With astronomy being celebrated around the globe this month, join the fun by participating in a unique lunar observing challenge: track down 20 features once thought to show evidence of change from weather, geology, and even life.

Big Bridge or Broad Imagination?
Close up of feature #20, O'Neill's Bridge, in the "searching for changes in vain" lunar challenge. The bridge is most likely the rim of a small crater between the two promontories when viewed in low sunlight. North is up.
NASA / LRO with labels by the author

Global Astronomy Month (GAM) is well underway. As reported earlier by Sky & Telescope, the month-long celebration of astronomy organized every April by Astronomers Without Borders offers anyone a chance to make a connection with the sky, whether through a local star party, astro art and poetry contests, or tackling any of a half dozen observational challenges. Some of these can be accomplished in a night, while others might require up to a month.

Waxing from half to full in the evening sky this week and next, the Moon presents makes a bright and inviting target perfect for testing your visual mettle. In GAM's Shoot for the Moon observing challenge, you're invited to spy a sliver of the lunar far side by tracking the Moon's monthly libration cycle or gauging the change in the Moon's apparent size from perigee to apogee using your camera.

20 Ways to Know Your Moon
This map of the Moon, based on Lunar Reconnaissance Orbiter images, shows the location of all 20 lunar challenge objects. Even a small scope will show at least 15 of them.
NASA with labels by the author

One of the more intriguing Global Astronomy Month challenges I'd like to offer for your consideration involves identifying 20 features on the Moon once thought to show changes in appearance due to either vulcanism, life forms, or weather. The list was compiled by John Goss of the Astronomical League and titled The Search for Change on the Moon — a Search in Vain. It begins with Giovanni Cassini's "sighting" of a misty formation (cloud?) between two craters in 1671 and continues through the mid-20th century. A few were new to me like William Herschel's report of forests carpeting the floor of the crater Gassendi.

Imagined Metropolis
Early 19th-century astronomer Franz von Paula Gruithuisen sketched what he believed to be a lunar city in a hilly region near the crater Schroeter. Click for a photo of same.
Franz von Paula Gruithuisen

Did you know that American astronomer William Pickering suspected seeing snowstorms swirling around crater peaks and mountain ranges on six occasions? Our 21st-century knowledge of the Moon necessarily places restraints on what we might expect to see, but it wasn't always so. Long before we knew better, astronomers were freer to interpret shade, glare, and unique lighting circumstances as vegetation, snow, or even volcanoes.

Wrong or not, their surmises and mistaken impressions make for both a pleasant ramble across the lunar landscape and an opportunity to revisit a bygone era of observational astronomy.

Many of the 20 features will come into sight in the next week, while the optimal time for viewing others (at least in the evening sky) has recently passed. For these, mark your calendar for May 9, when the moon will once again be a waxing crescent, and complete the challenge. Many of the landmarks are prominent craters like Theophilus, Eratosthenes, and Plato, which are so full of good things to explore, they're worth a second or seventieth look any time the light's right. Others are more obscure and make for real challenges: Hyginus N, Gruithuisen's Wallwerk or "walled city," and O'Neill's Bridge.

Once Upon a Livelier Moon
Challenge crater #3 is Hevelius, suspected by 18th-century German astronomer Johann Schroeter to hold an active volcano. This is one of 20 descriptive maps, one for each feature. Click on the map to see them all.
John Goss

The photos here should assist in locating several of these less prominent features, but the best way to pinpoint each is to use either the Lunar Reconnaissance Orbiter's ACT-REACT Quick Map (my favorite) or Google Moon. In the first, you can undo the current lunar phase by unchecking the Sunlit Region box. To search for a particular feature, click on the "stack of papers (layers)" icon then click the Nomenclature link. Select the Search tab and type in a name. Zoom in and out using the "+" and "–" links in the upper left corner.

Mystery Dark Spot?
Try to spot the obscure crater Hyginus N, a short distance north of the famed Hyginus Rille. In 1877, Hermann Klein, Director of the Cologne Observatory, found a "fresh" dark patch nearby.
NASA / LRO with labels by the author

To help with planning, you can use the global map, which will also show the locations of the larger craters and isolated mountain challenges. After you've spotted a few, share your progress with Astronomers Without Borders and the world on Facebook, or tweet it using the hashtag #GAM2016 (@gam_awb). Oh, and we'd love to hear from you, too. Clear skies!


Image of Aqua4U


April 16, 2016 at 7:42 pm

Bob! Thanks so much for the link to the ACT-REACT Quick Map! A VERY handy tool and now my favorite moon map! Tonight the moon is 77% full and since I am basically a deep sky observer I usually wouldn't bother to take my telescope out.. but tonight.. I definitely will and besides, there could be a meteor impact on the shadowed side, bright enough to see?

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Image of Bob-Patrick


April 19, 2016 at 5:23 am

Bob K. ...

Ditto on the link to the ACT-REACT Quick Map.


...Bob P.

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