Telescope Reviews: Gassendi and environs 16 May 2008

Matt,

For the benefit of you and others interested in such matters, I took a moment to update the-Moon Wiki page on colongitude explaining, hopefully in fairly clear language, exactly what it is and is not. Feel free to add to and/or modify it if changes are necessary.

Colongitude is indeed a helpful (and traditional) way to describe the lunar lighting circumstances, but depending on the level of precision one is interested it may be regarded as giving, as you say, only a ballpark feel. This is especially true of observations far from the Moon's equator.

As explained on the Wiki page, this variation in appearances is due to a second parameter listed as the "solar inclination" in the Virtual Moon Atlas ephemeris output. That's actually just a fancy word for the Sun's latitude as seen from the Moon. Perhaps it's called "inclination" because the changing solar latitude twists ("inclines") the terminator relative to lines of constant longitude: the colongitude defines where the terminator crosses the equator, but what is in light and what is in shadow to the north and south of the equator depends on how the terminator is "inclined". Because of this effect, it's quite possible for two observations of a region taken at the same colongitude to look different; and conversely, for two observations taken at different colongitudes to look substantially the same.

As a practical example of this, the present observations of Gassendi were made when the Sun's selenographic latitude was +1.58°. Mardi's (at 06:16 UT on May 16, 2008) is at a colongitude of 42.96° (you don't need to know locations to calculate colongitudes) while Larry's (at 08:08 UT) is at a colongitude of 43.91°. Whether you regard the change in shadow lengths due to this 1° change in colongitude significant or not is, of course a matter of opinion. But if you were to look at the Moon again on November 9, 2008, when the Sun's latitude is -1.54° you will find the clockwise twist caused by the lower solar latitude causes a given lighting pattern to be seen at an earlier (smaller) colongitude. In fact, if you look at around 04:35 UT on Nov. 9, when the colongitude is 42.86°, you might expect to see something similar to the long shadows of Mardi's lighting, but instead you will see something much closer to the short shadows of Larry's: the prediction of repeat lighting based on colongitude alone is off by about two hours.

Again, having the lighting (based on matching colongitudes) be offset by two hours relative to the correct times (based on matching sun angles) may not seem like a big deal; but this is for Gassendi, which is only 18° from the Moon's equator. For features still farther north or south of the equator, the variation in appearance at a given colongitude becomes increasingly large. Near the poles, features can (because of a high solar latitude) be visible in sunlight in one lunation, yet never be visible in another lunation (regardless of colongitude) due to a lower solar latitude.

So if you're going to give the Sun's colongitude it's almost necessary to give the Sun's latitude as well.

But it's hard to anticipate exactly what information a reader/viewer may be interested in; so unless colongitude is particularly relevant for a specific image, there's something to be said for simply giving the date/time and observing location. From those they can calculate whatever else they may be interested in, just as you can. Unlike the computable geometric parameters, there are other things, such as the telescope size, magnification, filtering, exposure time (for a photo), etc., that can be equally important to the interpretation of an observation and are more important to provide, because if you don't explicitly mention them the reader cannot possibly deduce them.

--

One problem with adding the many parameters displayed in the VMA ephemeris to an observation is that VMA adjusts (as you point out) the input time to what it thinks is the correct UT before making its calculations, but the adjustment from local time to UT is not always correct (especially in older versions of VMA). In the VMA ephemeris output you will see lines listed as "Date" and "Date (DT)". I believe the first is the time you enter, and the second is what VMA thinks is the correct "ephemeris time" (which is basically UT plus an ephemeris offset of a few minutes -- sometimes called "DeltaT"). Users may incorrectly assume VMA is automatically displaying data for the UT corresponding to the local time they enter when in fact it is displaying the results for some other UT. In the past, this was particularly a problem with users asking VMA to look up the data for some past observation. VMA assumed it could obtain the correct UT by adding the current number of hours offset from local time to UT. But if the current offset is based on daylight savings time, and the old observation time was not (or vice versa), the results would be off by 1 hour. This problem has hopefully been solved in the current version of VMA, but some users still have the time offset incorrectly set. As a consequence, a significant amount of the colongitude, "age", etc. data posted on the web is incorrect, or at least unverifiable, unless the actual time of the observation is listed.

To be sure that you understand how VMA works it's probably best to manually set the offset to zero by unchecking the "Use Computer Date and Time Zone" box under "Configuration...General...". That forces VMA to give the results for a known UT input. When you switch back to local time (if you choose to do so), you need to manually verify that the offset from "Date" to "Date (DT)" is what you expect, putting "Date (DT)" (the thing the calculations are based on) close to the correct UT, and returning the same data, especially if you manually enter a date for a past (or future) observation. But please don't give "ephemeris time" instead of UT on your observations! That would lead to total confusion. "Ephemeris time" is used internally for the calculations, but it is not an "improved" version of UT.

Alternatively you can try calculating the colongitude using LTVT or with the JPL Horizons ephemeris system. The former displays the colongitude in the main screen (under "Computed Geometry", as well as giving you the equatorial longitudes of the Morning Terminator ("MT") and the Evening Terminator ("ET"). The latter will give you the precise longitude of the sub-solar point, which you have to subtract from 90° to obtain the colongitude. Hopefully VMA will give you the same number.

--

By the way, as to searching an archive of images for something with similar lighting, LTVT has a probably little-known Photo times search capability under "Files...". You simply present the program with a list of dates and times and it will automatically search through those to tell you the ones that show the Moon at a particular colongitude, or more importantly, those that show a particular feature (like Gassendi) at a definite sun angle. There was at one time a list of the dates and times of all the Consolidated Lunar Atlas plates available on the LTVT download page; there is now a more extensive list of calibration data for each, which permits an even more powerful Find photos search telling you if the feature of interest is actually included in the observation. Both these search functions work for drawings as well as photos.

-- Jim