display | more...

The axial inclination of a planet is the angle the axis around which the planet spins makes with the normal to the orbital plane.

In simpler terms: The axial inclination of the Earth is about 23.5°. This means that if you live within 23.5° of the Equator, you will see the sun at zenith sometime during the year. This area is called the Tropics, the Tropic of Cancer is 23.5° North of the equator and is the northernmost latitude at which you can see the sun at zenith, the Tropic of Capricorn is 23.5° South and is the southernmost latitude at which you can see the sun at zenith. Also, if you live withn 23.5° of the Poles, there will be some days where the Sun never sets and there will be some days where the sun never rises. The Arctic circle is 23.5° from the North pole, and is the southernmost point at which this happens, the Antarctic circle is 23.5° from the South pole and is the northernmost point at which this happens.

This angle appears again and again as you examine further. The highest the sun gets above the horizon at the poles is 23.5°. The lowest the sun gets at midday on the equator is 23.5° from zenith. The value tan(23.5°) is a minimum for certain dimensions of some sundials. The analemma is twice 23.5°, or 47°, 'tall'.

A planet with no axial inclination would be very boring indeed. Every day, everywhere on the planet, would be exactly half daylight and half night (except the poles). There would be no seasons.

A planet with an axial inclination of greater than 45° would be an interesting place to live. There would be regions where the sun reaches zenith sometimes, sometimes doesn't set for days, and sometimes doesn't rise for days. Weather would be extreme, and more than that it would alternate between extremes.

A planet with an axial inclination of 90° would be even more extreme. The poles would experience half a year of daylight and half a year of nighttime, like Earth, except in the middle of the daylight the sun would be standing still at zenith (instead of circling at 23.5° above the horizon). The equator would be the most hospitable (or at least, the most mild) place on the planet. The sun would skim the horizon at first, one revolution each 'day', then the circle the sun traces across the sky would tilt so it is above the horizon on one side and below on the other - half daylight, half night, every day. It would continue to tilt until the sun hits zenith at midday, continues so it gets lower on the other side of the sky, and after half a year it skims the horizon again - in the opposite direction! Then it continues tilting, there are days and nights again with the sun going the opposite direction across the sky, and back to the beginning after a year. How could one define 'East' or 'West' on such a planet (or 'North' or 'South')?

A planet with an axial inclination of between 90° and 180° is 'upside down', in that the direction of the spin is opposite the direction of the orbit. 180° is the maximum value the axial inclination can be.

When a planet is in tidal lock with the star it is orbiting (or a moon is in tidal lock with the planet it is orbiting), there is essentially no spin on the planet / moon, and the idea of axial inclination makes no sense - there's no axis!

Log in or register to write something here or to contact authors.