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Is there meaningful reason to distinguish 90 and 270 degree rotational axis?

Keith Thompson
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Gstestso
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    Could you give some context? Have you read a paper or article that makes this distinction? Do you think they're the same? – Warrick Oct 29 '15 at 06:26
  • I think the short answer is that it depends on handedness or (not sure this is a technical term), Mirror equality. Is there a difference between 9:00 PM and 3:00 PM? – userLTK Oct 29 '15 at 07:44
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    @userLTK -- Even better, is there a difference between 6:00 AM and 6:00 PM? – David Hammen Oct 29 '15 at 08:29
  • @Warrick Wikipedia says that Uranus is tilted 98 degrees, not 262 degrees. But Venus, Wikipedia says, is tilted 177 degrees, not 183 degrees. The assumption seems to be that the planets were not formed with retrograde rotation. – LocalFluff Oct 29 '15 at 13:22
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    if you are talking about the axial tilt of planets, then it is given as an angle between 0 and 180 degrees. – James K Oct 29 '15 at 14:50
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    Turning your head 90 degrees to the right is very different from turning it 270 degrees to the left... – DJohnM Oct 30 '15 at 22:27

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No, I don't believe there's any meaningful distinction.

A planet's axial tilt can be thought of as a 2-dimensional quantity: its angle relative to the ecliptic (or to whatever baseline you're using), and the direction in which it's tilted. When we talk about a planet's axial tilt as a number of degrees, we're only talking about the first component.

A planet whose rotation axis is perpendicular to the ecliptic would have a tilt of 0°. If its rotation axis is parallel to the ecliptic (rotating "on its side"), its tilt is 90°. If its axis is "vertical" but it's spinning backwards, its tilt is 180°. The range from 0° to 180° is enough to express all possible tilts, both prograde and retrograde.

Consider taking a planet with a 0° tilt and tilting its axis by 270°. The result is exactly the same as tilting it 90° in the opposite direction. Since the axial tilt is usually expressed as just the magnitude of the tilt, and not its direction, the distinction between 90° and 270° can be ignored.

And if we want to express the direction as well, we'll just say that it's tilted 90° in some specified direction.

We could have had a convention where the tilt ranges from 0° to just under 360°, with the direction specified more narrowly, but that would be less useful; the particular direction of a planet's axial tilt is less interesting than its magnitude. Also, the direction varies over time due to precession.

We could also have had a convention where the tilt ranges from 0° to 90°, and we also specify whether it's retrograde or not; then Venus would have a tilt of 3° rather than 177°.

Keith Thompson
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  • If the tilt is only between 0 and 180, how do we specify which rotation pole is "pointing" towards the star at a given time? Isn't there an ambiguity here that can be resolved by allowing tilt angles between 180 and 360? – ProfRob Oct 29 '15 at 21:20
  • @RobJeffries: Think of it as specifying a single point on the complete sphere that is the sky. That point is where the planet's north pole points. The axial tilt is an angle ranging from 0° (the north celestial pole) to 180° (the south celestial pole); it's just the "latitude" or declination. To specify exactly which point in the sky the planet's north pole points to, you need second coordinate that can range from 0° to 360° (or from -180° to +180°). – Keith Thompson Oct 29 '15 at 21:39