If you were to ask the question, "What is your favorite crater?" ... the answer would usually boil down to one of three: Copernicus (my choice), Tycho, or Clavius.
In an informal straw poll, the answer is most often the last: Clavius.
This should be no surprise. Clavius is something of a coincidental miracle that's easily seen by anyone with a halfway decent telescope.
The miracle? It has a series of craters inside, describing an almost perfect arc while going from small to large in a surprisingly even series.
On top of that, it has two notable craters on its rim, both named for Americans: Rutherfurd and (the newer-named) Porter.
Porter? Yup, Russell Porter, the inventive amateur who ended up working on just about every major telescope built during his life. Mr. Stellafane. That guy.
Clavius has the disadvantage of being placed very much in the south - not all that far from the southern limb. But it has two advantages to make up: it's almost on the meridian (just a few degrees west of the halfway mark) and it is Huge! In fact, it's one of the largest of the "walled plains" (great big honking craters) on the Moon. Side-to-side on the long dimension, it weighs in at 225 kilometers, or about 140 miles.
You could easily fit the entire San Francisco Bay Area in it, with room to spare.
Another very cool thing about it is just being able to say the name: Clavius. Rolls easily off the tongue and sounds very hifalutin.
Where did it come from? And what does that have to do with dots?
Christopher Clau (anybody who was anybody in those days had a latinate version of their name for publishing; in Clau's case, Clavius. Look how close he came to being Santa Claus, though!) was a German-born Jesuit mathematician who taught at the Collegio Romano for most of his career.
That's a pretty distinguished post, but not good enough to get your name on the Moon.
His reputation was the highest: he was considered one of the greatest scholars of his time, and assisted guys like Tycho Brahe, Johann Kepler, Galileo Galilei. He was called the "Euclid of the sixteenth century."
Even so, he was also the instigator of one of the funniest scandals of the late middle ages, by using his influence to solve a problem with the Julian calendar.
The Julian leap-year rule created 3 leap years too many in every period of 385 years.
As a result, the occurrence of the equinoxes and solstices slowly moved away from their calendar dates. The date of the spring equinox determines Easter so the church began to press for reform.
Clavius proposed that Wednesday, Oct. 4, 1582 (Julian) should be followed by Thursday, Oct. 15, 1582 (Gregorian).
He also proposed that leap years occur in years exactly divisible by four, except that years ending in 00 must be divisible by 400 to be leap years. This rule is still used today and is so accurate that no further reform of the calendar will be necessary for many centuries.
Problem solved, right? Sort of.
The people of Frankfurt rioted against the Pope and mathematicians who, they believed, had conspired together to rob them of 11 days!
Several articles later (published popularly) he managed to explain what had happened well enough to calm the panic... as it turns out he was also an excellent teacher, with several texts to his name.
He was also gifted with handiwork, and produced an instrument to measure fractions of angles, sundials and developed a quadrant for use in surveying.
But what's this dot stuff?
Okay, he was the first guy to use the decimal point.
So whenever you see that old ad from Sun about putting the "dot in dotcom" (whatever that means), you might look to the night sky for a bit more reality: it was more the Moon's Clavius than Sun's marketing.
If you'd like to look at it, just watch for a first-quarter Moon and glance at the southern limb through just about any scope.
That big thing you see will be it, and you'll have no problem figuring out which one is Clavius if the terminator is anywhere near it.
If you catch it at sunrise, the craterlets may not stand out as they will the next night, but it's an awesome sight anyway!
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