Shortest Day

[swill453]

Where I live;

At the winter solstice there is 6 hours 56 mins light (between sunrise and sunset)

At the summer solstice there is 6 hours 22 mins dark (between sunset and sunrise)

Why the (quite big) difference? Is it due to the curvature of the Earth or some there reason?

Answering my own question it seems the difference is caused by refraction. The sun is visible above the horizon even when it is below it;

https://en.wikipedia.org/wiki/Atmospheric_refraction

Surely that doesn't affect the actual sunrise/sunset times, just the amount of light that is visible after sunset?

It probably explains why in the north of the country the twilight lasts much longer. In midsummer it doesn't really get dark for long.

Scott.

[bungeejumper]

Nice sunrise this morning, anyway. We can see right across Salisbury Plain from here. If you're interested, the Youtube Stonehenge video is at https://www.youtube.com/watch?v=fd2GHhnI4Ys .

BJ

[Mike4]

Nice sunrise this morning, anyway. We can see right across Salisbury Plain from here. If you're interested, the Youtube Stonehenge video is at https://www.youtube.com/watch?v=fd2GHhnI4Ys .

BJ

Thank you for that lovely video. I love the way the A303 snaking past with its steady traffic even at dawn today brings scale, humanity and contrast to the scene.

It will be SUCH a shame if/when they get rid of that road. Millions of people get to see and marvel at Stonehenge as they happen to pass by in their daily lives (including me every few months), and the proposed tunnel will put a stop to all that.

[moorfield]

Yes I guess you are correct but it does not immediately come to mind at the moment. Dull grey days make it even shorter.

Well look on the bright side (pun intended). From tomorrow the days are going to get longer

It doesn't start getting lighter in the mornings for about a fortnight (latest sunrise in London happens after December 30th).

The evenings have already started to get lighter though.

Yes it's often forgotten that although the days now get longer the mornings continue to rise later well into January. Another 3 minutes later by the 28th, falling back those 3 minutes again only by the 9th or 10th. The sun will be setting another 15-20 minutes later by then, but no-one really notices that much yet. Hence the January blues for SAD sufferers like myself.

[Dod101]

Living where I do, it is well into January before we notice any difference and by the end of the month we can say that the days are stretching a bit.

Dod

[Bubblesofearth]

Surely that doesn't affect the actual sunrise/sunset times, just the amount of light that is visible after sunset?

It probably explains why in the north of the country the twilight lasts much longer. In midsummer it doesn't really get dark for long.

Scott.

I think most tables of sunrise/sunset times allow for the refraction phenomenon, i..e they record the times the sun appears to rise and set. Otherwise the times I quoted in the previous post should be more or less the same.

Interestingly if you look at the difference near the equator it'a a lot less. For Singapore it's 15 mins compared to 34 mins here. Presumably explained by the more acute (shallower) angle of descent /ascent of the sun.

BoE

[swill453]

I think most tables of sunrise/sunset times allow for the refraction phenomenon, i..e they record the times the sun appears to rise and set.

It's possible you're right, but it doesn't seem plausible to me.

Scott.

[vrdiver]

I think most tables of sunrise/sunset times allow for the refraction phenomenon, i..e they record the times the sun appears to rise and set.

It's possible you're right, but it doesn't seem plausible to me.

Depends on which definition is being used: Civil, Nautical or Astronomical...

https://www.timeanddate.com/astronomy/different-types-twilight.html

VRD

[scotia]

I recollect fishing off Reiss beach (Caithness) in the late evening/early morning around the summer solstice. The Sun dipped down in the NW, leaving a glow which turned into sunrise on the NE. It never got really dark. On the golf course above the dunes, they play a round of golf around midnight on the longest day.
As for the shortest day - I recollect a ferociously cold Wild North-easter down at Wick harbour. I didn't share Charles Kingsley's welcoming words.

[swill453]

I think most tables of sunrise/sunset times allow for the refraction phenomenon, i..e they record the times the sun appears to rise and set.

It's possible you're right, but it doesn't seem plausible to me.

