APOD: The Sky in 2021 (2022 Mar 21)

[APOD Robot]

The Sky in 2021

Explanation: What if you could see the entire sky -- all at once -- for an entire year? That, very nearly, is what is pictured here. Every 15 minutes during 2021, an all-sky camera took an image of the sky over the Netherlands. Central columns from these images were then aligned and combined to create the featured keogram, with January at the top, December at the bottom, and the middle of the night running vertically just left of center. What do we see? Most obviously, the daytime sky is mostly blue, while the nighttime sky is mostly black. The twelve light bands crossing the night sky are caused by the glow of the Moon. The thinnest part of the black hourglass shape occurs during the summer solstice when days are the longest, while the thickest part occurs at the winter solstice. Yesterday was an equinox -- when night and day were equal -- and the northern-spring equinox from one year ago can actually be located in the keogram -- about three-quarters of the way up.

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[XgeoX]

Very apropos that it is an hourglass shape!

“ Yesterday was an equinox -- when night and day were equal “
Let me fix that…
“Yesterday was an equinox -- when night and day were almost equal “
Much better.

The refraction of the sun by the atmosphere adds a few minutes, give or take, depending on your latitude which means more daylight than night!


Eric

[Ann]

Very apropos that it is an hourglass shape!

“ Yesterday was an equinox -- when night and day were equal “
Let me fix that…
“Yesterday was an equinox -- when night and day were almost equal “
Much better.

The refraction of the sun by the atmosphere adds a few minutes, give or take, depending on your latitude which means more daylight than night!


Eric

Quite so, Eric.

Please note the Blue Hour:

The Sky in 2021. Image Credit & Copyright: Cees Bassa (Netherlands Institute for Radio Astronomy)

---

APOD 21 March 2022 Blue Hour Netherlands annotated.png

The Blue Hour.

Note that the Blue Hour is always blue, regardless of whether the sky is clear or overcast.

Ann

[rstevenson]

Thanks Ann. I came here to ask why the equinox day was clearly larger than the night in this image, but including the blue hour evens it out nicely.

Rob

[Ann]

Thanks Ann. I came here to ask why the equinox day was clearly larger than the night in this image, but including the blue hour evens it out nicely.

Rob

You're welcome, Rob!

As for why the equinox day is longer than the equinox night, I believe it is because the Earth's atmosphere reflects daylight back over the horizon even after the Sun has set (or after a given spot on the Earth has rotated away from the Sun into the Earth's shadow, so to speak).

If the Earth hadn't had an atmosphere, the equinox day and night would indeed be the same length.

Ann

[Dan]

Latitude plays an important factor here. This was not made whilst on the equator.

[orin stepanek]

YearOfSky_Bassa_960_annotated.jpg

Today's picture is pretty neat!

artworks-000064777927-e3ahj5-t500x500.jpg

Kitty is being funny; but is pretty neat also!

[Chris Peterson]

Thanks Ann. I came here to ask why the equinox day was clearly larger than the night in this image, but including the blue hour evens it out nicely.

Rob

You're welcome, Rob! :D

As for why the equinox day is longer than the equinox night, I believe it is because the Earth's atmosphere reflects daylight back over the horizon even after the Sun has set (or after a given spot on the Earth has rotated away from the Sun into the Earth's shadow, so to speak).

If the Earth hadn't had an atmosphere, the equinox day and night would indeed be the same length.

Even then, if you want to get all picky (which is what I think the refraction issue is, as well), no. Because the equinox isn't a day, it's an instant. So what you really have is a pair of days on either side of it, each with slightly different day/night balances. And, of course, things are complicated by the fact that the Sun isn't a point source. The common convention is that "day" begins when the upper limb of the Sun contacts the horizon, and ends when it disappears below the horizon. So that tweaks the actual day length, as well.

[jan Tuli]

Isn't it interesting that an image that depicts time so uniquely is shaped like an hourglass

[Ann]

Isn't it interesting that an image that depicts time so uniquely is shaped like an hourglass

Analemma. Image: Giuseppe Donatiello.

---

The hourglass shape of the day- and night-time sky from the Netherlands in 2021 reminds me of an analemma. Of course, an analemma is shaped like a figure eight and not like an hourglass.

Wikipedia wrote:

In astronomy, an analemma (/ˌænəˈlɛmə/; from Ancient Greek ἀνάλημμα (analēmma) 'support')[a] is a diagram showing the position of the Sun in the sky as seen from a fixed location on Earth at the same mean solar time, as that position varies over the course of a year. The diagram will resemble a figure eight. Globes of Earth often display an analemma as a two-dimensional figure of equation of time vs. declination of the Sun.

The north–south component of the analemma results from the change in the Sun's declination due to the tilt of Earth's axis of rotation. The east–west component results from the nonuniform rate of change of the Sun's right ascension, governed by combined effects of Earth's axial tilt and orbital eccentricity.

Well... that's all Greek to me. But the picture is nice.

Ann

[Fred the Cat]

Very apropos that it is an hourglass shape!

“ Yesterday was an equinox -- when night and day were equal “
Let me fix that…
“Yesterday was an equinox -- when night and day were almost equal “
Much better.

The refraction of the sun by the atmosphere adds a few minutes, give or take, depending on your latitude which means more daylight than night!


Eric

Quite so, Eric

Note that the Blue Hour is always blue, regardless of whether the sky is clear or overcast.

Ann

"Like sands through the hour glass,

so are the days of our lives Days of our Lives."

The blue hour confines the chaos outside the night,

IMG_3777 (2).JPG

as out of focus, as reflections - frozen in time as ice.

[MarkBour]

Very apropos that it is an hourglass shape! . . .

Eric

Ahh, you are a person focused on the night.
To a daylight-focused person, it looks like a bulge.

Okay, honestly, regardless of whether you pick day or night,
the long-term pattern for Mid-Latitude day/night lengths is:

DayLengthMidLatitudes.png

and you only get an hourglass if you crop it in a particular way.

[XgeoX]

Very apropos that it is an hourglass shape! . . .

Eric

Ahh, you are a person focused on the night.
To a daylight-focused person, it looks like a bulge.

Okay, honestly, regardless of whether you pick day or night,
the long-term pattern for Mid-Latitude day/night lengths is:

DayLengthMidLatitudes.png

and you only get an hourglass if you crop it in a particular way.

If you crop it for 18 months you get a sexy lady!

Eric

[MarkBour]

If you crop it for 18 months you get a sexy lady!
Eric

I don't know the definition of keogram. A quick search does not find it in a dictionary or Wikipedia or Wiktionary. I wonder who coined the term, and if it is named after someone, or if "keo" is a reference to something. Some of the links in the caption give a definition by example. From them, I would say that the definition could be: a form of image sampling that plots one part of a set of still images (consistently the same part, such as one given vertical column), and then plots a sequence of these parts on the plane using one axis as a time axis.

So, the examples that were given of keograms in the links took a given column out of a bunch of photos, say one every 5 minutes over the course of a day and lined them up, placing each different photo sample from left to right. A single day example is below.

AllSkEye User Documentation has a lovely description at:
https://allskeye.com/userdocs/index.htm ... nction.htm
with these images:

Capture1.PNG

The top image shows one moment from an all-sky camera and a vertical column that is chosen as the sampled set. That same vertical column gets placed in the image below, and it moves to the right and does it again and again over the course of 24 hours.

But if that is a keogram, then, today's APOD is a keogram of keograms. Perhaps we'd call it a keogram-squared, or maybe a keokeogram. It has taken a set of keograms, one for each day of the year, taken a horizontal slice consistently from each of them and plotted time again, now on the vertical axis as we go through the year.

It's fun to look at this. Two of the most obvious things were pointed out in the caption -- (1) you can see the change in the times of day/night over the course of the year, locating the equinoxes and solstices. (2) You can see the presence of the Moon.

Other things I can see are to watch the changes in the brightness of the moon and its timing from night to night. And there are a couple of lone spots in a few nights that look like stars. It's hard to say, but perhaps those nights managed to capture a bright star or a planet in their sample.

Actually, as I was exploring this and realizing you can seem more if you look at it a while, I came across the nice twitter thread from Cees Bassa, the creator of this APOD image, which I could have found if I had just clicked on his name in the credits of today's APOD. (https://twitter.com/cgbassa/status/1479480674277068800)

A NOTE OF CORRECTION: The APOD caption says that today's image involved a sample once every 15 minutes, but according to the author's thread, it was actually once every 15 seconds. So this APOD involves a bit of data from 2.1 million images!