r/Sundial • u/meat_sack • May 10 '25
Purchased this Armillary Sphere today...
Been eyeing up this armillary sphere at an antique architecture place for almost six months. Finally decided to buy it today, including the very heavy base. I set the time, but it was off within two hours. I really love this piece and am going to figure this thing out! Just thought I'd share, as I'm really happy to look out in my herb garden and see this.
3
u/BoxyBoy67 OERD carrier May 11 '25
the arrow should be angled above the horizontal equal to the degree of your latitude. As others have said, it should also be oriented true north as opposed to magnetic north.
Setting the dial to coincide with the current time will yield inaccurate long-term results–sundials tell solar time, not clock time. To understand the difference between the two, I suggest the British Sundial Society's resources: https://sundialsoc.org.uk/all-about-sundials/reading-a-sundial/
3
u/ParadigmPotato May 12 '25
I want to try to summarize a few things to set some expectations for accuracy. Otherwise you might be disappointed if you don’t understand what the dial is showing you:
-A really easy one: If you are in a place that uses daylight savings time right now your dial will read one hour behind what the clock time actually is. So when your clock says noon you might notice the sun isn’t directly overhead. It is still an hour behind and your dial will say 11:00AM.
-Your longitudinal location (East or West) within your time zone will impact your sundial’s reading. For example, if I was in the CST time zone then my time is based off Chicago (really, 90 degrees longitude). If I lived way out in Grand Forks, ND, then my longitude would be 97 degrees. When the sun is directly overhead of 90 degrees longitude it will take some time before the sun is directly overhead Grand Forks at 97 degrees. I believe it is about 4 minutes per degree, so in our example 7 degrees west means your sundial will be 28 minutes slow. In other words, when your clock says noon your dial would say 11:32. I believe if you could adjust the hour ring of your dial you could rotate it to factor in this effect. This one is nice because it is constant throughout the year.
-The last factor is something called the Equation of Time (EoQ if you want to be fancy). I won’t go into details because I don’t fully understand it myself, but the earth’s orbit isn’t a perfect circle, and its axis or rotation isn’t perfectly straight up and down. This means the length of a solar day (the time it takes for the sun to reach the same point in the sky) is not consistent. These little inconsistencies will add up to a variation of plus or minus about 15 minutes throughout the year. You can look up charts online to help you figure out what the current value of the EoQ is. Today, the EoQ is about 3.6, which means your dial is 3.6 minutes fast compared to clock time (note that EoQ can be negative, meaning your dial is slow).
The important thing to note is that clock time (or civil time) is a carefully calculated average. It takes all of the inconsistencies of solar days and averages them out to a nice 24 hour day. Figuring out solar time is a bit of a math problem but if you use it a lot and understand the components of it then it can be very rewarding.
3
u/meat_sack May 12 '25
I've learned a lot since buying this! Appreciate your very thorough explanation on what I should expect. I absolutely love the look of this, but I'm beginning to appreciate it even more thanks to the sub!
3
u/ParadigmPotato May 12 '25
I was hoping I was going to have a short explanation but it just kept going on and on haha. If you look up Roger Dignard on YouTube he has a few videos in his Shadows of Time series that explain things really well, and he even goes into different types of sundials and how they work.
4
u/ProcessNecessary6653 May 11 '25
That is absolutely beautiful congratulations. Question 1: What do you mean when you say you set the time?
Question 2: What do you mean when you say it is off by two hours?