Depends on which definition is being used: Civil, Nautical or Astronomical...

https://www.timeanddate.com/astronomy/different-types-twilight.html

That seems to be definitions of twilight, not definitions of sunset.

BoE's assertion is that the sunset time itself is affected by refraction.

Scott.

[Bubblesofearth]

Depends on which definition is being used: Civil, Nautical or Astronomical...

https://www.timeanddate.com/astronomy/different-types-twilight.html

VRD

These definitions refer to the varying levels of light at different solar positions relative to the horizon. They could all apply to apparent (i.e. refracted) position.

BoE

[Bubblesofearth]

These definitions refer to the varying levels of light at different solar positions relative to the horizon. They could all apply to apparent (i.e. refracted) position.

BoE

The relevant Wiki link for refraction of sunlight;

https://en.wikipedia.org/wiki/Atmospheric_refraction

[vrdiver]

That seems to be definitions of twilight, not definitions of sunset.

BoE's assertion is that the sunset time itself is affected by refraction.

Scott.

From the link: https://www.timeanddate.com/astronomy/different-types-twilight.html

Civil dawn is the moment when the center of the Sun is 6 degrees below the horizon in the morning.

Nautical dawn occurs when the Sun is 12 degrees below the horizon during the morning.

Astronomical dawn is the time when the center of the Sun is at 18 degrees below the horizon.

Hope that helps.

VRD

[swill453]

From the link: https://www.timeanddate.com/astronomy/different-types-twilight.html

Civil dawn is the moment when the center of the Sun is 6 degrees below the horizon in the morning.

Nautical dawn occurs when the Sun is 12 degrees below the horizon during the morning.

Astronomical dawn is the time when the center of the Sun is at 18 degrees below the horizon.

Hope that helps.

Not really. If it used the words "sunrise" (or sunset) specifically then it might have helped.

(Though of course I don't expect it could.)

Scott.

[Lootman]

I recollect fishing off Reiss beach (Caithness) in the late evening/early morning around the summer solstice. The Sun dipped down in the NW, leaving a glow which turned into sunrise on the NE. It never got really dark. On the golf course above the dunes, they play a round of golf around midnight on the longest day.

I don't know if it is still a thing these days but back in the 1970s and 1980s it was common for students and young travellers to take an overland summer trek to North Cape in Norway, the northernmost point in Europe accessible by road. The road ends at a 1,000 foot high cliff which faces north to, well, nothing. A signpost there says something like "North Pole - 1,300 miles".

The sun of course never sets in the summer. For several weeks it just endlessly rotates just above the horizon. You lose all sense of day and night, and time. And as I recall it was one big party 24 hours a day, with kids from all over Europe.

By contrast I cannot imagine how bleak and miserable it must be there right now.

[vrdiver]

Not really. If it used the words "sunrise" (or sunset) specifically then it might have helped.

(Though of course I don't expect it could.)

Scott.

In that case you'd have to start with the twilight definitions:

Civil twilight occurs when the Sun is less than 6 degrees below the horizon. In the morning, civil twilight begins when the Sun is 6 degrees below the horizon and ends at sunrise. In the evening, it begins at sunset and ends when the Sun reaches 6 degrees below the horizon.

Whether that's an apparent or a real sunset (i.e. the refracted image of the sun or the actual sun) is not stated, so agreed, not as helpful as I thought!

However, I did find this from the Hong Kong Observatory*:
For example in Hong Kong, the atmospheric refraction causes the sunrise and sunset to appear about 2 minutes early and late respectively when compared to the situation without the atmospheric refraction. In fact, the times of sunrise and sunset announced by the Hong Kong Observatory have already included the effect of the atmospheric refraction so that users do not need to calculate the times by themselves.

Turning to the Greenwich Observatory** for a bit more help, I found the following definition:
The published times of sunrise and sunset refer to the moment when the Sun's upper limb, as affected by refraction, is on the true horizon of an observer at sea-level.

Further, it seems (and makes sense) that there is a "standard" refraction that's built into the sunrise / sunset tables, which differs from the actual refraction on the day, which varies according to air conditions, affected by temperature, cold air inversions etc. so the observer can record a different time to that expected anyway.

