Starship Asterisk*

Equinox on a Spinning Earth

Explanation: When does the line between day and night become vertical? Tomorrow. Tomorrow is an equinox on planet Earth, a time of year when day and night are most nearly equal. At an equinox, the Earth's terminator -- the dividing line between day and night -- becomes vertical and connects the north and south poles. The above time-lapse video demonstrates this by displaying an entire year on planet Earth in twelve seconds. From geosynchronous orbit, the Meteosat satellite recorded these infrared images of the Earth every day at the same local time. The video started at the September 2010 equinox with the terminator line being vertical. As the Earth revolved around the Sun, the terminator was seen to tilt in a way that provides less daily sunlight to the northern hemisphere, causing winter in the north. As the year progressed, the March 2011 equinox arrived halfway through the video, followed by the terminator tilting the other way, causing winter in the southern hemisphere -- and summer in the north. The captured year ends again with the September equinox, concluding another of billions of trips the Earth has taken -- and will take -- around the Sun.

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The video is kind of confusing. I mean, I get it, but I think that it might be more clear if the shadow were stabilized in a vertical position and Earth were shown tilting. Stabilizing Earth makes it look as if the light source, Sun, is going up and down. Hmm, but it is interesting to see it presented like this. The advantage for this method is that the relationship between day length in the two hemispheres is very evident and we can see the poles becoming completely darkened during their respective winters.

The line between day and night is always vertical!!!

"vertical" is definitely not the right word.
Vertical means perpendicular to the surface of the geoid (aligned with the gradient of the gravity field), or pointing to the center of mass of the Earth.
Therefore the terminator is always vertical (assuming the earth is a sphere, the plane of the terminator always cuts this sphere in two halves).

PhM wrote:The line between day and night is always vertical!!!

"vertical" is definitely not the right word.
Vertical means perpendicular to the surface of the geoid (aligned with the gradient of the gravity field), or pointing to the center of mass of the Earth.
Therefore the terminator is always vertical (assuming the earth is a sphere, the plane of the terminator always cuts this sphere in two halves).

You are right, but so is APOD Robot. It depends on the context. When describing a 2-D image, it is acceptable to refer to the up-down direction as the vertical direction. In these images, it just so happens that the Earth's axis is vertical, and the terminator appears vertical at the equinoxes, because Meteosat has an elongation of 90° in each frame.

This is a fantastic video.

Actually, day and night are not equally long tomorrow. They aren't even equally long today, because at least where I live, the sun rises at 6.13 a.m. and sets at 6.19 p.m. So today where I live, the Sun is above the horizon for twelve hours and six minutes.

I think this has to do with how the Earth's atmosphere scatters sunlight in such a way that the Sun is seen to be above the horizon when in fact it is just below it.

Ann

Ann wrote:Actually, day and night are not equally long tomorrow. They aren't even equally long today, because at least where I live, the sun rises at 6.13 a.m. and sets at 6.19 p.m. So today where I live, the Sun is above the horizon for twelve hours and six minutes.

I think this has to do with how the Earth's atmosphere scatters sunlight in such a way that the Sun is seen to be above the horizon when in fact it is just below it.

Ann

I'm not sure why but our equal night/day was on St. Pat's day when AM and PM were both at 6:37!

Ann wrote:Actually, day and night are not equally long tomorrow. They aren't even equally long today, because at least where I live, the sun rises at 6.13 a.m. and sets at 6.19 p.m. So today where I live, the Sun is above the horizon for twelve hours and six minutes.

I think this has to do with how the Earth's atmosphere scatters sunlight in such a way that the Sun is seen to be above the horizon when in fact it is just below it.

Ann

The Northern Hemisphere gets very nearly the same amount of daylight as the Southern at the equinoxes.

If sunrise and sunset were defined by the centre of the Sun on the horizon (elevation angle 0°), and there was no atmosphere, then day would be equal to night across Earth at the equinoxes. Edit: this also assumes that day and night are defined precisely by sunrise and sunset, which is somewhat arbitrary.

It took a second for my brain to adjust to the image, but what a unique perspective! I love it how we can extract art from the cold logic of science and technology. Humans are great.

Magnificent, and even more interesting: the weather patterns over the coasts of Namibia, around Madagascar, Erythrea, and just about one hundred other etc(s); the momentum of the Sahara anticyclone, etc...

Great visual effect! Shadow and light moving across a spinning globe.

FloridaMike wrote:It took a second for my brain to adjust to the image, but what a unique perspective! I love it how we can extract art from the cold logic of science and technology. Humans are great.

"Cold logic of science and technology"? Rubbish. More like spirit warming beauty.

Art is just our feeble attempt to interpret nature in different ways. We don't extract art from scenes of nature, but rather, glimpse a part of the Universe as it really is.

Chris Peterson wrote:

FloridaMike wrote:It took a second for my brain to adjust to the image, but what a unique perspective! I love it how we can extract art from the cold logic of science and technology. Humans are great.

"Cold logic of science and technology"? Rubbish. More like spirit warming beauty.

Art is just our feeble attempt to interpret nature in different ways. We don't extract art from scenes of nature, but rather, glimpse a part of the Universe as it really is.

The Mona Lisa and Flight of the Valkyries is just our feeble attempt to interpret nature in different ways? Rubbish. Art connects to something in us far older and deeper than our logic and this piece is magnificent. Not because it is an intrinsically beautiful image, it is not. This piece is magnificent because it allows us to connect a logical understanding of the cold hard facts to a deeper Human experience.

FloridaMike wrote:The Mona Lisa and Flight of the Valkyries is just our feeble attempt to interpret nature in different ways?

