I know the earth's orbit is eliptical, and I know it's tilt that affects the seaons.
I've read that earth is closes to the sun in January. Does this mean it's not equally close in July and furthest in march and October therefore not symetrical?

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It's an elliptical orbit, but the sun isn't at the center of the ellipse, but offset somewhat. As a result, perigee occurs in early January, with apogee in early July.

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When astronomers say the tilt of the Earth's axis affects the seasons, they mean that it is the most important factor. If the Earth's axis was perpendicular to the ecliptic, there would still be seasons, albeit very small variations.

The Earth's orbit is elliptical; however, the distance of the sun isn't a significant factor compared to the tilt of the Earth's axis when it comes to heating. This is why the southern hemisphere is hotter than the northern hemisphere*. The Earth is close to perihelion in January, which coincides with the Southern summer, but the Earth is near aphelion at the time of northern summer. In theory, the Southern Hemisphere should have larger temperature ranges.

*However, because the Southern Hemisphere has more water, it acts as a giant heat reservoir and temperature ranges there are actually smaller than in the North.

I appreciate the response, but the question isn't about seasons or seasonal heating...... it's simply if the orbit is symetrical, and I supposed I should add to that if it's centered. Perhaps I should assume by definition that it's not centered and symetrical becuase otherwise there wouldn't be a single closest point, there would be 2. Thinking about that is what led to the question.

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I must not PX. PX is the mind-killer. I will face my PX. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the PX has gone there will be nothing. Only I will remain.

Perhaps you are thinking that the orbit is an ellipse with the sun in the geometric center of the ellipse. This is incorrect. The sun is located at one of the foci (plural of focus) of the ellipse. There are two foci for an ellipse. Geometrically there is a midpoint on a line between these foci. The difference between the foci and the midpoint (of what would be the center of a circle) is called the eccentricity*. If an ellipse has an eccentricity of zero then it is called a circle.

Due to the Law of Gravitation, as we understand it, the orbit must be an ellipse. The orbit may be circular (or nearly so) or it can be eccentric. But either way the sun must be at a focus (single, or one of two) but not in between.

As far as seasons go, it would be nice if the earth's tilt caused the solstices to coincide with appehelion/perihelion, but they don't. Perhaps in the future, with the precession of the earth's poles, that may be the case.

*Actually, I forget if eccentricity is measured from focus to focus, or focus to midpoint, but I think you get the idea (one would be half of the other).

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J Pax

Perihelion, Earth's closest approach to the Sun, occurs near January 4. Aphelion, Earth's farthest point from the Sun, occurs near July 4. The actual calendar dates can vary by a calendar day. If one discounts small effects, then yes Earth's orbit is symmetric about these two points: for equal time intervals on either side of the two extremes, the distances are the same (again discounting tiny effects due to long term changes in Earth's orbit, and the Sun's small motion about the Sun-Jupiter center of mass and perturbations therefrom due to the pull of the other planets).

Keeping it generalized, not to scale, not looking for extream precision...
Figure 2 is true and Figure 1 is not?

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I must not PX. PX is the mind-killer. I will face my PX. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the PX has gone there will be nothing. Only I will remain.

The first is certainly wrong. The second looks odd but it emphasises that the sun is not at the geometric centre of the ellipse. It is at one focus.

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To add one clarification, in case you intended your picture to look this way - as Spaceman Spiff pointed out, the orbit IS symmetric along a line connecting the foci of the ellipse, which would also connect the closest and farthest points of the orbit. So in the second picure if you intend the 94 and 92 to be the farthest and closest points, the Sun would be offcenter toward the 92, but it would be STRAIGHT down in the orientation of your picture - it would not also be shifted right to left. The line connecting 94 and 92 would bisect the Sun. Like I said, I can't tell if you meant to make if offcenter both ways, though.

Wolf, I don't know how much you know about the geometry of ellipses, but for the benefit of those who know little...

If you have two fixed points A and B, an ellipse is the locus of points C that satisfy the equation, AC + BC = a constant. That is, when you add the distance between the point C and point A to the distance between point C and B, you'll always get the same answer for all points on the ellipse. Essentially, it means that the triangle ABC has a constant perimeter, but since AB is a constant, we can omit it from the equation.

So to draw an ellipse, take two thumbs tacks and place them at where you want the points A and B to be. Then encircle the thumb tacks and a pencil with a circle of string of length according to your choosing. Using the pencil to keep the string taut, draw a shape around the tacks, remembering the string must be kept taut. That shape will be an ellipse.

The points A and B are the foci of the ellipse. A circle is just a special kind of ellipse where the two foci are coincident (that is they are at the same). The geometric centre of the ellipse is the point halfway between the foci. We tend to denote the distance between the a focus and the centre as c. For a circle, c = 0.

You'll notice that the line connecting the two points that are furthest away, the major axis, is the line of the foci. The ellipse is symmetical about the major axis and also symmetrical about the minor axis. The minor axis is the shortest line that can be drawn between two points on the ellipse that goes through the centre. All points on this line are equidistant from the two foci. The major axis is perpendicular to the minor axis. We call the two points at the end of the major axis, the apses.

