<p>The first accurate determination of the rotation period and the first hints that the rotation period was different from the orbital period (as originally expected) was in 1965 from 430 MHz radar echo measurements from the now-collapsed Arecibo Observatory (RIP) in Puerto Rico. These were reported in a very short paper (1/2 page) in <em>Nature</em> by <a href="https://ui.adsabs.harvard.edu/abs/1965Natur.206Q1240P/abstract" rel="nofollow noreferrer">Pettengill and Dyce 1965</a>. This was followed by a slightly longer paper by Peale and Gold (<a href="https://ui.adsabs.harvard.edu/abs/1965Natur.206R1240P/abstract" rel="nofollow noreferrer">Peale and Gold, 1965, Nature</a>) giving the theoretical context which says (after discussing tidal friction expected from the Sun on Mercury):</p>
<blockquote>
<p>For a planet on a circular orbit the final condition would then be one of synchronous rotation like the motion of the Moon with respect to the Earth. Mercury's motion around the Sun takes 88 days and for synchronous rotation the sidereal period would thus be 88 days also. The observed value of 59 ± 5 days differs markedly from this (see preceding communication).</p>
</blockquote>
<p>(the <code>preceding communication</code> is the radar measurement of Pettengill and Dyce)</p>
<p>As the OP says, the later <a href="https://ui.adsabs.harvard.edu/abs/1965Natur.208..575C/abstract" rel="nofollow noreferrer">November 1965 Nature paper</a> by Giuseppe Colombo seems to be the first to raise the possibility of the rotation period being 2/3rds of the orbital period and therefore it being in a 3:2 spin-orbit resonance:</p>
<blockquote>
<p>IN a recent communication by S. J. Peale and T. Gold the rotational period of Mercury, determined from radar Doppler-spread measurements to be 59 +/- 5 days ....snip...
A very nearly uniform rotational motion of 58.65 sidereal-day period, that is 2/3 of the orbital period, may indeed be a stable periodic solution.</p>
</blockquote>
<p>This is borne out by the 1967 paper by <a href="http://adsabs.harvard.edu/full/1967AJ.....72..351D" rel="nofollow noreferrer">Dyce, Pettengill &amp; Shapiro 1967AJ..72.351</a> (free to view) which discusses the determination of the rotation periods of both Venus and Mercury via radar and discusses what lead to the misleading earlier optical results for the rotation period of 88 days. It also states (page 358):</p>
<blockquote>
<p>In fact Colombo (1965) pointed out the possibility that Mercury's rotation period might be 58.65 days i.e. exactly two-thirds of its rotation period. This value of the rotation period would apparently make Mercury unique in the solar system in having its axial motion locked to its orbital motion in such a manner.</p>
</blockquote>

CC BY-SA 4.0
2021-01-15T01:16:14.260Z
2021-01-15T01:16:14.260Z
astrosnapper
<p>When was it confirmed that Mercury has a 3:2 spin-orbit resonance and by whom (research group/radio observations...)? The first suggestion was made by Giuseppe Colombo in 1965.
Its proximity to the sun posed some problems in nailing the orbit and rotation of Mercury, but what made it eventually possible to measure it? Was there a breakthrough or development of a technology or was it just &quot;steady scientific progress&quot;?</p>

CC BY-SA 4.0
2021-01-13T20:40:31.087Z
I am surprised a little bit on the close vote - I am very curious to the details of a good answer to this very ontopic and interesting question.
peterh
CC BY-SA 4.0
2021-01-13T23:55:46.677Z
I would presume that sending a space probe to Mercury (Mariner 10 in 1973) closed the case once and for all.
Pierre Paquette
<history><mercury><orbital-resonance>
Mercury's spin-orbit resonance
theWrongAlice
# Mercury's spin-orbit resonance

(Asked by: username_0 on 2021-01-13)

When was it confirmed that Mercury has a 3:2 spin-orbit resonance and by whom (research group/radio observations...)? The first suggestion was made by Giuseppe Colombo in 1965.
Its proximity to the sun posed some problems in nailing the orbit and rotation of Mercury, but what made it eventually possible to measure it? Was there a breakthrough or development of a technology or was it just "steady scientific progress"?

**Tags:** history, mercury, orbital-resonance
> **Comments:**
> **username_1**: I am surprised a little bit on the close vote - I am very curious to the details of a good answer to this very ontopic and interesting question.  
> **username_2**: I would presume that sending a space probe to Mercury (Mariner 10 in 1973) closed the case once and for all.  

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## 1 answers

### Answer by username_3 on 2021-01-15. Score: 4 | Accepted answer

The first accurate determination of the rotation period and the first hints that the rotation period was different from the orbital period (as originally expected) was in 1965 from 430 MHz radar echo measurements from the now-collapsed Arecibo Observatory (RIP) in Puerto Rico. These were reported in a very short paper (1/2 page) in *Nature* by [Pettengill and Dyce 1965](https://ui.adsabs.harvard.edu/abs/1965Natur.206Q1240P/abstract). This was followed by a slightly longer paper by Peale and Gold ([Peale and Gold, 1965, Nature](https://ui.adsabs.harvard.edu/abs/1965Natur.206R1240P/abstract)) giving the theoretical context which says (after discussing tidal friction expected from the Sun on Mercury):

> For a planet on a circular orbit the final condition would then be one of synchronous rotation like the motion of the Moon with respect to the Earth. Mercury's motion around the Sun takes 88 days and for synchronous rotation the sidereal period would thus be 88 days also. The observed value of 59 ± 5 days differs markedly from this (see preceding communication).

(the `preceding communication` is the radar measurement of Pettengill and Dyce)

As the OP says, the later [November 1965 Nature paper](https://ui.adsabs.harvard.edu/abs/1965Natur.208..575C/abstract) by Giuseppe Colombo seems to be the first to raise the possibility of the rotation period being 2/3rds of the orbital period and therefore it being in a 3:2 spin-orbit resonance:

> IN a recent communication by S. J. Peale and T. Gold the rotational period of Mercury, determined from radar Doppler-spread measurements to be 59 +/- 5 days ....snip...
> A very nearly uniform rotational motion of 58.65 sidereal-day period, that is 2/3 of the orbital period, may indeed be a stable periodic solution.

This is borne out by the 1967 paper by [Dyce, Pettengill & Shapiro 1967AJ..72.351](http://adsabs.harvard.edu/full/1967AJ.....72..351D) (free to view) which discusses the determination of the rotation periods of both Venus and Mercury via radar and discusses what lead to the misleading earlier optical results for the rotation period of 88 days. It also states (page 358):

> In fact Colombo (1965) pointed out the possibility that Mercury's rotation period might be 58.65 days i.e. exactly two-thirds of its rotation period. This value of the rotation period would apparently make Mercury unique in the solar system in having its axial motion locked to its orbital motion in such a manner.


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