Comments on: It’s summer. What’s “noon” to you?

By: Space Shuttle & ISS Visible in Night Sky | This Crazy Cosmos

Space Shuttle & ISS Visible in Night Sky | This Crazy Cosmos — Fri, 22 Feb 2008 23:20:20 +0000

[...] First Day of Summer ~ June 21, 2007: blogs.earthsky.org Earth & [...]

By: Daniel Clements

Daniel Clements — Mon, 24 Sep 2007 16:03:50 +0000

The difference between solar noon and your clock noon is very interesting. Did you know this was discovered many years ago and the effective calculations were put into a mechanical clock at the begging of the 18th century. We have actually a clock that can be seen on our website number 100 in clocks section, that shows this so called,’equation of time’ It is very rare, take a look, it is quite amazing.

Regards

Daniel

http://www.pendulumofmayfair.co.uk

Actual Link I believe is at http://www.pendulumofmayfair.co.uk/view.asp?pid=272&cat=Longcase Clocks

By: Larry Sessions

Larry Sessions — Fri, 22 Jun 2007 14:20:33 +0000

Hey, Bruce! You’re probably more up on this than I am, so I will happily defer to you. There is a variation due to the inclination, but up until now I never knew the amount. It has to do with the sun’s apparent motion relative to the celestial equator, and my assumption was always that this is less of a factor than the distance (hence orbital speed) variation through the year. I suspect that this is because the orbital variation seems typically to be listed first, so I just assumed that it was more important. I also assume that the additional variations of 13-14 and 21-22 seconds are scaled off the equation of time (but you know what assumptions can make out of you!).

HOWEVER (!) I’ve just found a graphic on the Greenwich Observatory/Nautical Maritime Museum website that definitely shows that the variation due to the tilt certainly contributes a greater amount to the overal variation. Take a look here: The equation of time.

Thanks for pointing that out!

Larry

By: Bruce McClure

Bruce McClure — Thu, 21 Jun 2007 17:07:56 +0000

Hi Larry!

As a sundial enthusiast, it’s hard for me not to get excited about the concept of “noon”. The discrepancy between clock noon and solar noon, in fact, does some mind-bending things around solstice time. For instance, today’s June 21 solstice brings the longest day of the year (as measured by duration of daylight) to the northern hemisphere. Yet, at mid-northern latitudes, the earliest sunrise of the year came about a week ago; and the latest sunset of the year will come about one week after today’s summer solstice.

That’s because the day as measured from solar noon to solar noon exceeds 24 hours at this time of year. I believe it’s something like 13 to 14 seconds longer than 24 hours. If, however, the Earth revolved around the sun in a perfect circle (and traveled at a constant speed), I believe these June days would be about 21 to 22 seconds longer than 24 hours.

The longest days of the year, as measured from solar noon to solar noon, happen around the December solstice. Then, the solar day is about 30 seconds longer than 24 hours. Again, I believe that figure would be something like 21 to 22 seconds, if the Earth revolved around the sun in a perfect circle and at a constant speed.

I don’t recall reading anything about the inclination of the Earth’s axis in regards to the varying times of the sun’s transit, but I believe axial inclination has more of an effect than the Earth’s changing orbital speed. I believe the solar days are longer around the solstices and shorter around the equinoxes, primarily because of axial inclination.

For now, enough of my musings . . .

Bruce