It appears to have been a known issue for a while.

EVALUATING THE EFFECTIVENESS OF CURRENT ATMOSPHERIC REFRACTION MODELS IN PREDICTING SUNRISE AND SUNSET TIMES***
While the author of the table is unnamed, indications are that Newton is the origin of the 34' value.

VRD


*https://www.hko.gov.hk/en/education/astronomy-and-time/astronomy/00493-effect-of-atmospheric-refraction-on-the-times-of-sunrise-and-sunset.html

**https://www.rmg.co.uk/stories/topics/when-dawn-dusk-twilight

***https://digitalcommons.mtu.edu/cgi/viewcontent.cgi?article=1776&context=etdr

[Nimrod103]

Nice sunrise this morning, anyway. We can see right across Salisbury Plain from here. If you're interested, the Youtube Stonehenge video is at https://www.youtube.com/watch?v=fd2GHhnI4Ys .

BJ

Thank you for that lovely video. I love the way the A303 snaking past with its steady traffic even at dawn today brings scale, humanity and contrast to the scene.

It will be SUCH a shame if/when they get rid of that road. Millions of people get to see and marvel at Stonehenge as they happen to pass by in their daily lives (including me every few months), and the proposed tunnel will put a stop to all that.

If the Druids wanted peace and quiet for their religious ceremonies, why did they build Stonehenge so close to the main road?

[1nvest]

Nice sunrise this morning, anyway. We can see right across Salisbury Plain from here. If you're interested, the Youtube Stonehenge video is at https://www.youtube.com/watch?v=fd2GHhnI4Ys .

BJ

Thank you for that lovely video. I love the way the A303 snaking past with its steady traffic even at dawn today brings scale, humanity and contrast to the scene.

It will be SUCH a shame if/when they get rid of that road. Millions of people get to see and marvel at Stonehenge as they happen to pass by in their daily lives (including me every few months), and the proposed tunnel will put a stop to all that.

If the Druids wanted peace and quiet for their religious ceremonies, why did they build Stonehenge so close to the main road?

Mick Jagger insisted as he didn't like rolling stones across grassland.

OT: https://thetruesize.com/ is a bit of fun - drag down countries to the equator for a indication of the true size relative to others. Dropping the US (light blue) next to Africa and dropping the UK on top of that

[Mike4]

Nice sunrise this morning, anyway. We can see right across Salisbury Plain from here. If you're interested, the Youtube Stonehenge video is at https://www.youtube.com/watch?v=fd2GHhnI4Ys .

BJ

Thank you for that lovely video. I love the way the A303 snaking past with its steady traffic even at dawn today brings scale, humanity and contrast to the scene.

It will be SUCH a shame if/when they get rid of that road. Millions of people get to see and marvel at Stonehenge as they happen to pass by in their daily lives (including me every few months), and the proposed tunnel will put a stop to all that.

If the Druids wanted peace and quiet for their religious ceremonies, why did they build Stonehenge so close to the main road?

Actually in BJ's link to the English Heritage live stream of the sunrise, I imagined we were going to see some sort of Druid ceremony worshipping the Winter Solstice sunrise oe something deep and meaningful.

But no, just a couple of hundred people randomly milling about, and kids playing!

[Dod101]

I recollect fishing off Reiss beach (Caithness) in the late evening/early morning around the summer solstice. The Sun dipped down in the NW, leaving a glow which turned into sunrise on the NE. It never got really dark. On the golf course above the dunes, they play a round of golf around midnight on the longest day.
As for the shortest day - I recollect a ferociously cold Wild North-easter down at Wick harbour. I didn't share Charles Kingsley's welcoming words.

Your summer story works when there is a lack of cloud cover but otherwise the best you can say is that it is as if God was holding a lantern above the clouds. I have not been in Caithness on the shortest day nor do I particularly want to. A wind of any sort would be welcome right now though, to blow away the cloud cover and mist we have had (in Perthshire) for the last several days.

Dod