Eclipses vs. Occultations…Let’s Keep Them Separate

We astronomers are a funny lot. We tend to use words in weird ways. We like to use one word, like color, to describe any of several different concepts and thereby become the source of misconceptions, like saying Sun is a yellow star when we know that its actually cottony white to the eyes of astronauts. (As you might expect the atmosphere plays a role in this misconception too.)  We can, and should, do better if for no other reason than for our introductory students for whom I will always steadfastly advocate.

My current fuming is over the way we teach eclipse phemonema and the words we use therein. When we teach lunar phases, we know that many students invoke clouds as the culprit. They also invoke Earth’s shadow and the concept of something “blocking” Moon’s light as the cause of the disk’s changing visual appearance. They even somehow manage  to conflate “shadows” and “blocking” to explain lunar phases. This has always baffled me given that both are quite easily visually dispelled with kinesthetic demonstrations. Still, I think there’s a more subtle problem here.

Shadows are intimately relevant to eclipse phenomena. In fact, we can define an eclipse as one object standing in the shadow of another object. If you think about it, this has nothing to do with light being blocked. It just means what it says; one object is in the shadow of another object. The object being eclipsed is the one inside the shadow.

Blocking of light is a different mechanism than standing in another object’s shadow, and as a different mechanism there should be a different name for it. Indeed there is, and that name is occultation. When Moon passes in front of a more distant object, that object is occulted becuase its light is physically blocked. It is true that that object’s shadow is also cast onto Earth, but that isn’t why we see the light blocked. There is physical material in the way. The bodies’ relative angular sizes play a role in occultations too. If the near object, the one blocking the farther object’s light, has an angular size that is significantly smaller than the farther object’s angular size, we call the event a transit and say that the near object is “transiting the disk” of the farther object.

Actually, eclipses, occultations, and transits are special cases of a syzygy, when three celestial objects lie on a line. Maybe there would be some benefit to introducing this term as a general phenomenon and then discussing how the objects’ relative apparent sizes distinguish the various events. I don’t know.

Amateur astronomers who regularly observe Jupiter and the antics of its four largest natural satellites have no trouble distinguishing between eclipses and transits. If they see, say, Io in sillouhette against Jupiter’s disk they always unambiguously refer to that event as a transit, which is correct. If they see Europa move into Jupiter’s shadow they always unambiguously refer to that event as an eclipse. If they, and we, can get it right for Jupiter, then we can and should get it right for other combinations of objects. Contrary to what I was recently told by a journal editor, words are indeed all we have to express understanding, even with graphical information. We still think internally with words (although I welcome insight from psychologists with information to the contrary). Words matter.

My bottom line is that blocking light and immersion in a shadow are different mechanisms and as such, they deserve different names with the assumption that using the same name for both will confuse students.



Leave a Reply

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

This site uses Akismet to reduce spam. Learn how your comment data is processed.