APOD: Equinox on a Spinning Earth (2021 Sep 22)

[APOD Robot]

Equinox on a Spinning Earth

Explanation: When does the line between night and day become vertical? Today. Today 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 featured time-lapse video demonstrates this by displaying an entire year on planet Earth in twelve seconds. From geosynchronous orbit, the Meteosat 9 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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[gmPhil]

Vertical?! hmm.. ok, we know what you mean

[orin stepanek]

giphy.gif (5.91 MiB) Viewed 2988 times

Happy Equal knosks everyone!

[mfavret]

Hi!
I'm french and I hope that you will excuse my poor English.
Vertical ?!?
Vertical is a line oriented by the gravitational field of Earth. Rising from the center of our planet, then perpendicular to a horizontal ground. There is nothing like that when you look at the Terminator on earth from space.
Hum, a line from the bottom up ? But in space there is no bottom neither up ! Why showing Earth with north pole up and South pole down ? There is absolutely no Reason. In Australia, I saw a globe with South pole up and it's a very different point of view.
We must remember that this type of orientation (north pole up) is absolutely cultural, because dominating nations (Europe first, then USA) are in the north hemisphere and they forced all the World to adopt their own point of view.
The only good way to explain equinoxe need reference to north and South pole. Vertical as Nothing to do with that, except to consider that Sun is exactely on the vertical from equator ! ;o)
Michel FAVRET

[neufer]

Vertical ?!?

Vertical is a line oriented by the gravitational field of Earth. Rising from the center of our planet, then perpendicular to a horizontal ground. There is nothing like that when you look at the Terminator on earth from space.
Hum, a line from the bottom up ? But in space there is no bottom neither up ! Why showing Earth with north pole up and South pole down ? There is absolutely no Reason. In Australia, I saw a globe with South pole up and it's a very different point of view.
We must remember that this type of orientation (north pole up) is absolutely cultural, because dominating nations (Europe first, then USA) are in the north hemisphere and they forced all the World to adopt their own point of view.
The only good way to explain equinoxe need reference to north and South pole. Vertical as Nothing to do with that, except to consider that Sun is exactely on the vertical from equator ! ;o)
Michel FAVRET

• Vertical in the video.
  Vertical in the ecliptic coordinate system.

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<<The ecliptic is the plane of Earth's orbit around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the background of stars. The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System bodies have orbits with only slight inclinations to the ecliptic, using it as the fundamental plane is convenient. The system's origin can be the center of either the Sun or Earth, its primary direction is towards the vernal (March) equinox, and it has a right-hand convention. It may be implemented in spherical or rectangular coordinates.>>

[Chris Peterson]

Hi!
I'm french and I hope that you will excuse my poor English.
Vertical ?!?
Vertical is a line oriented by the gravitational field of Earth. Rising from the center of our planet, then perpendicular to a horizontal ground. There is nothing like that when you look at the Terminator on earth from space.
Hum, a line from the bottom up ? But in space there is no bottom neither up ! Why showing Earth with north pole up and South pole down ? There is absolutely no Reason. In Australia, I saw a globe with South pole up and it's a very different point of view.
We must remember that this type of orientation (north pole up) is absolutely cultural, because dominating nations (Europe first, then USA) are in the north hemisphere and they forced all the World to adopt their own point of view.
The only good way to explain equinoxe need reference to north and South pole. Vertical as Nothing to do with that, except to consider that Sun is exactely on the vertical from equator ! ;o)
Michel FAVRET

While there might be more technically accurate descriptions (lying on a line of longitude, lying on a great circle that passes through the poles), I'd wager that in the context of this video, "vertical" makes sense to far more of APOD's audience than any other terminology.

[johnnydeep]

I don't know why I have such a problem with this. And using "vertical" to describe what's going on is a terrible choice IMHO.

My way of understanding this is that the plane of the terminator is always perpendicular to the plane of Earth's orbit around the Sun (the ecliptic plane). But only at the equinoxes does the plane of the terminator also intersect the Earth's north-south pole.

[Chris Peterson]

I don't know why I have such a problem with this. And using "vertical" to describe what's going on is a terrible choice IMHO.

My way of understanding this is that the plane of the terminator is always perpendicular to the plane of Earth's orbit around the Sun (the ecliptic plane). But only at the equinoxes does the plane of the terminator also intersect the Earth's north-south pole.

That's true. And an awful lot of people would likely scratch their heads for a long time trying to work out what it means.

[johnnydeep]

I don't know why I have such a problem with this. And using "vertical" to describe what's going on is a terrible choice IMHO.

My way of understanding this is that the plane of the terminator is always perpendicular to the plane of Earth's orbit around the Sun (the ecliptic plane). But only at the equinoxes does the plane of the terminator also intersect the Earth's north-south pole.

That's true. And an awful lot of people would likely scratch their heads for a long time trying to work out what it means.

Scratching my head was exactly what I was doing trying to understand the video and what "vertical" meant

[neufer]

I don't know why I have such a problem with this. And using "vertical" to describe what's going on is a terrible choice IMHO. My way of understanding this is that the plane of the terminator is always perpendicular to the plane of Earth's orbit around the Sun (the ecliptic plane). But only at the equinoxes does the plane of the terminator also intersect the Earth's north-south pole.

That's true. And an awful lot of people would likely scratch their heads for a long time trying to work out what it means.

Scratching my head was exactly what I was doing trying to understand the video and what "vertical" meant

The (spring equinox marble egg) ray of light on March 25

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The Pisa Cathedral, with the Leaning Tower of Pisa.

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Huygens isochronous cycloidal pendula with different amplitudes.

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<<At the end of the 10th century Pisa established March 25 as the beginning of its new year. This date was considered very important because it is both the Feast of the Annunciation (9 months before December 25) and it falls very close to the spring equinox. To mark the beginning of the Pisan new year a system was devised in the Pisa Cathedral whereby a beam of light shines through a round window on the south side of the nave and, precisely at noon on March 25, lands on the same spot every year: on top of a shelf affixed to a pylon on the opposite side of the church. This shelf rests on a marble egg, a symbol of birth and new life.

The lamp at the center of the nave is called Galileo's lamp, because a legend says that the great scientist formulated his theory of isochronism of the pendulum while watching its oscillations from the roof of the nave. (In 1673, Christiaan Huygens showed that pendulums needed to move on a cycloid path in order to be isochronous.)

Between 1589 and 1592, Galileo Galilei, who lived in Pisa, is said to have dropped two cannonballs of different masses from the tower to demonstrate that their speed of descent was independent of their mass, in keeping with the law of free fall. The primary source for this is the biography Racconto istorico della vita di Galileo Galilei (Historical Account of the Life of Galileo Galilei), written by Galileo's pupil and secretary Vincenzo Viviani in 1654, but only published in 1717. (The first Grand Masonic Lodge was founded on St John's (Midsummer's) Day, 24 June 1717.)>>