# I can't understand why the sun moves across the ecliptic

1. Aug 16, 2009

### Wattever

I can't understand why the sun moves across the ecliptic :(

I've been searching for hours and I've found nothing more than "because of the earth's revolution around the sun", so I'm assuming this is extremely obvious - but I can't understand it. I understand why the declination angle of the sun would change (because of the earth's axial tilt together with its revolution) but I don't understand the horizontal motion, and I understand that if we consider the earth not to be rotating the sun would appear to revolve around the earth (so we wouldn't be able to see it for half of the year from a specific position) but since it is rotating, at each point along the earth's orbit around the sun this specific point will be facing the sun at 24 hour intervals so the viewing angle should not be changed (if we ignore the difference in declination)! Can someone please explain this to me?

2. Aug 16, 2009

### D H

Staff Emeritus
Re: I can't understand why the sun moves across the ecliptic :(

Start by thinking of the Earth's motion from the perspective of the Sun. The Earth's motion about the Sun is very close to an ellipse. After accounting for the motion of the Earth and Moon about each other, the Earth's motion about the Sun is extremely close to being an ellipse. An ellipse is a two dimensional object -- they lie on a plane. The plane defined by the Earth's mean motion around the Sun is the ecliptic plane. From the perspective of the Sun, the center of the Earth will be extremely close to being on this plane. (The Earth is not exactly on the ecliptic plane because of the motion of the Moon and the Earth about each other and because the motion of the Earth+Moon about the Sun is not truly an ellipse. For example, the Sun and Jupiter orbit each other, and Jupiter perturbs the orbits of the Earth.)

Now switch perspective to the Earth. Because the Earth is very close to being on the ecliptic plane from the perspective of the Sun, the Sun is necessarily very close to being on the ecliptic plane from the perspective of the Earth.

3. Aug 16, 2009

### Wattever

Re: I can't understand why the sun moves across the ecliptic :(

I still have the same problem with that. I don't know if by sun's perspective you mean the sun (or the person on the sun looking at the earth) is rotating so as to be facing the earth as it moves or if the person is looking straight ahead and only the earth is moving. If it's the latter, I understand that the earth will appear to move, but it will be completely hidden for half a year - in the simulation I saw of the sun moving along the ecliptic it keeps on moving in the same horizontal direction endlessly, it doesn't disappear at all. If it's the former, why would it appear to move at all? Any two points can be facing eachother such that each appears to be centered to the other. Only the distance between them will differ because of the orbit being an ellipse. Even if the earth is moving in a straight line but the sun is always rotating so as to be facing it at any given time, the earth will appear centered all the time, is that wrong?

4. Aug 16, 2009

### D H

Staff Emeritus
Re: I can't understand why the sun moves across the ecliptic :(

Forget about the rotation of the Sun and Earth about their axes. Those are distractions that are leading you to be confused. Think only of the orbit.

5. Aug 16, 2009

### negitron

Re: I can't understand why the sun moves across the ecliptic :(

Perhaps this graphic will help:

http://img26.imageshack.us/img26/1707/800pxeclipticpath.jpg [Broken]

Now, imagine the Earth at various points in its orbit and see how the Sun will have a different constellation behind it at each one.

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6. Aug 16, 2009

### Integral

Staff Emeritus
Re: I can't understand why the sun moves across the ecliptic :(

Hang a ball from the ceiling so that it is about eye height when you are standing. Now slowly move around the ball noting what it is that you CANNOT see because it is blocked from your view by the ball. Note that as you move around the ball from a given spot the SAME object is always blocked by the ball.

As far as the sun is concerned the earths path does not change year after year it is like we are on a rail circling the sun. Now imagine mile markers on this rail we can label each marker with a day of the year. So every year on a certain day the earth is in very nearly the same location in its orbit with respect to the sun. Every year when you pass a marker you will be looking at the same stars at midnight, and at noon NOT be seeing stars lost in the glare of the sun. That set of stars which is blocked by the sun over the year are exactly those that we call the ecliptic.

7. Aug 16, 2009

### Wattever

Re: I can't understand why the sun moves across the ecliptic :(

I've been saying that if I ignore the rotation of the sun and earth about their axes the sun should disappear for half of the year, like http://www.math.nus.edu.sg/aslaksen/applets/earth_fixed/earth_fixed.html" [Broken]!

OK, the constellation behind the sun changes, but then shouldn't it appear as if the zodiac is the one moving and not the sun?! Because as I said the sun can always be centered in the sky (due to the rotation of the earth about its axis), as if the earth and the sun are glued to the two ends of a stick and the stick is rotating about its axis.

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8. Aug 16, 2009

### Integral

Staff Emeritus
Re: I can't understand why the sun moves across the ecliptic :(

The effect we are talking about is not easily observed. To see it you need to be well versed in the constellations. When you have learned to name every constellation in the night sky on any night of the year you then should be able to see the motion of the sun. However this motion takes months to see as there is little night to night change.

To claim it is the sun moving vs the stars is a pointless question both views can be used without confusion. Just keep in mind the real geometry and spend some time observing the motion of the stars.

Last edited by a moderator: May 4, 2017
9. Aug 16, 2009

### Wattever

Re: I can't understand why the sun moves across the ecliptic :(

It's not observation that I'm concerned with here, it doesn't really matter if I can't observe it (I'm not claiming it doesn't happen, I just can't understand it).

How come? If at a certain month the sun appears straight in front of my window and behind it some constellation, at a different month the sun would still appear straight in front of my window but the constellation behind it would be different. Hence the zodiac is the one that appears to be moving. But if it happens that I have to look sideways to see the sun, then the sun is the one that appears to move.

