From my related question, I'm still in search of historical knowledge of the outer planets. That is, how our understanding of them developed over time.

So right now I'm hunting for the highest quality photos of Uranus and Neptune that we had before Voyager flew by them.

I've looked here since it has a good one of Jupiter, but couldn't find any for Neptune or Uranus. I'm starting to wonder if those last 2 planets would even show up as resolvable disks, given the huge distances and relatively small telescopes of the era. I think the Hale Telescope at the Palomar Observatory was just 5 m wide in its primary mirror.

After googling this for so long, I've come to the conclusion that the best bet would be old astronomy books, which sadly, I do not have. Can anyone help me out?

  • $\begingroup$ It no doubt was taken with Hale, but I can't really find anything good about it. Uranus would only be about 18 urads (3.7 arcseconds) from Earth at best. I can't find what Hale's resolution was historically, but I believe it wasn't much. $\endgroup$ – PearsonArtPhoto Jul 19 '15 at 11:43
  • $\begingroup$ Maybe the astronomers at astronomy.SE can provide such an image? Planets are often observed from Earth while spacecrafts visit them. $\endgroup$ – LocalFluff Jul 19 '15 at 12:11
  • $\begingroup$ @LocalFluff it's possible, but "astronomy" in their case could be more like study of stars/space only. Space exploration can and has been done with telescopes for a long time, but if it proves necessary, I will migrate the OP. $\endgroup$ – DrZ214 Jul 19 '15 at 20:08

My 1965 Sourcebook On The Space Sciences indicates that they are resolvable:

The polar flattening of [Uranus] is clear from its telescopic image and the ellipticity determined from the motion of the nearest satellite is 1/18. The result derived from direct measurement of the equatorial and polar diameters is about 1/12, but since the maximum angular diameter of Uranus as seen from Earth is only 4 sec of arc, it is evident that this value of the ellipticity cannot be very accurate...

In the telescope, Uranus is seen as a somewhat flattened spherical body with a greenish color... at certain times a whitish equatorial zone and other bands, similar to but fainter than those of Jupiter and Saturn, can be observed.

Like Uranus... Neptune has a greenish color... Observers have occasionally reported seeing faint irregular markings and a bright equatorial zone on Neptune, but the situation is uncertain.

However, there are no photos of either in the book, unlike Jupiter and Saturn, which both have fairly detailed mid-century photos from Hale-Palomar.

The 1964 English translation edition of Flammarion Book Of Astronomy has photos of Uranus from 1936! A very small disc is visible, with no evident features. No photos of Neptune are in the book, possibly because it would show merely a point.

enter image description here

Interestingly, the book has a variety of pictures of Jupiter and Saturn from the 1910s through the 1950s, from different observatories (Flammarion, Lowell). Remarkably, they all show comparable levels of detail in Jupiter, with the main difference, as far as I can tell, being the quality of the photographic film grain.

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    $\begingroup$ Interesting since the true flattening seems to be 1/44. 1/18 is closer to those of Jupiter and Saturn at 1/16 and 1/10, so maybe the interpretation was biased. Or maybe Uranus' 98° tilt made observations harder during the mid 20th century. Found this paper by Cook from 1979 about flattening of U and N with a much better value. Derived from physics instead of observation, but with ref's to other papers. $\endgroup$ – LocalFluff Jul 19 '15 at 16:19
  • $\begingroup$ Nice find, it only whets my appetite even more for the photos tho! And that's another question I had, if we knew about Uranus' 90 degree tilt before Voyager? If so, we must have been able to resolve details on the disk somehow. $\endgroup$ – DrZ214 Jul 19 '15 at 20:05
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    $\begingroup$ Yup, the same book has U's axial tilt at 98 degrees, but determined by Doppler shift of spectral lines rather than direct observation -- but that accurate determination implies quite good resolution if I understand the technique correctly! $\endgroup$ – Russell Borogove Jul 19 '15 at 20:44
  • $\begingroup$ @DrZ214 William Herschel discovered Titania and Oberon already in 1787. The plane of their orbits should at least strongly have suggested that Uranus itself was tilted correspondingly. $\endgroup$ – LocalFluff Jul 19 '15 at 21:32
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    $\begingroup$ @DrZ214, the book page has two images (only the bottom strip of the first image is visible at the top of the JPG. The first (not visible) image would have been taken on December 19, while the one seen in the picture was taken on December 30. $\endgroup$ – BowlOfRed Aug 11 '15 at 7:13

I believe it's so hard to find a good picture because there wasn't one. The best telescope in the world in the 1980s was Hale telescope at Palomar Observatory. It was a 5m telescope, but subject to atmospheric effects.

Uranus has a resolution of about 18 urads from Earth, or about 3.7 arc seconds.

Pre-adaptive optics, the best one could hope for in terms of angular resolution was around 1 arc second. Given that, it seems that Uranus would be little more than a blur, resolvable, but with no real detail. Neptune would be even worse.

Keep in mind that for the most part, all of the fancy tricks that can be used today to observe planets weren't around. There was no adaptive optics, no digital cameras recording hundreds of frames to find a "lucky" frame, and other similar things. This was a serious challenge of the day, to get good pictures of items like planets.

Also keep in mind that Neptune and Uranus are less colorful than Saturn and Neptune, making photos of them less interesting than would otherwise be obtained.

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    $\begingroup$ At the top of page 190 (scroll up to the top of the page) there's a pair of pictures taken with Hale, one without AO and one with: books.google.com/… Definitely resolvable but not clear. $\endgroup$ – 1337joe Jul 19 '15 at 15:07
  • $\begingroup$ Good find. I only suspected there wasn't much, good find on the actual images. $\endgroup$ – PearsonArtPhoto Jul 19 '15 at 15:26
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    $\begingroup$ @1337joe Not bad but if it was taken with CCDs then it doesn't really count because we didn't have CCDs before Voyager (AFAIK). And IIRC, CCDs detect 90% of light whereas photo film only gets 10%! However, the fact that it's Neptune in that photo suggests to me that Uranus could very well be resolvable as a much clearer disk. $\endgroup$ – DrZ214 Jul 19 '15 at 20:12

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