<p>First, don't think that equinox is when the day and night are the same. It is the moment, when the sun's declination is zero, or when the sun is directly over the equator. Let me explain ...</p>
<p>Earth is always rotating around the Sun, so <a href="https://en.wikipedia.org/wiki/Declination" rel="nofollow noreferrer">declination</a> of the Sun is always changing (except at the solstices when it stops for a minute and goes in the other way). As @JohnHolst has pointed out, Sun's declination is changing for 1 angular minute per hour (mark 1 mm on the 3 m of distance – this is nearly 1 angular minute). An equinox doesn't last for a day. It lasts for a moment when the Earth is totally aligned and straight with the sun:
<em>(this figure shows the geocentric model, which is easier to represent and imagine)</em></p>
<p><a href="https://i.sstatic.net/OZffQ.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/OZffQ.png" alt="declination = 0" /></a></p>
<p>Also, Wikipedia says:</p>
<blockquote>
<p>An equinox is commonly regarded as the <strong>instant of time</strong> when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun. This occurs twice each year, around 20 March and 23 September. <strong>In other words, it is the <em>moment</em> at which the center of the visible Sun is directly above the equator.</strong></p>
</blockquote>
<p>P. S.</p>
<p>You can look <a href="https://astro.unl.edu/classaction/animations/coordsmotion/eclipticsimulator.html" rel="nofollow noreferrer">this simulation</a>.</p>

CC BY-SA 4.0
2019-05-04T15:40:56.390Z
My question is if this declination changes so much from the day of the equinox to the next. From the images online it is not easy to visualize this
Jim
CC BY-SA 4.0
2019-05-04T17:49:31.157Z
It changes a little bit, but enough, that it isn't equinox anymore.
User123
CC BY-SA 4.0
2019-05-04T18:43:23.233Z
This "little" that is enough is hard for me to visualize.
Jim
CC BY-SA 4.0
2019-05-05T01:10:41.047Z
@Jim. You are correct that the change from one day to another is not that large. But the definition of the equinox is not based on the length of daylight, or the sun being approximately over the equator. The definition is the Sun's declination is 0 degrees 0 arcminutes 0 arcseconds. At the time of the March equinox, the declination changes by approximately 1 arcminute per hour (or 1 arcsecond per minute of time). Thus, the time of the equinox can be stated very precisely.
JohnHoltz
CC BY-SA 4.0
2019-05-05T09:23:55.800Z
@JohnHoltz: This helps. Though `..equinox is not based on the length of daylight,..` but everywhere I read it say equinox means equal nights. So I thought that just based on the name it is about the length of daylight. Is this term a misnomer then and just kept for historical reasons? Also `the Sun's declination is 0 degrees... ` you mean  from the equator?
Jim
CC BY-SA 4.0
2019-05-05T09:25:44.470Z
@User123: `when the Earth is totally aligned and straight.` but the earth is always tilted.
Jim
CC BY-SA 4.0
2019-05-05T18:19:50.850Z
@Jim If this will help you: declination of the Polaris is always (near) 90 degrees - anywhere you are.
User123
CC BY-SA 4.0
2019-05-05T21:21:48.983Z
@Jim. You're correct that equinox originally meant "equal day and night". If this definition is used, and if the length of day is sunrise to sunset, then the date of the equinox would be different (by a few days) depend on your latitude, and the March equinox would occur before Mar 20. At some time in the past, the definition was changed to when the Sun is at 0 declination. (Declination is one of 2 celestial coordinates, and 0 declination is directly over the Earth's equator.)
JohnHoltz
CC BY-SA 4.0
2019-05-06T09:12:24.270Z
+1 for 'equinoxes last for a moment'.  They are the instantaneous point when the change occurs
Dave Gremlin
CC BY-SA 4.0
2019-05-06T11:10:27.697Z
@JohnHoltz Declination is one of 5 celestial coordinates, not 2.
User123
CC BY-SA 4.0
2019-05-06T13:26:35.793Z
@User123 Good point. I was thinking of two coordinates, declination and right ascension, are required to locate an object on the sky.
JohnHoltz
CC BY-SA 4.0
2019-05-06T16:15:20.527Z
@JohnHoltz: `At some time in the past, ...` how do you define past? This definition seems to me possible only with modern technlogy. Could 150 years ago realize that the sun is at 0 declination at that date? Also by declination do you mean the angle with the celestial equator?
Jim
CC BY-SA 4.0
2019-05-10T08:16:27.823Z
@Jim You can measure the Sun's declination with a well-built sundial. You can get a more accurate measurement if you have a telescope, and we've had those for over 400 years.
PM 2Ring
CC BY-SA 4.0
2020-06-25T14:19:43.567Z
@Jim *but everywhere I read it say equinox means equal nights.* — Where everywhere? You shouldn't look for some popular science articles. (where things are too simplified)
User123
CC BY-SA 4.0
2019-05-04T15:29:38.980Z
2020-06-25T14:22:55.433Z
2020-06-25T14:22:55.433Z
User123
<p>Two further points to make are that:</p>

<p>(1) because of the way we define sunrise and sunset as being when the entire Sun is below the horizon not just the centre of the Sun.</p>

<p>(2)  When the Sun is close to the horizon the Sun’s ray are refracted (bent) by the Earth’s atmosphere so that the Sun appears to be slightly higher in the sky that it would be if the Earth had no atmosphere. For more details see <a href="https://en.wikipedia.org/wiki/Atmospheric_refraction" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Atmospheric_refraction</a>. This means that the Sun <strong><em>appears</em></strong> to be above the horizon when it is actually just below it.</p>

<p>Because of these facts there are always more than 12 hours of daylight and thus slightly less than 12 hours of night on the dates of the equinox.</p>

<p>The dates when there are almost exactly 12 hours between sunrise and sunset are know as the equiluxes and vary according to latitude.</p>

<p>For more details see my recent blog post of this topic. <em>(Apologies if this seems like self promotion, it just pulls it all the information together about equinoxes and equiluxes into one single place.)</em></p>

<p><a href="https://thesciencegeek.org/2018/03/04/20-march-2018-the-equinox/" rel="nofollow noreferrer">https://thesciencegeek.org/2018/03/04/20-march-2018-the-equinox/</a></p>

CC BY-SA 4.0
2019-05-16T16:54:56.210Z
Can you rephrase/elaborate the ` Due to the refraction of the near horizontal rays of the Sun near the horizon, `? I don't understand what this means
Jim
CC BY-SA 4.0
2019-05-17T11:46:52.163Z
@jim I have done so. I hope this helps
Science Geek
CC BY-SA 4.0
2020-06-21T19:48:06.783Z
amazing facts !! thanks !!!!!!!!!!!
Fattie
CC BY-SA 4.0
2020-06-25T20:15:13.397Z
@ScienceGeek sent a bounty, thanks :)
Fattie
CC BY-SA 4.0
2019-05-15T12:38:48.480Z
2019-05-17T11:43:07.783Z
2019-05-17T11:43:07.783Z
Science Geek
<p>The easiest graph to consider is the graph of day-length. If the day-length varies from 8 to 16 hours, there can only be two equinoxes:
<a href="https://i.sstatic.net/XmFEw.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/XmFEw.png" alt="enter image description here"></a></p>

<p>At the time of equinox, the day length changes at the fastest rate, by a few minutes in temperate regions, compared to the solstices, which change by a few seconds every day. So, the equinox doesn't exist perfectly, the night time will be 11.57, 12.01, 12.04... It can be off by a minute, but the next day, it is changed by a few minutes...
march 20th: 
<a href="https://www.timeanddate.com/sun/france/paris?month=3&amp;year=2019" rel="nofollow noreferrer">https://www.timeanddate.com/sun/france/paris?month=3&amp;year=2019</a></p>

<p>Don't try to understand with words, try to understand with images and objects in your hands: This image shows you. On the equinox, does the north pole and the south pole face away from the sun? <a href="https://youtu.be/tX3Y5bzNDiU?t=35" rel="nofollow noreferrer">https://youtu.be/tX3Y5bzNDiU?t=35</a></p>

<p>Don't over think it, rotations are confusing, just have a basic idea that the north pole becomes parallel with the sun on the equinox, and points away from the sun in winter, and towards the sun in summer. </p>

<p>Perhaps you can pick up an apple or a fruit and put it near a lightbulb / lamp and do the movement mechanics for yourself. try youtube for "seasons earth animation" <a href="https://youtu.be/WLRA87TKXLM?t=5" rel="nofollow noreferrer">https://youtu.be/WLRA87TKXLM?t=5</a></p>

CC BY-SA 4.0
2019-05-04T16:50:34.603Z
I have seen that video. I don't get how in 1:17 we see that the sun is 90 degrees of the equator but as it shows to move, I don't understand why for the next 24 hours there is such a difference in the angle
Jim
CC BY-SA 4.0
2019-05-04T17:13:47.167Z
Can you elaborate on the graph a bit?
Jim
CC BY-SA 4.0
2019-05-04T19:10:29.267Z
Ok I added some info. you are right by the way, the equinox changes changes slowly and it can be stuck in between two days, for example, one night will last 11.58 hours, and the next day can last 12.02 hours. with the day being about 12 hours but almost never to the very second, and it can be either the day or the night which is closest to being 12.00 hours, it depends where you are on the world when the pole is half way between facing towards and away from the sun.
predatflaps
CC BY-SA 4.0
2019-05-04T19:18:44.847Z
Thanks for the update. The sun is a sphere so the light rays emitted should be the same in all directions. Hence if the tilt of the earth on the equinox moves the next day since it won't move towards the sun, the rays should be 90 degrees to the equator as well after the "official" equinox. I'll see the videos you pasted again in slow motion
Jim
CC BY-SA 4.0
2019-05-04T19:24:58.203Z
E.g in the video https://www.youtube.com/watch?v=tX3Y5bzNDiU&feature=youtu.be&t=35 what is the difference of the angle of the sun to the equator from 0:35-0:36? It seemds to me it should be 90 degrees till 0:36 even though the earth has moved a lot from 0:35. This is the part that is confusing to me
Jim
CC BY-SA 4.0
2019-05-04T19:26:43.373Z
If you notice that video actually shows a yellow spot on the equator from 0:35 to 0:36. So why that doesn't show 90 degrees = equinox?
Jim
CC BY-SA 4.0
2019-05-05T16:59:46.580Z
@Jim If you look at a graph if the Sun's declination vs time, you get a sine wave (almost). The maximum & minimum of that wave are the solstices, and the points where the declination is zero (where that sine wave crosses the horizontal axis) are the equinoxes. At the maximum & minimum points of the wave, it has a slope of zero, so the rate of change of the Sun's declination is the slowest near the solstices.
PM 2Ring
CC BY-SA 4.0
2019-05-05T17:00:01.783Z
@Jim (cont) And at the equinox points the slope of the wave is at its maximum absolute value, so the rate of change of the  declination is at its maximum. So at dates near the equinox the change in day-length is at its maximum.
PM 2Ring
CC BY-SA 4.0
2019-05-04T16:38:41.827Z
2019-05-10T08:05:48.880Z
2019-05-10T08:05:48.880Z
predatflaps
<p>The earth is smoothly and constantly moving around the sun. At the moment of the equinox, a line from the center of the sun to the center of the earth passes through the equator. That only lasts an instant. Before that instant, the line is on one side of the tilted equator, and after that, it's on the other side. The day labeled &quot;the equinox&quot; is just the day on which that instant happens.</p>
<p>In any case, a single day is enough to make a measurable difference in any definition of &quot;exposure to the sun&quot;. For instance, depending on the latitude, the length of time from sunrise to sunset can change by several minutes from one day to the next, near the equinoxes.</p>

CC BY-SA 4.0
2020-06-26T07:46:42.933Z
`a line from the center of the sun to the center of the earth passes through the equator.` this sentence is what is confusing to me as in my mind this line should exist always. I suspect that the meaning is different than an arbitrary straight line from the center one sphere (sun) to another (earth)
Jim
CC BY-SA 4.0
2020-06-26T02:59:27.990Z
2020-06-26T02:59:27.990Z
Mark Foskey
<p>The earth is tilted at 23.5 degrees which means that in the solstices the north/south hemisphere is more radiated by the sun and the tilt to the sun is maximum.    </p>

<p>In the equinoxes, the tilt is not towards the sun but to the "side" so the equator has direct exposure to the sun. As a result, we have 12 hours on day and night on the equinoxes.   </p>

<p>What I don't understand is why is it only for these 2 days (equinoxes) that this happens. In the next day(s) I think the equator should have the same exposure to the sun as the equinoxes. Has the tilt changed so much?<br>
I couldn't find any animation that shows the exact path of the tilt throughout the year.<br>
Can someone please help me understand this?</p>

CC BY-SA 4.0
2019-05-05T09:24:40.587Z
@BillDOe: `I guess you could think of the solstices ` you mean equinoxes right?
Jim
CC BY-SA 4.0
2019-05-05T23:37:51.540Z
Yeah, I meant equinoxes. (Hate when I do that.) I deleted it and added new corrected comment. My apologies.
BillDOe
CC BY-SA 4.0
2019-05-05T23:38:35.960Z
I guess you could think of the equinoxes as that moment when the plane of the poles is exactly at right angles to a line drawn from the Sun to the Earth. "Plane of the poles" being a plane defined by the poles of the Earth and a line drawn through the Earth perpendicular to the Plane of the Ecliptic.
BillDOe
<the-sun><earth><ecliptic><equinox><solstice>
Why is the equinoxes only 2 days and not more?
Jim
# Why is the equinoxes only 2 days and not more?

(Asked by: username_0 on 2019-05-04)

The earth is tilted at 23.5 degrees which means that in the solstices the north/south hemisphere is more radiated by the sun and the tilt to the sun is maximum.

In the equinoxes, the tilt is not towards the sun but to the "side" so the equator has direct exposure to the sun. As a result, we have 12 hours on day and night on the equinoxes.

What I don't understand is why is it only for these 2 days (equinoxes) that this happens. In the next day(s) I think the equator should have the same exposure to the sun as the equinoxes. Has the tilt changed so much?  
I couldn't find any animation that shows the exact path of the tilt throughout the year.  
Can someone please help me understand this?

**Tags:** the-sun, earth, ecliptic, equinox, solstice
> **Comments:**
> **username_0**: @BillDOe: `I guess you could think of the solstices ` you mean equinoxes right?  
> **username_1**: Yeah, I meant equinoxes. (Hate when I do that.) I deleted it and added new corrected comment. My apologies.  
> **username_1**: I guess you could think of the equinoxes as that moment when the plane of the poles is exactly at right angles to a line drawn from the Sun to the Earth. "Plane of the poles" being a plane defined by the poles of the Earth and a line drawn through the Earth perpendicular to the Plane of the Ecliptic.  

---

## 4 answers

### Answer by username_2 on 2019-05-04. Score: 8

First, don't think that equinox is when the day and night are the same. It is the moment, when the sun's declination is zero, or when the sun is directly over the equator. Let me explain ...

Earth is always rotating around the Sun, so [declination](https://en.wikipedia.org/wiki/Declination) of the Sun is always changing (except at the solstices when it stops for a minute and goes in the other way). As @JohnHolst has pointed out, Sun's declination is changing for 1 angular minute per hour (mark 1 mm on the 3 m of distance – this is nearly 1 angular minute). An equinox doesn't last for a day. It lasts for a moment when the Earth is totally aligned and straight with the sun:
*(this figure shows the geocentric model, which is easier to represent and imagine)*

[![declination = 0](https://i.sstatic.net/OZffQ.png)](https://i.sstatic.net/OZffQ.png)

Also, Wikipedia says:

> An equinox is commonly regarded as the **instant of time** when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun. This occurs twice each year, around 20 March and 23 September. **In other words, it is the *moment* at which the center of the visible Sun is directly above the equator.**

P. S.

You can look [this simulation](https://astro.unl.edu/classaction/animations/coordsmotion/eclipticsimulator.html).