Yup.

Chris Peterson wrote:
Art is just our feeble attempt to interpret nature in different ways.

I resemble that remark.

Vertical means perpendicular to the surface of the geoid (aligned with the gradient of the gravity field), or pointing to the center of mass of the Earth. Therefore the terminator is always vertical (assuming the earth is a sphere, the plane of the terminator always cuts this sphere in two halves).

I think they mean vertical to the ecliptic plane of the solar system.

therm wrote:Vertical means perpendicular to the surface of the geoid (aligned with the gradient of the gravity field), or pointing to the center of mass of the Earth. Therefore the terminator is always vertical (assuming the earth is a sphere, the plane of the terminator always cuts this sphere in two halves).

The Earth is only a sphere in one sense. We frequently are interested in just its surface, which is a non-Euclidian plane, a two dimensional surface. In that frame, "vertical" is readily interpreted as a line that passes through each pole.

Imagine yourself atop a precipice overlooking the vastness of a natural landscape. A sunrise to your east is illuminating the world in golden hues, birds are sounding off around you, and an ancient meandering river is just discernible through a morning fog clinging to the valley below. Behind you is a famous Turner painting on a stand of some sort. It's a masterpiece. You get to choose one to keep and view for the rest of your life. Which do you choose?

A painting offers two paltry dimensions and its meaning and beauty are derived from a fleeting and mutable pattern of neural signals confined within one's cranium. It is a momentary hijacking of the credit for what nature has spent a few billion years on. The artist's finest brush strokes are haphazard piles of molecules compared to the intricate microscopic arrangements found within a single leaf. Still, it is somewhat amazing that nature has concocted a way to observe and imitate itself by paintings or mathematical formulas, feeble as they may be.

(Moments after you make your decision, a bear plows through the painting, scares the crap out of you and you jump off the cliff to your death. Oops.)

geckzilla wrote:(Moments after you make your decision, a bear plows through the painting, scares the crap out of you and you jump off the cliff to your death. Oops.)

But fortunately, it is witnessed by a competent artist standing on yet a higher cliff, who immortalizes the moment on another canvas, capturing again a fleeting glimpse of the beauty of nature.

The funny side appears. That was good Chris.

Was there or will there ever be a time when precession would allow the Earth to spin at a 90 degrees angle to the sun for longer than mere moments thus eliminating the seasons?

I had read one day tidal forces will also affect the Earth so it too will be locked with the same side facing of the Earth facing the moon.

http://www.haydenplanetarium.org/tyson/ ... idal-force

Will this process affect precession possibly, one day, making both the tides and the seasons a thing of the past?

Ron-Astro Pharmacist wrote:Was there or will there ever be a time when precession would allow the Earth to spin at a 90 degrees angle to the sun for longer than mere moments thus eliminating the seasons?

No. First of all, precession isn't the issue. All precession does is change the direction the Earth's axis tilts, not the angle. The angle, currently about 23°, does vary somewhat over long periods, but not more than couple of degrees.

I had read one day tidal forces will also affect the Earth so it too will be locked with the same side facing of the Earth facing the moon.

Probably not. First of all, the oceans will evaporate in a couple of billion years, reducing most of the tidal effects. And in another couple of billion, the Earth and Moon will be inside the solar radius, something that neither is likely to survive. These things will occur sooner than tidal locking could.

Chris Peterson wrote:

FloridaMike wrote:The Mona Lisa and Flight of the Valkyries is just our feeble attempt to interpret nature in different ways?

Yup.

I think you are confusing one of the things that art does with a definition of what art is.

FloridaMike wrote:I think you are confusing one of the things that art does with a definition of what art is.

Who knows? In my view, humans are native engineers, and art is just one form of engineering.

Is it me or does anyone else notice lines in the dark (western hemisphere) part of the video after it plays?? It looks like a bunch of rings of saturn!! Perhaps there is some ghost anomalies the satelite picked up that weren't immediately visible to the naked eye

If this were an art theory discussion board and I were the moderator, my finger would be itching over the delete button.

Chris Peterson wrote:

FloridaMike wrote:It took a second for my brain to adjust to the image, but what a unique perspective! I love it how we can extract art from the cold logic of science and technology. Humans are great.

"Cold logic of science and technology"? Rubbish. More like spirit warming beauty.

Science borrows from art and art borrows from science. Both are essential to humanity. Hot and cold are relative terms.

geckzilla wrote:Imagine yourself atop a precipice overlooking the vastness of a natural landscape. A sunrise to your east is illuminating the world in golden hues, birds are sounding off around you, and an ancient meandering river is just discernible through a morning fog clinging to the valley below. Behind you is a famous Turner painting on a stand of some sort. It's a masterpiece. You get to choose one to keep and view for the rest of your life. Which do you choose?

A painting offers two paltry dimensions and its meaning and beauty are derived from a fleeting and mutable pattern of neural signals confined within one's cranium. It is a momentary hijacking of the credit for what nature has spent a few billion years on. The artist's finest brush strokes are haphazard piles of molecules compared to the intricate microscopic arrangements found within a single leaf. Still, it is somewhat amazing that nature has concocted a way to observe and imitate itself by paintings or mathematical formulas, feeble as they may be.

Mainly since the advent of photography, the art of painting changed, to include depictions of abstraction, and other things not necessarily observable in nature. My house is full of bright, colourful impasto oil paintings of beautiful but "unnatural" depictions of nature. No less inspiring than my nature and astronomy photographs.

Anthony Barreiro wrote:If this were an art theory discussion board and I were the moderator, my finger would be itching over the delete button.

Yup.