Equinox confusion

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Gwenno
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Equinox confusion

Post by Gwenno »

I always believed that , as the name suggests, that on the day of either equinox, Sunrise to sunset would be 12 hours. However, according to the BBC weather page for my area, today we have 12 hours and 1 minute Sunrise to sunset, and on 21st of March we have 12 hours 13. Is Paddy's day the equinox this year? Is it in fact the day when the sun lies exactly vertically above the equator at 12 noon, i.e. Not necessarily when sunrise to sunset is 12 hours?
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Clogs
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Re: Equinox confusion

Post by Clogs »

Gwenno wrote:I always believed that , as the name suggests, that on the day of either equinox, Sunrise to sunset would be 12 hours. However, according to the BBC weather page for my area, today we have 12 hours and 1 minute Sunrise to sunset, and on 21st of March we have 12 hours 13. Is Paddy's day the equinox this year? Is it in fact the day when the sun lies exactly vertically above the equator at 12 noon, i.e. Not necessarily when sunrise to sunset is 12 hours?

Yes, but also no.
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tabascoboy
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Re: Equinox confusion

Post by tabascoboy »

Atmospheric refraction of sunlight for one thing.
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inactionman
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Re: Equinox confusion

Post by inactionman »

Might be worth factoring in that the sun is a large sphere, not a point source of light.
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Re: Equinox confusion

Post by TB63 »

inactionman wrote:Might be worth factoring in that the sun is a large sphere, not a point source of light.
Tell that to the flattards. .
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Fangle
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Re: Equinox confusion

Post by Fangle »

inactionman wrote:Might be worth factoring in that the sun is a large sphere, not a point source of light.
And the earth is not completely spherical.
OB..
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Re: Equinox confusion

Post by OB.. »

On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the Sun, atmospheric refraction, and the rapidly changing duration of the length of day that occurs at most latitudes around the equinoxes.
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Yer Man
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Re: Equinox confusion

Post by Yer Man »

https://en.wikipedia.org/wiki/Twilight# ... l_twilight

Twilight on Earth is the illumination of the lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth's surface is neither completely lit nor completely dark. The word twilight is also used to denote the periods of time when this illumination occurs.[2]
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lilyw
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Re: Equinox confusion

Post by lilyw »

Yer Man wrote:https://en.wikipedia.org/wiki/Twilight# ... l_twilight

Twilight on Earth is the illumination of the lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth's surface is neither completely lit nor completely dark. The word twilight is also used to denote the periods of time when this illumination occurs.[2]
Crepuscular - one of the truly great words in English
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A5D5E5
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Re: Equinox confusion

Post by A5D5E5 »

Fangle wrote:
inactionman wrote:Might be worth factoring in that the sun is a large sphere, not a point source of light.
And the earth is not completely spherical.
Yes, but this is often overstated. There is only about 0.25% difference between the polar and equatorial radius so I doubt it makes any difference.
Gwenno
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Re: Equinox confusion

Post by Gwenno »

I accept stuff about twilight etc which is why I specified time of sunrise to sunset, rather than amount of 'daylight' . I like the Wikipedia explanation which amounts to 'it is because it is' - fair enough :lol:
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6.Jones
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Re: Equinox confusion

Post by 6.Jones »

A5D5E5 wrote:
Fangle wrote:
inactionman wrote:Might be worth factoring in that the sun is a large sphere, not a point source of light.
And the earth is not completely spherical.
Yes, but this is often overstated. There is only about 0.25% difference between the polar and equatorial radius so I doubt it makes any difference.
We could calculate that. It'd make a change from calculating mortality rates on the Coronavirus thread.
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message #2527204
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Re: Equinox confusion

Post by message #2527204 »

lilyw wrote:
Yer Man wrote:https://en.wikipedia.org/wiki/Twilight# ... l_twilight

Twilight on Earth is the illumination of the lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth's surface is neither completely lit nor completely dark. The word twilight is also used to denote the periods of time when this illumination occurs.[2]
Crepuscular - one of the truly great words in English
The French word for dusk
Yer Man
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Re: Equinox confusion