If you consider one of the apses, the distance to the nearest focus and the distance to the furthest focus add together to equal the length of the major axis. But of course, remembering the definition of an ellipse, the distance between each focus and a point on the ellipse must add to this for all points. Hence, the constant described above is equal to the size of the major axis. However, we tend to state use the semi-major axis, half the major axis, more and we denote it with by a. Similarly, we tend to denote half the semi-minor axis by b, although we don't use it often.

So, going back to the definition of the ellipse. For two fixed points A and B, an ellipse is the locus of all the points C where AC + BC = 2a.

An interesting result of this is that since the two ends of the minor axis are equidistant from each focus, it follows that the distance must be the semi-major axis, a.

So we have a, the semi major axis, and c, the distance between the centre and a focus. How do we relate them? If we take the ratio c/a, we get the eccentricity, e, which is essentially a measure of how stretched the ellipse is. For a circle, c = 0, so e = 0. As c gets bigger, and it must always be less than a or we no longer have an ellipse, the ellipse becomes more and more stretched. Eccentricity is a useful ratio because it defines the shape of the ellipse. It is common that we define the size and shape of an ellipse by the semi-major axis and the eccentricity. We usually disregard c, since we know that if we ever needed it, it is given by c = ea.

So we have a, which is the semi-major axis, which is half the distance between the furthest points on the ellipse. We have b, which is the semi-minor axis, which is half the distance between the closest diametrically opposed points, which is given be b² = a² - c² = a²(1-e²). We have c which is have the distance between the foci. which is given by c=ea. We have e, which is the eccentricity, which is the determiner of the shape of the ellipse.

One last value of importance is the parameter, p, sometimes called the semilatus rectum. This is the distance between a focus and a point on the ellipse where the line between the two is perpendicular to the major axis and parallel to the minor axis. It turns out that the parameter is given by p = a(1+e²). This is useful because in defining a polar function, which gives us an ellipse.

r = p/(1 + e cos t), where t is the angle between the pole and r is the radius at that angle. This equation has its origin at one focus of the ellipse NOT at the geometic centre. It can be seen that when t = 90°, r = p and hence the semi latus rectum is the distance straight up from the focus to the ellipse. If we didn't define the semi latus rectum, the polar equation would be r = a(1+e²)/(1 + e cos t), which is slightly more long winded. Therefore, having it is useful because it simplifies the equation and has physical significance of its own.

Since a circle is just where the two foci are coincident, c=0 and hence e=0. Therefore, from the polar equation, we can see that for a circle, r is a constant and is given by r = a = p = b.

The polar equation is useful to us because in application to two body orbital mechanics, we find that the minor mass orbits the major mass in the path of an ellipse where the major mass is at one focus of the ellipse.

Now that we are in a situation where we consider one focus more important than the other, some more terms come to mind. We must remember that we are dealing with ellipses were 0 < e < 1. As can be seen, for t = 0°, r is a minimum and hence at this point on the ellipse, the orbiter is closest to the attractor. We call this point the periapsis and usually donate is rp. From the polar equation of the ellipse, it is given by, rp = p/(1+e) = a(1-e). At the same time, for t = 180°, the distance between the prime focus and the ellipse is greatest. This point is called the apoapsis and is where the orbiter is furthest from the attractor. It is given by, ra = p/(1-e) = a(1+e).

For a circular orbit, since the two foci are coincident at the centre, the attractor is at the centre and hence the periapsis and apoapsis are undefined.

For those who dare to read further, you may be wondering what happens when e is not less than 1 but in fact equals or is greater than 1. This is where it gets freaky. You can try to plot the graphs of various conic sections, the general term for shapes that follow the aforementioned polar equation, on graph paper or on a graphic calculator.

For a conic to have e=1, two possible conditions are possible. The first is degenerate conic, which is a straight line, where p = 0 and things get irritating. The other possibility is the ultimate extension of an ellipse. If you keep the parameter constant, but increase the eccentricity and therefore the semi major axis, you are effectively stretching the ellipse further and further. As e approaches 1, a, and c for that matter, approaches infinity. At this point, we have a parabola. For e>1, the conic is a hyperbola, which is even screwier, has negative c and a and two different curves.

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bmillsap, didn't mean to offset the sun to the left.
Glom, more than I ever knew there was to know about ellipses.

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I must not PX. PX is the mind-killer. I will face my PX. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the PX has gone there will be nothing. Only I will remain.

That's partly because the second looks like the orbit of the Earth is supposed to be perfectly circular, with the sun offset from the center.

The orbit of the Earth is elliptical, as in the first picture -- but the sun is offset from the center, as in the second picture.

Actually, the orbit of the Earth is elliptical, more as in the second picture.

Assuming that the Earth goes from 92mm (million miles ) to 94 mm, the semimajor axis is 93 mm. Using the info Glom set out so handily, the semiminor axis would be the square root of 93^2 minus 1^2. That's 92.995 mm. Almost circular--except the sun is offset down from the center by a million miles.

It's a bit more than that, but you get the picture.