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I am reading a book on spherical astronomy. Surprisingly, my problem is not with the mathematics. The author mentions several times words like 'eastward movement' and 'westward movement'. For example, the Earth revolves around the Sun in an eastward direction. When I look at the diagram from the perspective of someone above the north pole of the Sun, the Earth seems to go around the Sun in an anticlockwise manner. My problem is I don't understand what is meant by eastward movement ? Can someone explain what this means without ambiguity ? Is there some sort of a right-hand rule that one can follow such as Fleming's right-hand rule in electricity ?

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From the point of view of an observer hovering above the Earth's surface, the rotation of the Earth is eastward. That is, points on the surface appear to move in an easterly direction as the Earth spins.

Extending this to the orbital motion, there are at least two possible interpretations. One is that the spin angular momentum and orbital angular momentum are in the same direction. This is roughly true (and certainly closer to true than the opposite). Another interpretation would be that the Earth moves around in its orbit such that it heads in an easterly direction when referred to a longitude on the Sun. This is also approximately true.

If you are asking what is a general definition of the north pole, then I suppose it could be the direction of the angular momentum vector. Then yes indeed, the right hand grip rule tells you the spin is eastward as it relates to north.

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  • $\begingroup$ The question that I'm asking is how do we know that the earth spins in the easterly direction when looking from space. $\endgroup$ – medwatt Dec 16 '15 at 10:21
  • $\begingroup$ @medwatt The Sun rises in the east. If you are asking me how do we decide what is the north pole and what is the south, well that's just a human convention. I suppose you might argue that the north pole is defined by the angular momentum vector. $\endgroup$ – Rob Jeffries Dec 16 '15 at 11:30
  • $\begingroup$ @RobJeffries If one stands on the equator of the Earth and faces the North Pole, the Sun rises in the east (to the right). That's the Earth's rotation. But if one hovers over the equator of the Sun and faces its North Pole, the Earth will rise above the horizon from the west (to the left). That's the Earth's orbit. So Earth's rotation is westward but its orbit is eastward?? It's a bit dizzying how this is defined. $\endgroup$ – LocalFluff Feb 14 '16 at 16:18
  • $\begingroup$ Of course, if one stands on the rotating equator of the Sun, it would be the other way around. This should now be clear as a crystal sphere. $\endgroup$ – LocalFluff Feb 14 '16 at 16:41
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Image showing the revolution of earth

Eastward movement mentioned in the book that you're reading means that if you view the earth revolving around the sun from a vantage point in the solar system and if you kept the north pole upwards in your frame of reference, you would observe the earth going towards the right, around the sun. And if you moved to a point in the solar system where you can see the north pole of the earth as well as of the sun, you would see the earth moving anti clockwise in its orbit.

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  • $\begingroup$ There's still ambiguity. The Earth goes equally to both right and left, east and west, since it orbits in an ellipse and returns to the same point each year. $\endgroup$ – LocalFluff Feb 14 '16 at 16:29
  • $\begingroup$ I understand that you are having trouble getting the perspective that i am trying to develope. $\endgroup$ – Aniansh Feb 14 '16 at 16:32
  • $\begingroup$ If you view the earth from outside its orbit, you would view it going east in its orbit. North being the direction in which the north pole of the earth is pointing. $\endgroup$ – Aniansh Feb 14 '16 at 16:33
  • $\begingroup$ Okay, so say that I stand on the north pole of Uranus. Would I see the Earth move to the right or eastwards or how? $\endgroup$ – LocalFluff Feb 14 '16 at 16:37
  • $\begingroup$ Let's try the north pole of mars(the poles of uranus are a bit odd and would further complicate the problem). So, imagine yourself standing on the north pole of mars. If the earth is between the sun and mars, you will see that the earth goes towards the right( eastwards). And if the sun is in between earth and mars, you would see the earth going towards the left. $\endgroup$ – Aniansh Feb 14 '16 at 16:42

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