The Difference between ‘Above the Horizon’ and ‘In the Sky’

[Cross-posted to Revise & Dissent]

Here’s another paper I’ll have to cite, Time-Space Context of Moon-Related Beliefs by Jaak Jaaniste. It’s downloadable as a PDF paper from Folklore: The Electronic Journal of Folklore which I listed yesterday. I’ll have to read it a few times, but there are several ideas in it which are really interesting.

The most salient section for me is found on pages 190-1:

The psychology of seeing causes us to perceive a “half of the skies” above which we lose contact with the horizon to be about 20 degrees (Eelsalu 1996).

Thus – if the moon moves at more than 20 degrees above the horizon, it is simply “in the sky” and its exact position is considered of no importance. And, if the moon moves at less than 5 degrees above the horizon, it is significantly concealed by earthly objects.

This is because I don’t know if urban astronomy was a problem in the ancient world. The reason it might be is that a lot of sites I’m looking at are in urban contexts, even built into the hearts of cities which might have made observing the horizon difficult.

The horizon is important because the height of the horizon will change the day the sun rises over a particular point. The impact of the horizon varies on the latitude of the site. At the equator the sun rises vertically, so there’s no change, but the further towards the Earth’s poles you travel the more of an effect it has.

Diagram of how horizon height affects sunrise day or position in the Northern hemisphere.

So my concern looking at urban sites is trying to work out where the horizon is. Is it the natural horizon that matters, even if it’s blocked by a building? Is the the visible horizon that matters and, if it is, how to you reconstruct that? It would vary greatly on what buildings were in front of you and how high your own viewpoint was. I’ve been thinking of urban horizons as a problem and this is Jaaniste’s view.

Thus – if the moon moves at more than 20 degrees above the horizon, it is simply “in the sky” and its exact position is considered of no importance. And, if the moon moves at less than 5 degrees above the horizon, it is significantly concealed by earthly objects.

p191.

What I’m thinking now is that perhaps it isn’t a problem at all.

The sky at the horizon isn’t as full of stars as the sky above your head. The reason for this is atmospheric extinction. Near the horizon starlight has to pass through a lot more atmosphere than above your head, so stars dim. Too much atmosphere and they dim so much you can’t see them. As a rough guide a star has to rise a degree above the horizon for each step of magnitude before it is visible. So for example Spica, the sheaf of corn in Virgo’s hand, is a first magnitude star, so you’d expect to rise one degree before you could see it. Sualocin and Rotanev in Delphinus are closer to fourth magnitude, so you’d expect to wait until they were four degrees above the horizon before you could see them. Therefore if you’re arguing that people have an interest in stellar astronomy, and I’ll be arguing later in the year that the ancient Greeks did, building houses in front of a temple isn’t a problem.

In fact one of the key observations in ancient Greece may have been looking for a New Moon, to signal the start of a new month. At this time of the month the Moon will be a wispy silver fingernail in a bright sky. Concentrating on such a difficult object makes it difficult to get a reference, and a reference is handy if you want to prove that it’s there and not just a trick of the light. A built up horizon could arguably make referencing easier as in “No you idiot! Not there! Look there, over Alexander’s house.”

New Moon at 1.3 days. Photo (cc) JP Stanley.

Because the constellation names we use today derive from ancient Greece it’s tempting to look back and apply modern exact astronomy to the study of the ancient world. The depends on the idea that modern astronomy is a development of ancient astronomy. Unfortunately the speculations of relatively few philosophers aren’t accurate records of what people typically thought about astronomy in the ancient world. As Jaaniste points out classical astronomy is even less helpful once you move away from Mediterranean latitudes, but that might not be obvious if you live around 40° north where the classical experience of the night sky would match your own.

It’s an interesting argument and because it’s available via open-access you can read the paper yourself.

Update: Apologies to the people who download the PDF as a podcast. I’m still trying to get my head round the idea I can’t link to PDFs.