earth's axis are normal (90 degrees) or aligned with one another; the equinoxes and the solstices.
For example: starting at the vernal equinox, when it is zero. The next day the earth has moved along its orbit. If the earth had rotated in place the sun would appear overhead again 24 hours later. But the earth has also moved along its orbit. Looking back at the sun you would find that a small angle of difference (and time) exists between your position on the earth and the observed overhead position of the sun compared to the day before. That starts to build a small time difference that accumulates over the days until you reach the mid-point in orbit between the vernal equinox and the winter solstice. At that mid point the observed angle starts to close again (reducing the time difference) until it is zero again at the winter solstice. That happens 4 times through the year. The time adjustment will grow or decrease leaving each equinox and solstice depending on the direction of the earth's motion along its orbit relative to the sun. It depends on if you are moving away or towards the sun at that point in the orbit.
Hopefully this helps, it is hard to visualize this phenomena.
@@rogerdignard5438 Thankyou very much!
This was a beautifully clear explanation of the equation of time! And it was impressively concise. Thanks so much for it. I have one quick question. I understand why eccentricity produces an irregularity in apparent solar time (because of Kepler's second law) but why does obliquity produce an irregularity?
Coz the maths is floored(Crap).
They haven’t found a single constant yet..2023. 🎉
My math is confusing me somewhere. I seem to be getting the correct answer when I invert the sign for the EoT. I have an astronomical app that I've set for November, when the EoT shows +14. My meridian offset is 8 minutes, that is, I'm about 2 degrees west of my timezone central meridian. When I move the dial to show solar noon (local apparent time), clock time shows 11:54. The way to get from 12:00 to 11:54 is to *subtract* the +14 minutes from the EoT and then *add* the 8 minutes from the offset. Does that sound right? I thought you add the EoT (positive or negative) and subtract the offset (if you're west of the central timezone meridian)…?
It would take me some time to recreate your issue. Make sure that your EOT is selected for what you are converting: solar to clock or clock to solar as that will affect if it is a plus or minus value. Also, did you watch my video "Sundials and Longitude"? Together these two videos should give you what you need to work with. Thanks for
sharing this interest, it can be challenging at times to visualize the variables.
I know this is 2 yrs on, but perhaps I can help a little. Take 'sundial time' +/- EOT to get 'local mean time'. But your app says +14 in November, so it's backwards from the convention used in some fields. Looking at the curve shown in this video, in November your app should give you -14, not +14. He briefly mentioned that some apps give an 'inverted curve' (@3:58), and that sounds like what happened to you.
It's not unusual for those dealing with celestial navigation to have terms and such slightly different than those folks dealing with astronomical sightings. (like how navigators talk of 'sidereal hour angle' for a star, but astronomers use 'right ascension'. Sorry it's confusing, but that's just how life is sometimes.
Is it safe to assume that you can't ever build and position a sundial to accurately tell time without doing the calculations you cover in this video with your sundial being as much as 15 minutes or more off?
At 2:20, why does a day get longer or shorter based on the speed of its orbit around the sun when days are based on rotation? Does the earth take more or less time to spin at perihelion and aphelion? Please explain.
Earth travels faster when near the sun, the spin on its own axis doesn't really change much. The suns relative position as observed from earth changes faster because of this. Try doing drawings to convince yourself.
Sir, Considering your astronomical knowledge and interest, I have a question for you! It is said that earth has a fixed period of rotation about its axis with respect to stars and this period called as Sidereal day is 23 hrs and 56 minutes. I know that earth rotational time with respect to sun differs round the year and hence we have the eq of time. My question is that how the Siderial time remains constant w.r.t stars as the earth is not only rotating around it axis but also revolving around the sun in orbit. So earth is expected to be a bit displaced w.r.t stars as it moves/revolves in its orbit daily? Why this displacement has no effect on the Sidereal time?
I don't think that I know enough to answer this with any authority. An astronomer would understand this better than me. My assumption is that the extreme distance to the reference point for measuring a sidereal year may ignore any relatively small deviations in earth orbit.
Think of Sidereal Time as the time it takes the Earth to rotate a full 360 degrees. This value does not change relative to the Earth's position in it's orbit around the Sun. The Earth always spins around it's own axis at about the same speed and therefore Sidereal day is always about 23hrs 56min. You need to remove all other variables and only look at the 360 degree rotation in isolation from all other factors that can influence the time. Obliquity and eccentricity only factor in when finding the Equation of Time. The EoT will vary because obliquity changes the sub-solar location of the Sun relative to Earth and eccentricity changes the speed the Earth travels around it's orbit of the Sun. The obliquity can vary the sub-solar position by as much as just shy of 47 degrees over the course of 6 months. When the sub-solar location is at the Tropic of Cancer (23.4 deg N) meridian on the Summer Solstice- the Sun's apparent movement is slower than at the Equator (0 deg) meridian on the Equinox because the circumference of the Tropic of Cancer is smaller than that of the Equator and therefore the apparent sun travels less distance for any given time.
Relationship to finance?
1 Minute = 100 Sekunden
1minute was not 60 sehende, thats a lie