<p>Not exactly. </p>

<p>During either of the equinoxes there is a moment when the line between center of earth and the sun aligns with the equator. This doesn't coincide with prime meridian in any way though; it may happen at any meridian whatsoever that happens to coincide with the line.</p>

<p>Of course if it happens the sun is in zenith above prime meridian at that moment, your conditions will be satisfied. But depending on precision you allow, that may be a very short time window. It's all about how precisely you want that to be.</p>

<p>Near Equinox Earth tilts by about 0.25 degree of latitude per day, meaning that is the maximum angle by which the line will be off from equator while crossing the prime meridian on the same day as when Equinox happens. 0.25 degree of latitude is about 27 kilometers, so this is the maximum error. Of course on some years, that will be much less, it's just that Earth tilt is correlated with length of year, zenith line crossing of prime meridian is correlated with time of day, and time of day is not really correlated with length of year.</p>

CC BY-SA 3.0
2014-10-18T21:38:52.240Z
@barrycarter: Could you explain? *The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator.* That seems to imply they are always in sync, since one is defined by the other. Did you mean the ecliptic plane? In that case, it intersects the equator plane, and during equinoxes the intersection line happens to pass the Sun (or, if you nitpick at inexactness of the Ecliptic - the Sun passes the Earth equator plane twice a year.) At these moments Earth center, the Equator and the Sun are colinear, but the meridian is entirely random.
SF.
CC BY-SA 3.0
2014-10-20T00:47:46.580Z
@barrycarter: In fact, I meant the momentary celestial equator for given equinox day - the moment when the point on Earth where the Sun is in zenith crosses the Earth's equator.
SF.
CC BY-SA 3.0
2014-09-05T11:03:42.767Z
2016-09-29T08:59:56.200Z
2016-09-29T08:59:56.200Z
SF.
<p>I did a bit of googling but couldn't find the exact answer, so are the following 3 points collinear or not ?</p>

<p>1_center of earth</p>

<p>2_ascending(or descending) node of sun</p>

<p>3_point of intersection of prime meridian and equator(point of zero longitude on equator).</p>

CC BY-SA 3.0
2014-07-20T23:46:34.740Z
No. They can align at some special times, though. Why do you think they should be collinear?
Cheeku
CC BY-SA 3.0
2014-07-21T05:12:54.480Z
its not possible, all these points are stationary w.r.t to earth,, so they all remain collinear or never get aligned, they can't be aligned at some times and not at other times,
Salman Azmat
CC BY-SA 3.0
2014-07-21T16:01:05.190Z
The ascending node of the sun is not stationary w.r.t the earth. The earth rotates, so the prime meridian rotates, meaning that the intersection of it and the equator also rotates. The vernal equinox, which is the ascending node of the sun, is fixed in the sky (at least on small timescales), and hence it does not rotate.
Takku
CC BY-SA 3.0
2014-07-21T16:19:14.927Z
oh, right,   I meant to ask that at the time when sun passes through vernal equinox does these points get aligned ?
Salman Azmat
CC BY-SA 3.0
2014-07-22T13:54:21.063Z
@SalmanAzmat: Not exactly. During either equinox there is a moment when the line between center of earth and the sun aligns with the equator (as opposed to just crossing it twice daily), and if it happens the sun is in zenith above prime meridian at that moment, your conditions will be satisfied. But depending on precision you allow, that may be a very short time window. It's all about how precisely you want that to be. Each equinox the line crosses the prime meridian within about 0.25 degree of latitude, or about 27 kilometers from equator. Some years it's less, some - more.
SF.
CC BY-SA 3.0
2014-09-05T10:12:31.853Z
@SF. Plase promote your comments to an answer.
Walter
<the-sun><orbital-elements><satellite>
Are ascending node of sun, the point of intersection of prime meridian and equator and center of earth all collinear?
Salman Azmat
# Are ascending node of sun, the point of intersection of prime meridian and equator and center of earth all collinear?

(Asked by: username_0 on 2014-07-20)

I did a bit of googling but couldn't find the exact answer, so are the following 3 points collinear or not ?

1\_center of earth

2\_ascending(or descending) node of sun

3\_point of intersection of prime meridian and equator(point of zero longitude on equator).

**Tags:** the-sun, orbital-elements, satellite

> **Comments:**
> **username_1**: No. They can align at some special times, though. Why do you think they should be collinear?  
> **username_0**: its not possible, all these points are stationary w.r.t to earth,, so they all remain collinear or never get aligned, they can't be aligned at some times and not at other times,  
> **username_2**: The ascending node of the sun is not stationary w.r.t the earth. The earth rotates, so the prime meridian rotates, meaning that the intersection of it and the equator also rotates. The vernal equinox, which is the ascending node of the sun, is fixed in the sky (at least on small timescales), and hence it does not rotate.  
> **username_0**: oh, right,   I meant to ask that at the time when sun passes through vernal equinox does these points get aligned ?  
> **username_3**: @username_0: Not exactly. During either equinox there is a moment when the line between center of earth and the sun aligns with the equator (as opposed to just crossing it twice daily), and if it happens the sun is in zenith above prime meridian at that moment, your conditions will be satisfied. But depending on precision you allow, that may be a very short time window. It's all about how precisely you want that to be. Each equinox the line crosses the prime meridian within about 0.25 degree of latitude, or about 27 kilometers from equator. Some years it's less, some - more.  
> **username_4**: @SF. Plase promote your comments to an answer.  

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## 1 answers

### Answer by username_3 on 2014-09-05. Score: 2

Not exactly.

During either of the equinoxes there is a moment when the line between center of earth and the sun aligns with the equator. This doesn't coincide with prime meridian in any way though; it may happen at any meridian whatsoever that happens to coincide with the line.

Of course if it happens the sun is in zenith above prime meridian at that moment, your conditions will be satisfied. But depending on precision you allow, that may be a very short time window. It's all about how precisely you want that to be.

Near Equinox Earth tilts by about 0.25 degree of latitude per day, meaning that is the maximum angle by which the line will be off from equator while crossing the prime meridian on the same day as when Equinox happens. 0.25 degree of latitude is about 27 kilometers, so this is the maximum error. Of course on some years, that will be much less, it's just that Earth tilt is correlated with length of year, zenith line crossing of prime meridian is correlated with time of day, and time of day is not really correlated with length of year.

> **Comments:**
> **username_3**: @barrycarter: Could you explain? *The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator.* That seems to imply they are always in sync, since one is defined by the other. Did you mean the ecliptic plane? In that case, it intersects the equator plane, and during equinoxes the intersection line happens to pass the Sun (or, if you nitpick at inexactness of the Ecliptic - the Sun passes the Earth equator plane twice a year.) At these moments Earth center, the Equator and the Sun are colinear, but the meridian is entirely random.  
> **username_3**: @barrycarter: In fact, I meant the momentary celestial equator for given equinox day - the moment when the point on Earth where the Sun is in zenith crosses the Earth's equator.  

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