Telescope Reviews: Two amazing views, actually three

Jim,

I can't add much about satellite/balloon transits; but in connection with your observation of sunrise over the central peaks of Copernicus I am slightly curious about what you're using for a "colongitude calculator"?

From your verbal description you saw no peaks a little after (your local) sunset, but did see them approximately two hours later. If I'm not mistaken, sunrise over Copernicus occurred on Monday evening (rather than Tuesday) and sunset in Houston would have been at around 7:45 pm CDT (00:45 UT on April 15, 2008).

You are certainly on the right track that events like this are related to terminator position, but colongitude (or the "longitude of the morning terminator" to which you seem to be converting it) doesn't take into account the twist of the terminator due to the Sun's changing (called "Solar inclination" in VMA). For example, when you made your observation at 7:45 pm CDT (Houston sunset), the sun's colongitude was 21.83° which puts the morning terminator at 21.83°W. However, this refers only to the longitude at the point where the terminator crosses the Moon's equator. The Sun's latitude was +1.3°, giving the terminator a counterclockwise twist. Even for a crater as little distant from the equator as Copernicus (9.7°N), this twist causes the expected longitude of the terminator (at that latitude) to be different -- the actual longitude of the theoretical terminator near Copernicus at 7:45 pm CDT would have been about 22.1°W. Because the actual longitude of the terminator near the crater of interest varies in this way, the changing twist can throw features there into light or shadow even though the official colongitude is the same. And most craters are much farther from the equator than Copernicus, so the variations there will be much larger.

As a result, stating the colongitude (without giving the "solar inclination") is only a very rough predictor of when a given lighting condition will be observed at points away from the equator. In general, a much better approach is to state the sun angle at which you observed the event. The sun angle is the Sun's altitude above the local horizon on the Moon, and is computed for the exact location of interest. It takes into account both the colongitude and the Sun's latitude. When the sun angle repeats the lighting will be nearly the same.

In the present case it should be possible to reconstruct what you saw using photos from the LPOD Photo Gallery. In particular it contains a pair of photos taken by George Tarsoudis and one by Helmut Groell, all taken on June 5, 2006. They appear as shown in the attachment, where the sun angle at the center of Copernicus is listed below each photo.

George's photos have been enhanced to try to bring the peaks out of the shadows; but even so, they show them less clearly than Helmut's. The Sun's latitude here is +1.5° (very slightly greater than in was on April 14/15). The red line indicates the direction of the shadow cast by the high spot on Copernicus' east rim that was formerly known as Copernicus Alpha. The central peaks manage to avoid most of this shadow, and it looks like, under these circumstances, they first become visible when the sun angle at the center of Copernicus is around +2.9°. On the evening of Monday, April 14, you should have seen something similar to the first photo (with no peaks visible) at 9:18 CDT, and something similar to the second (with peaks in sunlight) at 9:55 CDT. This seems consistent with your statement that you first saw the peaks when you looked two hours after sunset, although I have trouble reconciling this with your computation that the terminator was at 21.9°E.

The presence of Copernicus Alpha complicates the prediction of sunrise events for other dates when the Sun's latitude is more to the south. A more southerly solar latitude would be expected to swing the shadow around to the north delaying the appearance of the peaks. You can see a photographic example of a Copernicus sunrise event of this sort (where the peaks had to emerge from the shadow of Copernicus Alpha) in Consolidated Lunar Atlas Plate D17. Copernicus Alpha adds an extra 500 meters or so to the elevation of the rim over which the sunlight has to stream, and over a distance of 55 km that adds about 0.5° to the required sun angle. Hence when the Sun is to the south (as it is in this photo), sunrise over the Copernicus central peaks is probably delayed until the sun angle (at the center of Copernicus) is around +3.5°.

Prediction of sunset events is easier because the rim on the west side is much more uniform in height. The LPOD Photo Gallery contains a pair of photos taken by Maximilian Teodorescu which establish that the last whiff of the central peaks is seen when the setting Sun falls below an angle of +2.5° (again, measured at the IAU center of Copernicus).

As you mention, you will have an opportunity to see this sequence from Houston on April 29, 2008, although perhaps a little closer to your local sunrise than you're predicting. Copernicus should look like the first of Max's photos at 10:18 UT (5:18 am CDT), and like the second of Max's photos -- with the peaks barely visible -- at 11:27 UT (6:27 am CDT). Since the Sun doesn't rise in Houston until around 6:45 am CDT, these events should be visible (weather permitting), but the brightening sky may interfere a little with seeing the final disappearance of the peaks.

If you have a Windows PC, and should you be interested in making additional predictions of this sort, you might wish to consider the freeware Lunar Terminator Visualization Tool. It will give you a readout of what the sun angle was over a particular feature at the time of your observation, and can give you a list of past and future times when the sun angle would be the same. Using the sun angles mentioned above you should be able to predict exactly when, in future years, you will be able to observe sunrise and sunset over the Copernicus central peaks.

-- Jim Mosher