# Difference between axial tilt and inclination

1. Oct 13, 2009

### Philosophaie

What is the difference between axial tilt and inclination? I know that axial tilt is the angle between the perpendicular to the elliptical plane and the north pole with the right time of year facing the sun. I guess the Inclination to the Sun's Equator is self explanatory. The other two I have no idea even after reading their Wiki articles. Could someone please explain?

axial tilt(Earth)= 23.49693deg @2000 epoch
inclination to the elliptic(Earth) = N/A
Inclination to the Sun's Equator(Earth) = 7.155deg
Inclination to the Invariable Plane(Earth) = 1.57deg

2. Oct 14, 2009

### Chronos

Axial tilt is the angle of the planetary rotational axis relative to its orbital plane around the sun. Inclination is the angle of its orbital plane relative to the solar rotational axis.

3. Oct 14, 2009

### Janus

Staff Emeritus
In astronomy, the most common measurement for inclination of planets is relative to the ecliptic or the plane of the Earth's own orbit.

Of course this gives a zero (or Not Applicable) value for the Earth's own inclination. However sometimes you will see a small non-zero value given. This value is relative to the inclination at some particular reference date (like the vernal equinox for the Julian year 2000).

The invarible plane is the average plane through which all the planets orbit, weighted by angular momentum. If you average out the angular momentum of the Solar system (ignoring the Sun). you get a plane through which you can say the system rotate in. This is the invarible plane.

4. Oct 14, 2009

### Philosophaie

The Inclination based on the elliptic is the angle between the line between the (Earth's center of mass and Sun's center of mass) and the Sun's axis of rotation. The data enclosed says that Earth is N/A. Why?

Does not the Inclination change or at least the angle changes due to the fixed nature of the tilt dynamics of the planets and the sun. The axial tilt does not change except it's orientation towards the sun.

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• ###### Inclination to Elliptic, Sun's Equator or Invariable Plane.xls
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Last edited: Oct 14, 2009
5. Oct 16, 2009

### Philosophaie

Now we are forgetting the whole Barycenter concept. I think the plane of the axial tilt is still going to be the same no matter where the center of mass is, within a few hundreth of a degree. The axial tilt produces the same seasons year round as the Earth travels around the Sun. The Inclination is the angle the elliptic makes with planes of the Sun.

6. Oct 16, 2009

### Janus

Staff Emeritus
The ecliptic is the plane of the Earth's orbit and has nothing to do with the Sun's axis of rotation unlike the inclination based on the Sun's equator which does.

It is N/A because it is meaningless to talk about the tilt of the Earth's orbit wth respect to the Earth's orbit.

7. Oct 17, 2009

### Philosophaie

The Line of Nodes is the point where the path of the plane of the planet and the moon intersect. The sun and its planets also base their orbital calculations on the Longitude of the Ascending Node and this Line of Nodes. The ancients based their observations on the Cusp of the Constellation Aries (or in Pisces now) at the equatorial center of the sky during the Vernal Equinox. This point is where the Longitude of the Ascending Node from is most calculation of Ellipses and most other orbital calculations are based. Where is this plane come into play in the calculations of the axial tilt and inclination?

Last edited: Oct 17, 2009
8. Oct 17, 2009

### Janus

Staff Emeritus

For moons orbiting planets, the inclination is measured with respect to the planet's equator. However, for planet's orbiting the Sun the most common and most convenient reference is the ecliptic, or the plane of the Earth's own orbit. In this case, the Line of Nodes is the path where the plane of the planet of interest's orbit intersects the plane of the Earth's orbit. The position of the ascending and descending nodes are measured from the position of the Vernal Equinox.

Axial tilt is measured with respect to the planet's own orbit. Thus Jupiter has an inclination of 1.31° with respect to the Earth's orbit, and an axial tilt of 3.13° with respect to a line perpendicular to its own orbit.