10. Aug 16, 2009

### Integral

Staff Emeritus
Re: I can't understand why the sun moves across the ecliptic :(

So you want to talk about how something appears but do not wish to observe it? Interesting.

Just what is it that you SEE moving?

11. Aug 16, 2009

### ideasrule

Re: I can't understand why the sun moves across the ecliptic :(

Yes, it definitely does look like that. That doesn't change the fact that in reality, the effect is produced by Earth's revolution around the Sun, not the zodiac's revolution around the Earth.

12. Aug 16, 2009

### Wattever

Re: I can't understand why the sun moves across the ecliptic :(

What? Whether I can see it or cannot is completely irrelevant to this topic where I'm looking to understand why the motion happens, ergo your mentioning that it takes months to observe was off-point-- which was what I was trying to say.
I was watching the skyguide in Starry Night, which showed the sun moving (although now that I watched it again, the sun remains in the center of the screen). And again, what I actually saw is off-point. When I say that according to my understanding, the constellations are the ones that should appear to be moving, I mean that according to my understanding of the earth and sun's motion they're the ones that should appear to be moving!

Are you sure? If that's so then I get it! So that means the sun always rises and sets in the same position (same azimuth, different altitude) position each day, right?

13. Aug 17, 2009

### D H

Staff Emeritus
Re: I can't understand why the sun moves across the ecliptic :(

The sun rises and sets at the same altitude but different azimuth each day.

This is all about celestial coordinate systems, so a review.

A Euclidean three dimensional coordinate system comprises an origin and three orthogonal axes. Given one coordinate system, you can create infinitely many more coordinate systems by picking a different origin or by picking a different set of axes.

The concept of an inertial reference frame, or inertial coordinate system, is of extreme importance in Newtonian mechanics. Newton's laws of motion are only true in an inertial frame. (Alternatively, a reference frame is inertial if Newton's laws of motion are true in that frame.) The origins of all inertial frames are not accelerating and the axes of all inertial frames are not rotating with respect to some other inertial frame.

The center of mass of the solar system is pretty close to non-accelerating. The solar system is of course orbiting the galaxy, but this acceleration is fairly small, about 2×10-10 m/s2. Couple this origin with a non-rotating set of axes and you have a fairly good approximation of an inertial frame.

So, what is a non-rotating set of axes? A frame that rotates with the Earth's daily rotation is a rotating frame, obviously. Frames such as this can be quite useful. For example, meteorologists use an Earth-centered, Earth-fixed frame to model the weather. Astronomers use a horizon-based system with the origin at the center of a telescope to aim a telescope. The remote stars will appear to be moving in such a frame. Is this motion "real"? In the sense that you observe it, yes. All reference frames are equally valid in the sense that one can write the equations of motion for some object in any frame. In the sense that even the closest star is moving at about 10,000 times the speed of light and accelerating at about 200,000 m/s2 in such a frame, not really. There is no reason to think that the Earth's daily rotation has any measurable impact on the behavior of the remote stars. The same goes for the Earth's annual orbit about the Sun, and the solar system's orbit about the galaxy. This is particularly true for stars that are very, very far away such as extra-galactic quasars.

The remote stars give a clue as to what constitutes a non-rotating frame. A simple way to look to find one : Pick a set of axes in which these remote stars are not moving. (Actually, this is not that simple a task; it keeps hordes of graduate students perpetually busy.) One way to pick a set of axes is to pick a plane. Pick a normal to that plane as one axis. Now pick another axis that lies on the plane. The third axis is the cross product of the two already chosen. Two obvious planes are the Earth's equatorial plane and the Earth's orbital plane. The first choice leads to equatorial reference frames; the latter to ecliptic frames.

14. Aug 17, 2009

### chemisttree

Re: I can't understand why the sun moves across the ecliptic :(

I assume that the sun is observed at the same time during the day? Say, noon? If that's the case, you should compare the length of the day with the rotational period of the earth and know they are not equal. The solar rate of movement across the sky is not the same as the sideral rate. Sideral rate = 23 hours, 56 minutes, 4.1 seconds. Solar rate = ~24 hours. That difference is how much more daylight we enjoy due to the length the earth moves in it's orbit about the sun each day.

But why would the sun's east-west position (horizontal?) change throughout the year. Noon is defined as the midpoint between dawn and dusk but that's not how we set our clocks. But you can see that if you look at the dawn-dusk midpoint, real noon should vary and only be the same at the two equinoxes. So why isn't the day clock symmetrical about the dawn and dusk time interval... why isn't noon exactly between these two times every day? Look again at your simulation set at a slower speed. Notice how the shadow's angle relative to lines of longitude changes throughout the year.

Last edited: Aug 17, 2009
15. Aug 17, 2009

### Staff: Mentor

Re: I can't understand why the sun moves across the ecliptic :(

I don't think anyone picked-up on this before:
You're contradicting yourself here. If the sun and earth are tethered to a stick, then both of them are rotating as the earth revolves around the sun. Then, the sun stays in the same place to an observer on earth and the zodiacs rotate across the sky.

If you consider the earth to not rotate as it goes around the sun, then the zodiacs stay fixed and the sun moves around the earth.

I think you're going to need to follow the earlier suggestion and walk circles around a piece of furniture to confirm this. You could also tape pictures of the zodiacs to the walls to observe while walking around the piece of furniture.

16. Aug 18, 2009

### Wattever

Re: I can't understand why the sun moves across the ecliptic :(

Yes, obviously I realize that-- I explicitly said "due to the rotation of the earth about its axis" in the second paragraph, which was a reply to integral and negitron. The first paragraph was a reply to D H. Two different situations.

Anyway, I got it. The problem was just that by "the sun moves across the ecliptic" (which I'd encountered numerously) I understood that the azimuth position of the sun changes, which now I know does not.