General astronomy discussion

Re: Earth set on the Moon

Centaur — Sat, 15 Mar 2008 16:29:33 GMT

Jeff has it right regarding the difference between an occultation and a transit. Some folks refer to all mutual planetary obscuration events as occultations, but if a planet crosses another one that appears wider, that is actually a transit.

Interestingly, the word eclipse is only properly applied to an event in which a shadow obscures an object, such as a lunar eclipse. What is commonly called a solar eclipse is actually an occultation if it is total and a transit if it is annular. But common usage trumps that notion, so I’ll keep calling them solar eclipses.

As I stated earlier regarding the term setting, that is a holdover from times when it was not known that the effect is due to the Earth’s rotation. While it may be more correct to say that the Earth appears to occult the Sun at dusk, common usage and the appearance that the Sun traverses the sky have made setting the term one must use to be understood. The usage of the term setting to describe an unusual scene from an orbiting satellite would also allow listeners to more quickly grasp the situation rather than hearing of an occultation.

Re: Earth set on the Moon

chipdatajeffB — Fri, 14 Mar 2008 19:34:24 GMT

To occult is to block out, so since Venus doesn't block out the Sun (entirely) when it crosses our line of sight, it's called a transit (which means to pass in front of) ...

Re: Earth set on the Moon

Pioneer — Fri, 14 Mar 2008 18:12:24 GMT

Thank you for all the replies. I agree with all of it and I've updated the blog with a link to this thread.

I have one minor issue with calling this "an argument over semantics." Why is it that calling a "transit" [of Venus for instance] an "occultation" would be wrong (and not merely a semantic issue) but calling three different astronomical happenings with the same word "setting" is a semantic issue? (1. stationary observer 2. moving satellite 3. libration=moving along the sky from east to west.) Maybe because there are no specific words for each of these? Maybe there should be.

Thanks again for the answers.

Re: Earth set on the Moon

Centaur — Thu, 06 Mar 2008 03:31:18 GMT

Welcome to the discussion group, Pioneer.

The appearance of the Earth setting is from the perspective of a satellite orbiting the Moon as implied in the video narration. It would be no more wrong to refer to that as “setting” than it would be to say we see the Sun appear to be setting due to the Earth's rotation and not to actual movement of the Sun. The argument seems to be over semantics; I’ll side with those calling it a setting and will be understanding of those who disagree. In your blog, Mr. Drumm preferred the term occultation. But we could then say that once it was accepted that the Earth's rotates, we should have stopped saying the Sun sets and instead say it is occulted. Even Drumm admitted the linguistic point is insignificant.

The Earth can appear to rise or set for a stationary observer on the Moon due to libration, however the shadowing on the craters and the fraction of the Earth seen illuminated would greatly change due to the length of time involved, unlike what is seen in the video. Contrary to your assertion in the blog, the Earth subtends and angle of about 2° for an observer on the Moon, not 4°. Libration of the Moon in latitude is nearly 7°. The libration in longitude can be as much as 8°. But as previously stated, the effect seen in the video is due to the views from an orbiting satellite and not due to libration.

Re: Earth set on the Moon

chipdatajeffB — Thu, 06 Mar 2008 03:05:34 GMT

I understand your point.

However, there are some places on the lunar surface (within a few degrees of the lunar limb and poles) from which the Earth would appear to rise and set, due to what is called libration. By rise and set, I simply mean that at some times the Earth would appear to be rising from behind the horizon of the Moon and at other times it would appear to be setting behind the horizon of the Moon. That would not be the same as having the Earth rise, move all the way across the "sky", and then set again ... but what else would you call it?

Earth set on the Moon

Pioneer — Thu, 06 Mar 2008 02:02:56 GMT

I would appreciate comments from experts on this topic. This is related to my post here. Do you agree that the photo does not represent Earth set and can you confirm my calculations?

Thanks in advance.