There are so many adjectives appropriate to describing deep sky objects: beautiful, awe-inspiring, spectacular - just to name a few. But one word you will not often find on that list would be dramatic. For something to be
dramatic, there has to be something actually going on. And for the most part, what we see beyond the confines of our Solar System has all the trappings of eternity. Other than the occasional supernova, nothing happens that is discernable to the human eye - at least, nothing notable over the lifespan of any single individual. With rare exception, what you observe tonight looks pretty much the same as it did 500 years ago, and will remain unchanged for the next 500. And that's actually part of its charm. There's a comfort in the Unchanging Heavens, in their timelessness, in knowing that the ancient Babylonians were observing the same sights as ourselves, as will be our unknowable descendants.
Not so with the Moon. Oh, I know, "Nothing ever happens on the Moon".* It looks the same now as it did millions of years ago. The footprints of American astronauts will still be there ages after the very name of America has been long forgotten. Yes, in that sense, nothing does happen on the Moon. But in another sense, it is forever constantly changing - day by day, hour by hour, minute by minute. I am referring to its appearance to an Earthbound observer. The relentless, inexorable movement of the terminator from east to west with its attendant shadows creates a landscape of ever shifting contour. That lonely mountain with its truly spectacular miles-long isosceles triangle of a shadow will soften to a hard-to-detect whitish smudge over the course of an evening. A line of isolated points of light will emerge as the colossal rim of a crater while you watch. That deep well of purest black on a crater floor will be brilliantly illuminated before you know it. The wrinkled and convoluted floor of a Mare will appear smooth as glass in a matter of hours. And all of these mutations are visible with the most modest of equipment - no "light bucket" required.
And sometimes transience can be as appealing as permanence. I love knowing the Big Dipper will be there for my grandchildren, and for their grandchildren. But I also find joy in a sunset that will disappear forever in 15 minutes - and find it for that very reason. With the Moon, you can have it both ways. For just this once, you can both have your cake and eat it too. The Moon's features may be eternal, but your view of them is all too fleeting. And to the cognoscenti, there's pleasure in knowing that it's not simply a matter of waiting until next month to see the same pageant of shifting shadows across any given feature. Due to the librations and the eccentricity of the lunar orbit, the same pattern of shadows you see tonight will not recur for another 18 years. (At my age, that basically means that I am always seeing something on the Moon for the Last Time. So you can bet that I pay attention.)
And thus this posting's title -
Explorers! I grew up in Arizona, and have fond memories of driving through what was back then in the 1960s an endless wilderness (sadly gone today, all "developed"), seeing at every turn of the road a new horizon, a new distant range of mountains, a canyon or a river valley. That's how I feel today observing the Moon. The advancing line of sunlight is forever opening up new vistas, or at least a new way of looking at old ones. The manner in which that unnamed feature west of Kunowsky Crater revealed itself will not be repeated until 2035! And I was fortunate enough to have seen it... and wonder.
* Title of 1949 short story by Robert A. Heinlein
