Channel Ribs
Well-Known Member
There is about a week where the sunset times do not change and there are a likewise few days when the sunrise times do not change, why is it that they are not the same few days??
Why are the latest dawn and earliest dusk offset?
- Thread starter Channel Ribs
- Start date
Romeo
Well-Known Member
eliptical orbits?
FWB
N/A
shmoo
Well-Known Member
Equation of time is on pretty steep slope at this time of year.
That's wot does it.
Image from http://en.wikipedia.org/wiki/Equation_of_time
That's wot does it.
Image from http://en.wikipedia.org/wiki/Equation_of_time
earlybird
Well-Known Member
GMT is what it says; ie a MEAN time, which is usually different from sun time, which is irregular. Mean time is useful for a regulated society. As said, the difference is given by the Equation of Time.
chewi
Well-Known Member
Forgive my ignorance but how is it related to the equation of time?
The equation of time graph above shows clock time vs solar time and doesn't stray any more than 17 mins. even if clock time were relevant.
The OP is asking why latest sunrise disagrees with earliest sunset by as much as a week, and I don't see how the EOT explains that.. That phenomenon would exist without clock time even being invented, it must surely be a celestial phenomenon that has a celestial explanation.
The equation of time graph above shows clock time vs solar time and doesn't stray any more than 17 mins. even if clock time were relevant.
The OP is asking why latest sunrise disagrees with earliest sunset by as much as a week, and I don't see how the EOT explains that.. That phenomenon would exist without clock time even being invented, it must surely be a celestial phenomenon that has a celestial explanation.
earlybird
Well-Known Member
[quote
The OP is asking why latest sunrise disagrees with earliest sunset by as much as a week, and I don't see how the EOT explains that.. That phenomenon would exist without clock time even being invented, it must surely be a celestial phenomenon that has a celestial explanation.
[/ QUOTE ]
If clock time had never been invented, how would you measure latest sunrise and earliest sunset?
The OP is asking why latest sunrise disagrees with earliest sunset by as much as a week, and I don't see how the EOT explains that.. That phenomenon would exist without clock time even being invented, it must surely be a celestial phenomenon that has a celestial explanation.
[/ QUOTE ]
If clock time had never been invented, how would you measure latest sunrise and earliest sunset?
chewi
Well-Known Member
I think I understand you now Earlybird.
I think you are saying that if we were measuring the sunset/sunrise in true solar time that there would be no offset, and it's clock time that creates an apparent offset by predicting sunrise/sunset under its flawed approximation to solar time.
So I wonder does the offset really exist?
I think you are saying that if we were measuring the sunset/sunrise in true solar time that there would be no offset, and it's clock time that creates an apparent offset by predicting sunrise/sunset under its flawed approximation to solar time.
So I wonder does the offset really exist?
earlybird
Well-Known Member
[ QUOTE ]
So I wonder does the offset really exist?
[/ QUOTE ]
The offset is, by definition, the difference between solar time and mean solar time, so yes, it does exist if you live by mean time, as society does. Otherwise, all you would observe is the shortest day, (solstice ).
So I wonder does the offset really exist?
[/ QUOTE ]
The offset is, by definition, the difference between solar time and mean solar time, so yes, it does exist if you live by mean time, as society does. Otherwise, all you would observe is the shortest day, (solstice ).
chewi
Well-Known Member
I meant between offset between the date of "latest" sunrise and the date of "earliest" sunset, not the offset between solar time and clocktime.
Does that date offset really exist or has clock time fabricated it?
Does that date offset really exist or has clock time fabricated it?
earlybird
Well-Known Member
[ QUOTE ]
Does that date offset really exist or has clock time fabricated it?
[/ QUOTE ]
Without clock time, the date offset wouldn't exist because there would be nothing to measure it against. However clock time, or mean time, is a necessary invention because a solar day varies +/- from 24hrs depending on the Earth's position in orbit. That would be inconvenient to live by:- eg 10 knts in your boat would give differing journey times for the same trip on different dates!
Does that date offset really exist or has clock time fabricated it?
[/ QUOTE ]
Without clock time, the date offset wouldn't exist because there would be nothing to measure it against. However clock time, or mean time, is a necessary invention because a solar day varies +/- from 24hrs depending on the Earth's position in orbit. That would be inconvenient to live by:- eg 10 knts in your boat would give differing journey times for the same trip on different dates!
alan_d
Well-Known Member
The date offset and the variation between mean time and solar time are both aspects of the same thing - see the link already given to the analemma website.
Sunrise and sunset are symmetrical about local solar noon, but this may differ from noon civil time because of longitude and because of the analemma, described by the equation of time. The shortest day is an astronomical phenomenon, but times of sunset and sunrise are man-made constucts and are measured from an arbitrary noon, not the astronomical solar noon for that day and position. What you are describing is the offset between clock noon and solar noon.
