Sunrise on Gassendi
One of my long term observing projects has been participation in the Lunar Transient Phenomena ("LTP") program of the Association of Lunar & Planetary Observers ("ALPO") and the British Astronomical Association ("BAA"). This project seeks to determine the true nature of various apparently strange sights catalogued by the BAA and NASA over the years. The purpose is to narrow down the items in these catalogues so that astronomers might be able to focus on those relatively few that are most interesting and possibly related to changes or events that actually happened on the Moon, rather than in Earth's atmosphere, an observer's optics or an observer's mind.
On the night of May 28, 2015, the lunar terminator and libration created lighting conditions almost identical to those existing on October 7, 2011. On that night, an experienced lunar observer with the BAA was observing the prominent crater Gassendi when he thought he saw a point of light deep within the shadow on the crater's floor. He thought this faint point of light was about 2/3 of the distance from the center of the crater to its eastern wall, too far to be the top of the crater's central peak. I decided to check it out. Using my 11" Celestron SCT at 311x, I saw the point of light immediately deep within the shadow on the crater's floor. The Sun was just beginning to rise on Gassendi, most of which was beyond the lunar terminator and in the dark. The point was tiny, but brighter than indicated in the original LTP report, and seemed to me to be roughly halfway between the center and the crater's eastern wall. At this point, only a couple of the highest peaks on Gassendi's west wall were hit by the dawn light, making it difficult to guess the full shape and center of the crater. But the mysterious point of light in the LTP report was there. I rechecked the crater and photographed it after about 45 minutes. At this time, more sunlight was hitting the crater and its west wall, so that its shape was becoming more visible, as was the bright point. The following image was taken through my C11 with Celestron's Neximage 5 Solar System Camera in poor seeing conditions, haze and stiff breezes. It is a stack of the 10 best of 271 frames processed in Registax 6 and finished in Photoshop Elements 9. If you look closely at the bright point within the shadow on Gassendi's floor, you will see a second, fainter point just to the right. These points are the tips of the two highest of the crater's central peaks.
The crater Gassendi was named for Pierre Gassendi (1592-1655), a French theologian, mathematician and astronomer. Gassendi was rather brave for his time because he was an early supporter of Copernicus and his theory that the Sun was the center of the solar system. You will recall that this was against church doctrine at the time. Gassendi also corresponded with Kepler and Galileo (who the church had placed under house arrest). In 1631, Gassendi was the first person to observe a transit of Mercury across the solar disk. Gassendi crater is 110km across and 1,860m deep. Its floor is uplifted and cracked, except in the south where it has subsided into a basin (note how low the south wall is compared to the rest of the walls). The smaller crater attached to Gassendi's north wall is Gassendi A and is 33km across and 3,600m deep. To give you a better idea of what Gassendi looks like in sunshine, I've attached the photo below. It was taken in July, 2011 under a hazy sky with steady seeing. I used the C11 with my point & shoot Olympus digital camera attached to a 40mm eyepiece. The photo is a single frame taken with camera settings of f/4.5, ISO 100 in shutter priority at 1/8 of a second and processed in Photoshop Elements.
Gassendi is a fascinating crater to observe and is located on the north end of Mare Humorum, a lava-filled ancient impact basin about 380km wide. If you look very closely at the above image, you can see some of the rilles, or cracks, in the floor. I had hoped to get a better picture of the sunlit Gassendi with the Neximage 5 camera after taking the sunrise photo, but the weather didn't cooperate when the lighting conditions were optimal. I'll try again because I'd like to capture more of the rilles, hills and other details on the crater's floor and rims.
Jay Albert, June 4, 2015