# Relation between earth position in orbit and its tilt

pixel01
The Earth revolves 360 degrees around the sun and its tilt changes from 23.5 to -23.5 and then back to 23.5 degrees. My question is whether the Earth axis tilt is linearly propotional to its position in the orbit? We know that at 4 positions : 0o, 90o, 180o and 270o , it fits.
Thanks for any helps.

PS. Assume that the Earth orbit is perfectly round

Last edited:

Staff Emeritus
Gold Member
The Earth's axis of rotation remains constant throughout the year. That is why we have Polaris as the north star. It remains fixed in the sky. If the axis of rotation, that is the tilt of the Earth changed over the period of year, we would not have a fixed pole star.

What changes is the relationship between the sun and tilt of the earth, this is because the Earth moves in its orbit about the sun. In June the north hemisphere is pointed at the sun, in December the southern hemisphere is pointed at the sun. Not because the tilt of the Earth has changed but because the LOCATION of the Earth has changed. The tilt remains constant.

Since the relationship between the Earth's tilt and the sun is periodic, it cannot be linear. It is sinusoidal.

pixel01
The Earth's axis of rotation remains constant throughout the year. That is why we have Polaris as the north star. It remains fixed in the sky. If the axis of rotation, that is the tilt of the Earth changed over the period of year, we would not have a fixed pole star.

What changes is the relationship between the sun and tilt of the earth, this is because the Earth moves in its orbit about the sun. In June the north hemisphere is pointed at the sun, in December the southern hemisphere is pointed at the sun. Not because the tilt of the Earth has changed but because the LOCATION of the Earth has changed. The tilt remains constant.

Since the relationship between the Earth's tilt and the sun is periodic, it cannot be linear. It is sinusoidal.

Yeah you are right. I expressed the idea not clearly. Here I mean the 'relative tilt' of the Earth compared to the sun. Here on earth, we see the sun's position changes over time in a year, so its 'relative tilt' toward the sun must change as well.
In other way, my question is if the postion of the sun in sky is linearly propotional to the postion of the Earth in its orbit.

Staff Emeritus
Gold Member
Once again. No it is not linear, by its periodic nature it is sinusoidal. This is observable, note how fast the day length is changing now. We are just a few days way from the vernal equinox, the day length change rate is very nearly linear at this time, but that only occurs in the weeks surrounding the equinoxes. As we approach the solstice the rate of change slows down, to point that over the period of the month around the solstice the day length changes very little.

Staff Emeritus
Some globes display an elongated figure-8 symbol with date markings in the middle of the Pacific Ocean. There obviously is no real figure-8, elongated or otherwise, in the middle of the Pacific Ocean. So what is this thing? Pick a specific time of day, stick a pole in the ground, and plot the position of the tip's shadow at the chosen time every day over the course of a year. The resultant curve will be a slightly lopsided figure-8 called an "analemma".

An analemma was even featured in the movie "Castaway". Tom Hank's character (Chuck) drew one on the wall of his cave to serve as a calendar. Unfortunately, Chuck's watch failed early in the movie. Without a watch he could not have drawn the analemma on the cave wall (picky, picky, picky; at least the movie did have some science in it).

Last edited:
Homework Helper
… apples and tops …

Hi pixel01!
Alan Bergman said:
"The world is like an apple whirling silently in space"

The Earth is more-or-less on its own, so far as spin is concerned.

So it behaves like a spinning top (imagine a doubled one, a bit like a double cone), and its motion is entirely regular.

Well, almost entirely … see http://en.wikipedia.org/wiki/Precession#Of_the_Earth.27s_axis

(btw, do a google search for "the Earth is like an apple" - it's fascinating! )

Last edited: