<p>If the semi-major axis of an ellipse is <em>a</em> and the eccentricity is <em>e</em>,
then the distance from the center <em>C</em> to a focus <em>F<sub>1</sub></em> or <em>F<sub>2</sub></em> is <em>c = ea</em>.
Apoapsis is <em>a + c</em> and periapsis is <em>a - c</em>.</p>

<p><a href="https://en.wikipedia.org/wiki/Ellipse" rel="noreferrer">Ellipse</a> with <em>e</em> &approx; 0.78:
<br/><img src="https://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/Ellipse-param.svg/310px-Ellipse-param.svg.png" alt="diagram of ellipse"></p>

<p>For the <a href="https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html" rel="noreferrer">Earth's orbit</a> around the Sun, <em>a</em> = 149.6 million km and <em>e</em> = 0.0167, so</p>

<ul>
<li><em>c = ea</em> = 2.5 million km</li>
<li>aphelion = <em>a + c</em> = 152.1 million km</li>
<li>perihelion = <em>a - c</em> = 147.1 million km</li>
</ul>

CC BY-SA 4.0
2019-12-17T21:31:56.073Z
2019-12-17T21:31:56.073Z
Mike G
<p>The semi-major axis is half of the diameter of the major axis of an ellipse by definition. So how does that make it any different from apoapsis, the furthest point an object is from a parent body in its orbit?</p>

<p>Both are radii along the longest axis, surely.</p>

CC BY-SA 4.0
2019-12-17T12:04:15.833Z
The central body is at one focus of the ellipse, not at the centre.
Steve Linton
CC BY-SA 4.0
2019-12-17T15:23:41.887Z
Hi, it's always worth drawing -- or finding -- a diagram before asking basic questions.
Carl Witthoft
<orbit><planet><solar-system><asteroids>
What's the difference between semi-major axis and apoapsis?
Will
# What's the difference between semi-major axis and apoapsis?

(Asked by: username_0 on 2019-12-17)

The semi-major axis is half of the diameter of the major axis of an ellipse by definition. So how does that make it any different from apoapsis, the furthest point an object is from a parent body in its orbit?

Both are radii along the longest axis, surely.

**Tags:** orbit, planet, solar-system, asteroids

> **Comments:**
> **username_1**: The central body is at one focus of the ellipse, not at the centre.  
> **username_2**: Hi, it's always worth drawing -- or finding -- a diagram before asking basic questions.  

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## 1 answers

### Answer by username_3 on 2019-12-17. Score: 4

If the semi-major axis of an ellipse is *a* and the eccentricity is *e*,
then the distance from the center *C* to a focus *F1* or *F2* is *c = ea*.
Apoapsis is *a + c* and periapsis is *a - c*.

[Ellipse](https://en.wikipedia.org/wiki/Ellipse) with *e* ≈ 0.78:
  
![diagram of ellipse](https://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/Ellipse-param.svg/310px-Ellipse-param.svg.png)

For the [Earth's orbit](https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html) around the Sun, *a* = 149.6 million km and *e* = 0.0167, so

* *c = ea* = 2.5 million km
* aphelion = *a + c* = 152.1 million km
* perihelion = *a - c* = 147.1 million km


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