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Lunar Features Take Center Stage . . .

July 10, 2011

"Lunar X" as seen through a telescope on July 7, 2011.

"Rupes Recta" as seen through a telescope on July 8, 2011.

This last week at the SLL Observatory (July 7th – 9th), lunar features stole the show! This was in part because clouds teased us with their constant dance. But mostly, it was because there were some fortuitous re-discoveries of lunar features that had many passersby putting their smart phone cameras up to telescope eyepieces.

The images above were captured through a 10″ Dobsonian telescope using an iPhone.  “Dobsonian” is a fancy way of saying ‘reflector telescope on a really inexpensive mount.’  The reason I mention this is, the images are reversed from what the actual orientations in the night sky were.  Therefore, the images below were flipped up/down and reversed left/right so that they correctly correspond with how the moon looked to the naked eye on the nights they were taken.  We also had to constantly adjust the position of the scope to keep the moon in the field of view.  But we had plenty of time . . . and patience.

Side Note: You probably noticed that the % illumination increased from the 7th to the 8th.  Because the illumination is increasing, it is called a “waxing” moon.  In addition, in the course of these two days, the transition from 1st quarter to waxing gibbous was captured.  “Gibbous” is the term given to the phases of the moon that are between 1st and 3rd quarter.  Anyway, back to the story . . . !

If you click in the first image and look near the top along the terminator, you can make out a white “X” feature.  The resolution of the digital image does not do it justice.  Actually looking through the telescope, the X sticks out like a sore thumb.  We zoomed in a bit on this object and were able to snap a better image:

Corrected orientation and higher magnification of "Lunar X."

You will have to click on the thumbnail image to see, but it’s worth it!  Look along the bottom 1/4th of the terminator (remember, the image is reversed the one at the top of this page) and you’ll see a conspicuous white X.  The Lunar X is known by this name because that’s exactly what it looks like: one big X.  Officially, it is the result of three neighboring meteorite impact sites (craters Blanchinus, La Caille and Purbach) that share rim boundaries.  More formally, it is known as the Purbach Cross or the Werner X.  This feature is only observable for a handful of hours just prior to the 1st quarter–so we were lucky indeed to capture it without having planned to!  Interestingly enough, does not yet have a page describing this feature by any of these names . . .  There is, however, a nice short article on this feature here:

Corrected orientation and higher magnification of "Rupes Recta."

When this next feature was spotted on July 8th, fellow observers really began giving me a hard time, and I deserved it!  Not knowing what it was, I just started referring to it as “Maier’s Dash.” Presumptuous and naive, indeed.  You see, the likelihood of seeing a prominent feature of the moon that has not already been observed, cataloged and named is extremely rare.  Folks have been gazing up at the moon with telescopes since the 1660’s!

The feature is actually called Rupes Recta; it is a lunar fault escarpment.  To get a closer look, click on the image to the right.  Then look about 2/3 down along the terminator and you will see a feature resembling the silhouette of a dagger or sword.  What makes this feature conspicuous is that you’re tempted into thinking it’s a long shadow caused by a single tall peak.  However, nearby craters and peaks have shadows at an angle of 90 degrees to Rupes Recta’s “shadow.”  Although it looks like a formidably sized cliff (in height and length), it’s not nearly as steep as you might think.  It is only about three football fields deep, and this is spread out over a 1-2 miles width.  Its length is about 68 miles.  I must give credit to a visitor who had more patience, a steadier hand (and a better phone!) than I.  He captured this image of the same feature through our 12″ Meade telescope:

Rupes Recta as taken by a visitor (corrected for naked eye orientation)

If nothing else, these X- and dagger-shaped features revealed to me two senses of scale: 1) typical sizes of craters; and therefore a sense of size for the overall moon and 2) the power of today’s run-of-the-mill telescopes.  We were just using a 10″ Dobsonian without any really hefty eyepieces.  You can get these outfits today for about a $500 – $800; and a lot cheaper than that if you buy secondhand.

For clickable, high resolution online lunar maps, check out  The Lunar X is in section F-4, Rupes Recta is in section F-3.

Many thanks to the Starcreek Astronomical Society for helping with this month’s events!!


From → SLL Observatory

One Comment
  1. Christy permalink

    I’m loving these pics Steve!

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