Alan
Sunrise and sunset are symmetrical about local solar noon, but this may differ from noon civil time because of longitude and because of the analemma, described by the equation of time. The shortest day is an astronomical phenomenon, but times of sunset and sunrise are man-made constucts and are measured from an arbitrary noon, not the astronomical solar noon for that day and position. What you are describing is the offset between clock noon and solar noon.
Alan
duncanmack
Well-Known Member
Quote
"Sunrise and sunset are symmetrical about local solar noon"
No they're not - except at the solstices.
From winter solstice to summer solstice the afternoons are longer.
From SS to WS the mornings are longer. Think about it.
"Sunrise and sunset are symmetrical about local solar noon"
No they're not - except at the solstices.
From winter solstice to summer solstice the afternoons are longer.
From SS to WS the mornings are longer. Think about it.
stephenh
Well-Known Member
Nice one - thank you, I never thought about that - obvious when pointed out, like most things, but then I always seem to miss the obvious......
chewi
Well-Known Member
That's only because of daylight savings time moving noon to suit society.
Sunset and sunrise are surely symmetrical about celestial noon all year round?, the earth has to spin the same angle from dawn to noon as noon to dusk.
Sunset and sunrise are surely symmetrical about celestial noon all year round?, the earth has to spin the same angle from dawn to noon as noon to dusk.
VicS
Well-Known Member
Isn't the answer to Malthouses question something to do with the fact that the earth's axis also gyrates slowly. A bit like when a gyroscope or any spinning top starts to wobble.
chewi
Well-Known Member
I think Earlybird was right , I just didn't understand him first time round, but the dawn and dusk must be symmetrical about local solar noon.
To manifest anything else the earth would have to change its spin speed on a daily basis, eg faster in the morning than in the afternoon ( from somebodys perspective).
I don't think that happens to any sufficient extent to affect time as we measure it (ie hr/ mins. not to atomic accuracy). It isn't factored into solar navigation...
So the am and pm will be equal about local solar noon.
To manifest anything else the earth would have to change its spin speed on a daily basis, eg faster in the morning than in the afternoon ( from somebodys perspective).
I don't think that happens to any sufficient extent to affect time as we measure it (ie hr/ mins. not to atomic accuracy). It isn't factored into solar navigation...
So the am and pm will be equal about local solar noon.
peterb
Well-Known Member
I think it's reasonable to suggest that sunrise and sunset are not usually perfectly symmetrical about solar noon.
Think about conditions in spring, when the days are getting longer. Because the day is lengthening, the afternoon will be slightly longer than the morning. Similarly, in autumn the mornings will be longer than the afternoons. You'll only get true symmetry at the solstices.
Think about conditions in spring, when the days are getting longer. Because the day is lengthening, the afternoon will be slightly longer than the morning. Similarly, in autumn the mornings will be longer than the afternoons. You'll only get true symmetry at the solstices.
duncanmack
Well-Known Member
[ QUOTE ]
I think Earlybird was right , I just didn't understand him first time round, but the dawn and dusk must be symmetrical about local solar noon.
They're not.
To manifest anything else the earth would have to change its spin speed on a daily basis, eg faster in the morning than in the afternoon ( from somebodys perspective).
Rotation speed doesn't vary, but tilt does.
I don't think that happens to any sufficient extent to affect time as we measure it (ie hr/ mins. not to atomic accuracy). It isn't factored into solar navigation...
It is factored into solar nav - look at your almanac. Compare sunrise and sunset times between March and September.
So the am and pm will be equal about local solar noon.
[/ QUOTE ]
Not according to the almanac. /forums/images/graemlins/grin.gif
If you counter that the UT is a mean time, then I refer you to Peterb's post. He has worked it out! /forums/images/graemlins/grin.gif
Peter! Have a /forums/images/graemlins/smirk.gif
I think Earlybird was right , I just didn't understand him first time round, but the dawn and dusk must be symmetrical about local solar noon.
They're not.
To manifest anything else the earth would have to change its spin speed on a daily basis, eg faster in the morning than in the afternoon ( from somebodys perspective).
Rotation speed doesn't vary, but tilt does.
I don't think that happens to any sufficient extent to affect time as we measure it (ie hr/ mins. not to atomic accuracy). It isn't factored into solar navigation...
It is factored into solar nav - look at your almanac. Compare sunrise and sunset times between March and September.
So the am and pm will be equal about local solar noon.
[/ QUOTE ]
Not according to the almanac. /forums/images/graemlins/grin.gif
If you counter that the UT is a mean time, then I refer you to Peterb's post. He has worked it out! /forums/images/graemlins/grin.gif
Peter! Have a /forums/images/graemlins/smirk.gif
