Expert: Philip Stahl - 11/22/2004

Question
-------------------------
Followup To
Question -
I have a question for a book I'm writing, I'm hoping you can answer it. I want to have the axis of the planet in my book tilted differently.
What would the earth be like, for example, if the planet were not tilted at all? Or tilted in the opposite direction?


Last part of the Answer

It also, of course, means the elimination of - or rather reduction of "seasons". (Which as you know arise precisely as a result of the axial tilt). Now, with 0 degrees tilt, you have essentially no change of seasons over the year. (the axial tilt remains the same as the Earth goes round in its orbit, so there's no change in solar insolation, or the amount of sunlight received per latitude location).

Re: tilted "in the opposite direction" I am not sure what you mean. Perhaps you might clarify that.

Humm. It's been awhile since science class. I'm really having to use the old brain here. ;) So the sun is going to shine only on the middle of the planet if I understand this correctly. So the middle parts would stay warm and light, and the polls (and the areas where the sun doesn't shine) would stay frozen and dark, is this correct?  Would this create a sort of permenant tropics in the areas where the sun constantly shines?
You don't need to answer the other question about the opposite tilt, I think I've found what I need with your answer to my first question.

You have helped me immensly. Thank you.

Rayne

Answer
Hello.

The situation at poles and the rest of the planet (for the no tilt case) would be exactly the same as for the Earth now, when there is coincidence between ecliptic and celestial equator.

That is, the two dates for the *equinoxes* ("equal day" - "equal night") all over the world. At those two dates (Mar. 21 and Sept. 23) on our tilted world, day and night are equal. They are further equal for *all latitudes* - though of course the duration varies depending on the latitude. (I.e. the longest duration - 12 hrs. each- occurs at the equator. The others diminish with latitude going north and south of the equator.)

Do only the "middle parts" stay warm and light on March 21 and Sept. 23? I don't think so. You will still get a measure of warmth even as far north as Oslo or beyond, or as far south as Santiago, Chile, or Melbourne. But the point is that there is not the *extreme* of temperature that one will find say during the summer, and during the winter.

In other words, if you wish - you can think of a condition that will perpetually "mimic" the first day of Spring for ALL over the planet! (Or, if you prefer- the first day of Fall!)

It would seem that a permanent tropic is no more likely than this than it is now. (Though as global warming increases, as it is doing, that becomes an ever greater probability).

Re: the poles, as with the other temperate, N & S latitudes - they would be neither as *extremely* light (Sun visible 24 hrs. a day), nor as *extremely dark* (Sun not visible at all) as they would be in a (23.5 deg) tilted world.

Geometrically speaking, since the Sun is *directly over* the equator (0 degrees lat.) , and the poles are 90 degrees from the equator, just as the zenith (overhead point) is 90 degrees from the horizon, the Sun would sit perpetually on the observer's horizon at the poles.

You would have a weird kind of perpetual twilight.

Hope all this helps.