<p>The trajectory of Earth's orbit is shaped not only by Sun's gravity. Earth's orbit is being changed by different sources of gravity as well, for example center of our galaxy. This is why it's not perfectly round.</p>

CC BY-SA 3.0
2014-12-18T13:03:36.303Z
So, that means that the second foci of the elliptical orbit (one being the sun) of which kepler wrote is towards the center of the Milky Way.
Dheeraj Kumar
CC BY-SA 3.0
2014-12-18T13:41:31.710Z
Precisely, this is a good explanation of this phenomenon: http://www.astro.washington.edu/users/smith/Astro150/Tutorials/Kepler/
duke
CC BY-SA 3.0
2014-12-18T14:24:54.540Z
@duke How can the second foci be the center of the galaxy?
Yashbhatt
CC BY-SA 3.0
2014-12-18T15:22:15.167Z
@Yashbhatt I didn't say it is, it can't be. But the fact is that the center of Milky Way has a major impact on the location of second foci.
duke
CC BY-SA 3.0
2014-12-18T15:25:18.070Z
-1. This answer is incorrect.
David Hammen
CC BY-SA 3.0
2014-12-18T21:16:43.850Z
I suggest you calculate (an order-of-magnitude estimate should suffice) the gravitational influence on Earth of the galactic core relative to the influence of the Sun. I think you'll find that the galactic core's influence is negligible compared to the Sun, or even to the other planets. Earth's orbit could be elliptical rather than circular even in the absence of any bodies other than the Earth and the Sun; external influences aren't necessary to explain the eccentricity. -1. (And your link says nothing about the influence of the Milky Way.)
Keith Thompson
CC BY-SA 3.0
2014-12-18T21:30:23.067Z
Hmm. We're currently close to perihelion (which will be on Jan 4), and the Sun is in or near Sagittarius, which is in the direction of the galactic core. So the second focus of the Earth's orbit around the sun *is* in the direction of the galactic core. I believe this is purely coincidental. Can you demonstrate that it's not? (The same effect should apply to the other planets.)
Keith Thompson
CC BY-SA 3.0
2014-12-18T12:04:53.013Z
2014-12-18T13:03:19.327Z
2014-12-18T13:03:19.327Z
duke
<blockquote>
  <p>According to wikipedia and other sources, a planet and a star always move in a circular orbit around the common center of mass of the both bodies ...</p>
</blockquote>

<p>This is not true. In the absence of other gravitational sources, a planet and a star move in elliptical orbits about the common center of mass. Ancient scientists assumed circular orbits, but only because circles were somehow most pure. There were problems with this: It didn't match what those ancient scientists saw. So they modified these circular orbits by tacking on other circles. It was a mess.</p>

<p>Kepler cleaned this mess up by noting that the orbits of the planets are very close to elliptical rather than circular. This empirical result was later strengthened by Newton, who derived that elliptical orbits (not circular orbits) are a natural consequence of a force that follows an inverse square law. Gravitation is such a force.</p>

<p>The orbits of the planets about the solar system are not quite elliptical because planets are attracted to one another as well as to the Sun. Nonetheless, the orbits are still very close to elliptical. The Sun is more than 1000 times more massive than Jupiter. Jupiter, Saturn, and the other planets represent tiny perturbations on the nearly elliptical orbits of the planets about the Sun.</p>

<p>What about other perturbing effects, such as nearby stars and the Milky Way as a whole? These are immeasurably small. A passing star will perturb planetary orbits over the course of the tens of millions of years or more. The Milky Way as a whole has essentially no effect whatsoever. Theoretically, the effect most certainly does exist, but it is so small that it can be ignored, even over the lifespan of the solar system.</p>

CC BY-SA 4.0
2020-11-02T16:01:59.267Z
Venus' orbit isn't "very close to elliptical", it's quite circular. Fully circular orbits with exactly 0 eccentricity are rather impossible in this universe.
John
CC BY-SA 3.0
2014-12-18T15:39:03.787Z
"a planet and a star move in elliptical orbits about the common center of mass", is the common center of mass center of that ellipse?
Dheeraj Kumar
CC BY-SA 3.0
2014-12-18T15:40:04.557Z
@DheerajKumar no, or rather, it's the center only in the case where the ellipse is a circle (a special case)
Stan Liou
CC BY-SA 3.0
2014-12-18T15:40:17.983Z
Anyway, if one focus is the sun then what is the **second** focus?
Dheeraj Kumar
CC BY-SA 3.0
2014-12-18T15:46:35.800Z
@DheerajKumar There's nothing special at the second focus, but there's a derivation of how to find it at [Why are orbits elliptical instead of circular?](https://astronomy.stackexchange.com/questions/1234/why-are-orbits-elliptical-instead-of-circular/).
Stan Liou
CC BY-SA 3.0
2014-12-18T15:57:12.230Z
so orbit is elliptical basically due to conservation of angular momentum.
Dheeraj Kumar
CC BY-SA 3.0
2014-12-18T16:13:19.553Z
@DheerajKumar Well, if you get down to it, orbits are elliptical because of conservation of the [LRL vector](http://en.wikipedia.org/wiki/Laplace%E2%80%93Runge%E2%80%93Lenz_vector) and energy, and though one can show that LRL is conserved as a result conservation of angular momentum, in some hypothetical world it's possible for LRL to be conserved without angular momentum conservation. Orbits would still be elliptical in that case (Kepler's first law would hold), but Kepler's second law (equiv. to angular momentum conservation) would be broken.
Stan Liou
CC BY-SA 3.0
2014-12-18T15:36:55.920Z
2014-12-18T15:36:55.920Z
David Hammen
<p>According to wikipedia and other sources, a planet and a star always move in a circular orbit around the common center of mass of the both bodies, in case  of Earth and sun, this center of mass lies inside the sun. Therefore, Earth should orbit the sun in a circular orbit. So, my question is, why does the Earth orbits the sun in an Elliptical orbit?  </p>

<p>And, if it does, and one focus is the sun, then what is the other focus?</p>

CC BY-SA 3.0
2014-12-18T10:03:07.670Z
i would suggest you to go through kepler's first law. That would answer your question.
Strikers
<orbit><the-sun><gravity><solar-system><earth>
What is the orbit of Earth around the Sun?
Dheeraj Kumar
# What is the orbit of Earth around the Sun?

(Asked by: username_0 on 2014-12-18)

According to wikipedia and other sources, a planet and a star always move in a circular orbit around the common center of mass of the both bodies, in case of Earth and sun, this center of mass lies inside the sun. Therefore, Earth should orbit the sun in a circular orbit. So, my question is, why does the Earth orbits the sun in an Elliptical orbit?

And, if it does, and one focus is the sun, then what is the other focus?

**Tags:** orbit, the-sun, gravity, solar-system, earth

> **Comments:**
> **username_1**: i would suggest you to go through kepler's first law. That would answer your question.  

---

## 2 answers

### Answer by username_4 on 2014-12-18. Score: 10 | Accepted answer

> According to wikipedia and other sources, a planet and a star always move in a circular orbit around the common center of mass of the both bodies ...

This is not true. In the absence of other gravitational sources, a planet and a star move in elliptical orbits about the common center of mass. Ancient scientists assumed circular orbits, but only because circles were somehow most pure. There were problems with this: It didn't match what those ancient scientists saw. So they modified these circular orbits by tacking on other circles. It was a mess.

Kepler cleaned this mess up by noting that the orbits of the planets are very close to elliptical rather than circular. This empirical result was later strengthened by Newton, who derived that elliptical orbits (not circular orbits) are a natural consequence of a force that follows an inverse square law. Gravitation is such a force.

The orbits of the planets about the solar system are not quite elliptical because planets are attracted to one another as well as to the Sun. Nonetheless, the orbits are still very close to elliptical. The Sun is more than 1000 times more massive than Jupiter. Jupiter, Saturn, and the other planets represent tiny perturbations on the nearly elliptical orbits of the planets about the Sun.

What about other perturbing effects, such as nearby stars and the Milky Way as a whole? These are immeasurably small. A passing star will perturb planetary orbits over the course of the tens of millions of years or more. The Milky Way as a whole has essentially no effect whatsoever. Theoretically, the effect most certainly does exist, but it is so small that it can be ignored, even over the lifespan of the solar system.

> **Comments:**
> **username_0**: "a planet and a star move in elliptical orbits about the common center of mass", is the common center of mass center of that ellipse?  
> **username_7**: @username_0 no, or rather, it's the center only in the case where the ellipse is a circle (a special case)  
> **username_0**: Anyway, if one focus is the sun then what is the **second** focus?  
> **username_7**: @username_0 There's nothing special at the second focus, but there's a derivation of how to find it at [Why are orbits elliptical instead of circular?](https://astronomy.stackexchange.com/questions/1234/why-are-orbits-elliptical-instead-of-circular/).  
> **username_0**: so orbit is elliptical basically due to conservation of angular momentum.  
> **username_7**: @username_0 Well, if you get down to it, orbits are elliptical because of conservation of the [LRL vector](http://en.wikipedia.org/wiki/Laplace%E2%80%93Runge%E2%80%93Lenz_vector) and energy, and though one can show that LRL is conserved as a result conservation of angular momentum, in some hypothetical world it's possible for LRL to be conserved without angular momentum conservation. Orbits would still be elliptical in that case (Kepler's first law would hold), but Kepler's second law (equiv. to angular momentum conservation) would be broken.  
> **username_6**: Venus' orbit isn't "very close to elliptical", it's quite circular. Fully circular orbits with exactly 0 eccentricity are rather impossible in this universe.  

---

### Answer by username_2 on 2014-12-18. Score: -6

The trajectory of Earth's orbit is shaped not only by Sun's gravity. Earth's orbit is being changed by different sources of gravity as well, for example center of our galaxy. This is why it's not perfectly round.

> **Comments:**
> **username_0**: So, that means that the second foci of the elliptical orbit (one being the sun) of which kepler wrote is towards the center of the Milky Way.  
> **username_2**: Precisely, this is a good explanation of this phenomenon: http://www.astro.washington.edu/users/smith/Astro150/Tutorials/Kepler/  
> **username_3**: @username_2 How can the second foci be the center of the galaxy?  
> **username_2**: @username_3 I didn't say it is, it can't be. But the fact is that the center of Milky Way has a major impact on the location of second foci.  
> **username_4**: -1. This answer is incorrect.  
> **username_5**: I suggest you calculate (an order-of-magnitude estimate should suffice) the gravitational influence on Earth of the galactic core relative to the influence of the Sun. I think you'll find that the galactic core's influence is negligible compared to the Sun, or even to the other planets. Earth's orbit could be elliptical rather than circular even in the absence of any bodies other than the Earth and the Sun; external influences aren't necessary to explain the eccentricity. -1. (And your link says nothing about the influence of the Milky Way.)  
> **username_5**: Hmm. We're currently close to perihelion (which will be on Jan 4), and the Sun is in or near Sagittarius, which is in the direction of the galactic core. So the second focus of the Earth's orbit around the sun *is* in the direction of the galactic core. I believe this is purely coincidental. Can you demonstrate that it's not? (The same effect should apply to the other planets.)  

---