> **Comments:**
> **username_0**: My question is if this declination changes so much from the day of the equinox to the next. From the images online it is not easy to visualize this  
> **username_2**: It changes a little bit, but enough, that it isn't equinox anymore.  
> **username_0**: This "little" that is enough is hard for me to visualize.  
> **username_3**: @Jim. You are correct that the change from one day to another is not that large. But the definition of the equinox is not based on the length of daylight, or the sun being approximately over the equator. The definition is the Sun's declination is 0 degrees 0 arcminutes 0 arcseconds. At the time of the March equinox, the declination changes by approximately 1 arcminute per hour (or 1 arcsecond per minute of time). Thus, the time of the equinox can be stated very precisely.  
> **username_0**: @JohnHoltz: This helps. Though `..equinox is not based on the length of daylight,..` but everywhere I read it say equinox means equal nights. So I thought that just based on the name it is about the length of daylight. Is this term a misnomer then and just kept for historical reasons? Also `the Sun's declination is 0 degrees... ` you mean  from the equator?  
> **username_0**: @User123: `when the Earth is totally aligned and straight.` but the earth is always tilted.  
> **username_2**: @Jim If this will help you: declination of the Polaris is always (near) 90 degrees - anywhere you are.  
> **username_3**: @Jim. You're correct that equinox originally meant "equal day and night". If this definition is used, and if the length of day is sunrise to sunset, then the date of the equinox would be different (by a few days) depend on your latitude, and the March equinox would occur before Mar 20. At some time in the past, the definition was changed to when the Sun is at 0 declination. (Declination is one of 2 celestial coordinates, and 0 declination is directly over the Earth's equator.)  
> **username_4**: +1 for 'equinoxes last for a moment'.  They are the instantaneous point when the change occurs  
> **username_2**: @JohnHoltz Declination is one of 5 celestial coordinates, not 2.  
> **username_3**: @User123 Good point. I was thinking of two coordinates, declination and right ascension, are required to locate an object on the sky.  
> **username_0**: @JohnHoltz: `At some time in the past, ...` how do you define past? This definition seems to me possible only with modern technlogy. Could 150 years ago realize that the sun is at 0 declination at that date? Also by declination do you mean the angle with the celestial equator?  
> **username_5**: @Jim You can measure the Sun's declination with a well-built sundial. You can get a more accurate measurement if you have a telescope, and we've had those for over 400 years.  
> **username_2**: @Jim *but everywhere I read it say equinox means equal nights.* — Where everywhere? You shouldn't look for some popular science articles. (where things are too simplified)  

---

### Answer by username_6 on 2019-05-15. Score: 5

Two further points to make are that:

(1) because of the way we define sunrise and sunset as being when the entire Sun is below the horizon not just the centre of the Sun.

(2) When the Sun is close to the horizon the Sun’s ray are refracted (bent) by the Earth’s atmosphere so that the Sun appears to be slightly higher in the sky that it would be if the Earth had no atmosphere. For more details see <https://en.wikipedia.org/wiki/Atmospheric_refraction>. This means that the Sun ***appears*** to be above the horizon when it is actually just below it.

Because of these facts there are always more than 12 hours of daylight and thus slightly less than 12 hours of night on the dates of the equinox.

The dates when there are almost exactly 12 hours between sunrise and sunset are know as the equiluxes and vary according to latitude.

For more details see my recent blog post of this topic. *(Apologies if this seems like self promotion, it just pulls it all the information together about equinoxes and equiluxes into one single place.)*

<https://thesciencegeek.org/2018/03/04/20-march-2018-the-equinox/>

> **Comments:**
> **username_0**: Can you rephrase/elaborate the ` Due to the refraction of the near horizontal rays of the Sun near the horizon, `? I don't understand what this means  
> **username_6**: @jim I have done so. I hope this helps  
> **username_7**: amazing facts !! thanks !!!!!!!!!!!  
> **username_7**: @ScienceGeek sent a bounty, thanks :)  

