How To Find A Planet

By Jochen Broecker

To make this clear straight away: this entry will only marginally touch upon weather and climate, but it will not be entirely unrelated altogether. Since you are reading this blog you must be interested in the natural sciences in general, and this makes it likely that you are (or have been at some point in your life) interested in stars and planets. You will know the difference between stars and planets. You will also know that that some of our planets were known already in antiquity whist others were not, but rather were discovered only after “science” took hold in the 18th century. These were Uranus and Neptune (and Pluto if you count it as a planet), while Mercury, Venus, Mars, Jupiter, and Saturn were known to all civilisations with a recorded history. They appear, for instance, in what is known as the “mul.apin”, an astronomical compendium compiled around 1000BCE in Babylonia.

But what exactly do we mean by “known”? It is probably safe to assume that already in the paleolithic age people were aware of the fact that while most stars appear to have a fixed position relative to one another, some travel across the sky, faster or slower, in a stately progression along a relatively narrow path which the sun and the moon also take. Clearly, the key insight though is that these are actually just a few stars that reappear periodically. As a more specific example, that the morning star and the evening star are in fact the same object is a result which for me ranks high on the list of important scientific achievements. It bears all the hallmarks of great science: it relies on careful collection of long-term observations, requires a certain level of abstraction and creativity, and lacks any immediate practical use. This last point is important because it says something about the society if it sees value in such results nonetheless. Identifying the five classical planets may thus be seen as the starting point of astronomy as a science, or even science as a whole.

One way to appreciate scientific progress is to try and retrace the steps of the scientists. Clearly, this is not always feasible especially with regards to more recent scientific breakthroughs like nuclear fission or the Higgs boson (one might also easily forget that for each step science takes in the right direction there correspond numerous turns in the wrong direction). But with regards to early astronomy, nothing but clear skies and the naked eye are needed (and here is one overlap with meteorology). With a reasonably good set of binoculars, we are already in the same position as Galileo.

As with so many other things, I came to realise this through talking to my children. In late December we went through a period with Jupiter and Saturn standing low in the west just after the (early) sunset. For a year or so they had been fairly close to one another, so their relative movement became apparent. They also provided a point of reference for the Moon and for realising the big leaps the Moon makes across the sky from night to night. There’s a lot to explain and make sense of here.

Figure 1: An elaborate geometric construction explaining the apparent motion of Mercury from Earth by Ibn Al-Shatir (1304-1375). Similar epicycles were already present in Ptolemy’s model but Al-Shatir’s model had far better predictive power. Epicycles still appear in Copernicus’ heliocentric model and were rendered obsolete only by Kepler’s laws.

And that’s when I found that I had seen only four of the five planets in my life so far, with Mercury being the missing one. If you would like to try this yourself, use an interactive star map on the web such as (Unfortunately we are entering a few months where only Jupiter can be seen in the first half of the night; but Venus currently appears at around six o’clock in the morning and can be seen even during the day if you know where to look.) Mercury is surprisingly hard to observe as it always stays close to the Sun. As it is usually completely drowned by the glare of the sun, it can only be observed during a short period either just after sunset or just before sunrise. Light pollution strongly affects the sky close to the horizon which makes the problem even harder.

But it is possible even in a place like Reading. Here’s what I did. Firstly, a small pair of binoculars will help. Find a place with a clear view of the western horizon where the sun sets; a high building is good since even trees or houses might obstruct the relevant area in the sky. Now two things have to come together: Mercury has to be maximally far away from the sun, and the western horizon has to be clear of any clouds. The first condition will be met again at the end of April this year. The second condition is fairly hard to predict even on the day. The skies might have been the bluest of blue during the day, yet even a small puff of steam in the wrong place might spoil it, since once the sun is down, atmospheric boundary layer activity will come to a rest, meaning that the cloud might remain there for a very long time (here’s another overlap with meteorology). I was unlucky on two days but on the third I was successful (I now know how it feels when the “launch window” of a space mission starts to close). Deep on the western horizon, which still had a tint of orange, I saw a bright white spot, brighter and whiter than Saturn which was close by. Mercury is more or less like a supersized version of our Moon; I can confirm that the light it reflects is very similar.

Having ticked off the classical planets, the next one on my list is Uranus. It is visible to the naked eye (without light pollution) and had made its way into star catalogues long before W. Herschel peeked at it through his telescope. Yet when I said earlier that “knowing” an object in the sky does not mean we recognise it as a planet, Uranus is a case in point. Uranus is a faint spot that moves very slowly in front of an ocean of other faint spots. Even Herschel did not recognise Uranus’ motion. Thanks to his magnificent homemade telescope, he realised that it was an extended disc, whilst stars appear as spots even in modern telescopes. The insight that it was actually a planet came only after more astronomers started looking at it. So I don’t expect to see more than a faint spot, and if it was only for my own skills and observations, rather than for the insights of our forefathers, I’d never know that Uranus is a planet, nor even what a planet is, probably.

This entry was posted in Astronomy, Climate, History of Science. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *