4 ms·
> It’s important to know that there are exactly 86400 (246060) UT1 seconds in a UT1 day. If the rotation of the Earth speeds up/slows down, the definition of a
by black6 5y ago
> It’s important to know that there are exactly 86400 (246060) UT1 seconds in a UT1 day. If the rotation of the Earth speeds up/slows down, the definition of a UT1 second changes since the number of seconds in a day is constant.
I recently took an introductory Latin course and learned that ancient Roman time operated on the same principle. As the days got longer in the summer, so did the absolute length of time encompassed by an hour. Hard to make a sundial to automatically compensate, I imagine.
- bonzini 5y agoI don't remember the details but it's actually just as easy. They're called unequal hours, Babylonian hours or Italic hours. More precisely the difference between two sunrises or two sunsets was 24 hours, so the hours were longer in winter and spring as days get longer, and shorter in summer and fall as days get shorter.
- fanf2 5y agoA sundial measures the angle between a point on the surface of the earth, the centre of rotation of the earth, and the sun. Because the earth rotates at a constant-ish speed, a sundial naturally shows equal hours. It is possible, but tricky, to make an unequal-hours sundial: https://www.cl.cam.ac.uk/~fhk1/Sundials/Newnham/write-up.pdf https://www.cl.cam.ac.uk/~fhk1/Sundials/Newnham/write-up.pdf https://www.cl.cam.ac.uk/~fhk1/Sundials/Newnham/article01.pdf https://www.cl.cam.ac.uk/~fhk1/Sundials/Newnham/article01.pd... https://www.cl.cam.ac.uk/~fhk1/Sundials/Newnham/article02.pdf https://www.cl.cam.ac.uk/~fhk1/Sundials/Newnham/article02.pd... https://www.cl.cam.ac.uk/~fhk1/Sundials/Selwyn/Selwyn.pdf https://www.cl.cam.ac.uk/~fhk1/Sundials/Selwyn/Selwyn.pdf