Post by Yer Man »

Gwenno wrote:I accept stuff about twilight etc which is why I specified time of sunrise to sunset, rather than amount of 'daylight' . I like the Wikipedia explanation which amounts to 'it is because it is' - fair enough :lol:
The simple answer is the bending of the sun's rays in the atmosphere.
The effect is that we can "see" the Sun a few minutes before it actually peeks above the horizon - and for an extra few minutes after it dips below.
If we had no atmosphere then the day wouldn't be stretched at both ends.
Gwenno
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Re: Equinox confusion

Post by Gwenno »

Yer Man wrote:
Gwenno wrote:I accept stuff about twilight etc which is why I specified time of sunrise to sunset, rather than amount of 'daylight' . I like the Wikipedia explanation which amounts to 'it is because it is' - fair enough :lol:
The simple answer is the bending of the sun's rays in the atmosphere.
The effect is that we can "see" the Sun a few minutes before it actually peeks above the horizon - and for an extra few minutes after it dips below.
If we had no atmosphere then the day wouldn't be stretched at both ends.
Would that account for the 16 minutes extra on 21/3/20? I think this year it is officially 20/3/20, and the difference is still 13 minutes.
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LandOTurk
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Re: Equinox confusion

Post by LandOTurk »

lilyw wrote:
Yer Man wrote:https://en.wikipedia.org/wiki/Twilight# ... l_twilight

Twilight on Earth is the illumination of the lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth's surface is neither completely lit nor completely dark. The word twilight is also used to denote the periods of time when this illumination occurs.[2]
Crepuscular - one of the truly great words in English
Latin of course. Crepuscule is dusk.

Aren't there 3 types of dawn and dusk? Nautical and ??
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LandOTurk
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Re: Equinox confusion

Post by LandOTurk »

LandOTurk wrote:
lilyw wrote:
Yer Man wrote:https://en.wikipedia.org/wiki/Twilight# ... l_twilight

Twilight on Earth is the illumination of the lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth's surface is neither completely lit nor completely dark. The word twilight is also used to denote the periods of time when this illumination occurs.[2]
Crepuscular - one of the truly great words in English
Latin of course. Crepuscule is dusk.

Aren't there 3 types of dawn and dusk? Nautical and ??
Wiki

Technically, the three stages of dusk are as follows.

At civil dusk, the center of the Sun's disc goes 6° below the horizon in the evening. It marks the end of civil twilight, which begins at sunset. At this time objects are still distinguishable and depending on weather conditions some stars and planets may start to become visible to the naked eye. The sky has many colors at this time, such as orange and red. Beyond this point artificial light may be needed to carry out outdoor activities, depending on atmospheric conditions and location.
At nautical dusk, the Sun apparently moves to 12° below the horizon in the evening. It marks the end of nautical twilight, which begins at civil dusk. At this time, objects are less distinguishable, and stars and planets appear to brighten.
At astronomical dusk, the Sun's position is 18° below the horizon in the evening. It marks the end of astronomical twilight, which begins at nautical dusk. After this time the Sun no longer illuminates the sky, and thus no longer interferes with astronomical observations
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A5D5E5
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Re: Equinox confusion

Post by A5D5E5 »

6.Jones wrote:
A5D5E5 wrote:
Fangle wrote:
inactionman wrote:Might be worth factoring in that the sun is a large sphere, not a point source of light.
And the earth is not completely spherical.
Yes, but this is often overstated. There is only about 0.25% difference between the polar and equatorial radius so I doubt it makes any difference.
We could calculate that. It'd make a change from calculating mortality rates on the Coronavirus thread.
Well some people could, but not me - for some reason I just can't get my head around celestial mechanics.

On an almost completely unrelated point, but for some reason typing celestial mechanics prompted me to remember it - James Clark Maxwell won a prize for solving the problem of the stability of Saturn's rings. At the time it was unknown whether the rings were solid, fluid or lots of individual lumps. He proved it was the latter by deriving and solving the differential equations and showing that other constructions were unstable. An early taster of his prowess with this branch of mathematics which has always wowed me.
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