---

### Answer by username_8 on 2019-05-04. Score: 5

The easiest graph to consider is the graph of day-length. If the day-length varies from 8 to 16 hours, there can only be two equinoxes:
[![enter image description here](https://i.sstatic.net/XmFEw.png)](https://i.sstatic.net/XmFEw.png)

At the time of equinox, the day length changes at the fastest rate, by a few minutes in temperate regions, compared to the solstices, which change by a few seconds every day. So, the equinox doesn't exist perfectly, the night time will be 11.57, 12.01, 12.04... It can be off by a minute, but the next day, it is changed by a few minutes...
march 20th:
<https://www.timeanddate.com/sun/france/paris?month=3&year=2019>

Don't try to understand with words, try to understand with images and objects in your hands: This image shows you. On the equinox, does the north pole and the south pole face away from the sun? <https://youtu.be/tX3Y5bzNDiU?t=35>

Don't over think it, rotations are confusing, just have a basic idea that the north pole becomes parallel with the sun on the equinox, and points away from the sun in winter, and towards the sun in summer.

Perhaps you can pick up an apple or a fruit and put it near a lightbulb / lamp and do the movement mechanics for yourself. try youtube for "seasons earth animation" <https://youtu.be/WLRA87TKXLM?t=5>

> **Comments:**
> **username_0**: I have seen that video. I don't get how in 1:17 we see that the sun is 90 degrees of the equator but as it shows to move, I don't understand why for the next 24 hours there is such a difference in the angle  
> **username_0**: Can you elaborate on the graph a bit?  
> **username_8**: Ok I added some info. you are right by the way, the equinox changes changes slowly and it can be stuck in between two days, for example, one night will last 11.58 hours, and the next day can last 12.02 hours. with the day being about 12 hours but almost never to the very second, and it can be either the day or the night which is closest to being 12.00 hours, it depends where you are on the world when the pole is half way between facing towards and away from the sun.  
> **username_0**: Thanks for the update. The sun is a sphere so the light rays emitted should be the same in all directions. Hence if the tilt of the earth on the equinox moves the next day since it won't move towards the sun, the rays should be 90 degrees to the equator as well after the "official" equinox. I'll see the videos you pasted again in slow motion  
> **username_0**: E.g in the video https://www.youtube.com/watch?v=tX3Y5bzNDiU&feature=youtu.be&t=35 what is the difference of the angle of the sun to the equator from 0:35-0:36? It seemds to me it should be 90 degrees till 0:36 even though the earth has moved a lot from 0:35. This is the part that is confusing to me  
> **username_0**: If you notice that video actually shows a yellow spot on the equator from 0:35 to 0:36. So why that doesn't show 90 degrees = equinox?  
> **username_5**: @Jim If you look at a graph if the Sun's declination vs time, you get a sine wave (almost). The maximum & minimum of that wave are the solstices, and the points where the declination is zero (where that sine wave crosses the horizontal axis) are the equinoxes. At the maximum & minimum points of the wave, it has a slope of zero, so the rate of change of the Sun's declination is the slowest near the solstices.  
> **username_5**: @Jim (cont) And at the equinox points the slope of the wave is at its maximum absolute value, so the rate of change of the  declination is at its maximum. So at dates near the equinox the change in day-length is at its maximum.  

---

### Answer by username_9 on 2020-06-26. Score: 1

The earth is smoothly and constantly moving around the sun. At the moment of the equinox, a line from the center of the sun to the center of the earth passes through the equator. That only lasts an instant. Before that instant, the line is on one side of the tilted equator, and after that, it's on the other side. The day labeled "the equinox" is just the day on which that instant happens.

In any case, a single day is enough to make a measurable difference in any definition of "exposure to the sun". For instance, depending on the latitude, the length of time from sunrise to sunset can change by several minutes from one day to the next, near the equinoxes.

> **Comments:**
> **username_0**: `a line from the center of the sun to the center of the earth passes through the equator.` this sentence is what is confusing to me as in my mind this line should exist always. I suspect that the meaning is different than an arbitrary straight line from the center one sphere (sun) to another (earth)  

---

