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How do we know that Mercury, Earth, Mars, Jupiter, Saturn, and Neptune rotate counterclockwise and Venus, Uranus, and Pluto rotate clockwise? How do scientists determine the direction of rotation of planets?

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    $\begingroup$ There are two aspects to this question. 1: How do we observe the rotation? 2: How do we define which direction is clockwise for a given celestial body? $\endgroup$
    – PM 2Ring
    Aug 16, 2022 at 8:16
  • $\begingroup$ please also note that depending on from which pole of the solar system you're looking at a given planet (except Uranus), the rotation could be either clockwise or counterclockwise. $\endgroup$
    – mysterium
    Aug 16, 2022 at 8:34
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    $\begingroup$ Everything spinds clockwise. It just depends on if you're looking at it from the wrong end. $\endgroup$ Aug 16, 2022 at 16:55

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I'll focus first on the question of the title: "how do we find / measure rotation?"

The easiest method is the same as everyone else: look and see. Take images some time apart and you will see in which direction distinctive surface features moved around the rotation axis. The easiest might be Jupiter with its great red spot, but every planet has surface (but also for atmospheric features like storms) which move.

For planets with an atmosphere with little features you can employ spectroscopy and measure the velocity via Doppler shift. Easiest for these measurements is to pick a strong emission or absorption line for one of the main elements in that planet and look at the spectrum spatially resolved. You then will find that it is slightly blue-shifted on one side and slightly red-shifted on the other side. There is a nice description how to achieve that even on your own with slightly advanced amateur equipment.

Now, once we saw or measured the rotation, we know how the 3D-orientation of the rotational axis, we can look at what is prograde and counterclockwise: We consider prograde rotation when the rotational axis of the planet roughly (maximum +-90°) align with the rotational axis of its orbital plane. You can take your right hand: your fingers point in direction of rotation, then your thumb points in the direction of the rotational axis. In our Solar system that's towards the North and only deviates for Venus (177°) and somewhat for Uranus which kinda rolls on its orbit with an obliquity of 98°.

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    $\begingroup$ I assumed Jupiter would be a very complicated example because don't we have to account for the spot moving across the "surface" of Jupiter due to winds in addition to the rotation of the planet? Seems like Mars would be the easiest example because the "canals" are easily seen in visible light telescopes and the atmosphere is transparent. $\endgroup$ Aug 16, 2022 at 18:50
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    $\begingroup$ Venus and Neptune were presumably a bit tricky to measure visually, because they're both featureless fuzzballs with odd rotations. $\endgroup$
    – Mark
    Aug 17, 2022 at 3:03
  • $\begingroup$ @ToddWilcox the problem for gas giants is the definition of "surface". So you have to use whatever you can measure (e.g. speed at 1-bar 'surface' or optical depth 1), which may be the clouds or the motion of the gas of the atmosphere to define rotation. Like on Earth, of course, the different latitudes experience different winds, so you can apply a reasonably-weighted average in order to get a global rotational speed. @ Mark: yes. You have to employ spectroscopy and use Doppler measurements in these cases. $\endgroup$ Aug 17, 2022 at 8:58
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This question about the direction of rotation is a bit ambiguous, as "direction" can be interpreted as retrograde/prograde -or- as the direction of the north pole axis.

This answer is about the direction of a planet's axis of rotation. When it comes to the direction of the rotation, the method that most people use is described in the following paper:

Cartographic Coordinates and Rotational Elements

Essentially, the idea is to calculate de right ascension and declination values of the north pole of each planet using formulas described in the document. Each formula is planet specific, due to its unique interactions with other bodies in the solar system. The values of the right ascension and declination are expressed using the planet's ecliptic equator (the intersection of the sphere and the International Celestial Reference Frame (ICRF), which is fixed to the stars and centered on the solar system barycenter).

enter image description here Once, the north pole is determined, the equatorial plane can be deducted (the plane orthogonal to the vector defined between the north pole and the planet center and containing the planet center) as well as the line of nodes that define the intersections of the ICRF plane and the planet's equatorial plane.

The last step is to properly orient the north pole to a given direction. This is achieved by calculating the angle W that specifies the position of the line of nodes. Again, the formula to calculate the value for W is planet specific. Using the value of W, the north Pole axis can be rotated around the ICRF z-axis (0,0,1), to position the line of nodes properly.

In each formula, the right ascension, the declination, and the position of the prime meridian are calculated using:

  • T = interval in Julian centuries (of 36525 days) from the standard epoch J2000
  • d = interval in days from the standard epoch J2000
  • The standard epoch is JD 2451545.0, i.e. 2000 January 1 12 hours TDB
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    $\begingroup$ Your answer would greatly improve if you could paraphrase the gist of your link (which indeed is a good resource). As it stands, this is a link-only answer which is frowned upon $\endgroup$ Jan 12, 2023 at 3:36
  • $\begingroup$ update: In addition to the link, I incorporated a brief description of the method used to calculate the planet's north pole direction. $\endgroup$
    – ChuckM
    Jan 13, 2023 at 17:41

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