# What is the axial tilt of a planet measured relative to?

I am very much a beginner on the astronomy front but I understand about planets having different axial tilts, hence why Venus turns the opposite direction from the Earth and Uranus turns sideways.

However, I am confused as to what the axial tilt bearing is measured from. For example, if all planets were in a perfect line from the sun, would they spin perfect in alignment on their own various axial tilts? Would some still be 'off-set' by a few degrees to one side or the other?

If you look at the Astronomical Almanac for the year 2011 as an example, in the table at the top of page E3, you find two measures of axial tilt. The third column is the declination of the planet's north pole for the mean equinox and equator of date 2011 January 0, 0 hours terrestrial time. So this is measured with respect to the Earth's orbit. The last column is the inclination of the planet's equator to the planet's orbit.

If you look at the right ascension column in the same table, you see that on that particular date the right ascensions are all different.

• THAT sounds like exactly what I need! Thank you very much Mr Ashton! – HugMyster Jul 6 '15 at 15:37

Draw a line perpendicular to the planet's orbit. Measure the angle between this line and the planet's axis. That's the axial tilt, and it doesn't matter what other planets are doing.

The Wikipedia page on axial tilt has some diagrams that help clarify this.

• That's informative, I didn't know that. So it is not relative to the ecliptic (except for Earth)? For example, Ceres' has an axial tilt of only 3°, but its orbit has an inclination of about 10°. So it still has "seasons" that make up for the difference (or the sum, I suppose)? – LocalFluff Jul 6 '15 at 14:38
• No, it's not relative to the ecliptic. And yes, Ceres still has 'seasons' (although neither of those percentages would really be enough to cause much of an effect). It may be helpful to imagine a planet with an orbital inclination of 90 degrees - which orbits the sun at right angles to the other planets. And then imagine what axial tilts of 0 and 90 degrees would physically mean in each of those situations. – chrispittman Jul 6 '15 at 14:48
• Just as a thought, doesn't Uranus fit in that category? It has an axial tilt of something huge like about 80°. – HugMyster Jul 6 '15 at 15:38
• Absolutely, yes. – chrispittman Jul 6 '15 at 15:57