Once you know that the Moon is visible in your sky, its second most important characteristic is its colongitude. This number defines the place where sunrise is occurring, that is the position of the day-night line called the terminator. Someone new to lunar observing usually thinks that the phase (% illumination) or age is crucial. As it turns out, these last two parameters only serve as a quick guide to what might be visible.
If there is a particular feature you are interested in seeing at sunrise, you will want to note the colongitude of the Sun, not the phase or illumination of the Moon. For example, the crater Bruce lies very close to the center of the lunar disk, and so would see sunrise at First Quarter. However, the phase of the Moon depends on the perspective of the observer (i.e. from the direction of the Sun, the phase is always full !). The elliptical orbit of the Moon creates different librations, enough that sunrise at Bruce can be a full day ahead or behind what you might expect from the phase.
Colongitude is a necessary number for doing follow-up studies of an area. For example, say you saw some neat shadows on some feature during an observing session and would like to repeat the observation on a future date. Use Lunar Calculator (by entering the date and time of your original observation and hitting “Calculate”) to generate the colongitude value at the time of your observation. Next, click on the colongitude button, then click on “Go”.
A window pops up to give you the dates/times over the next 24 months (2 months in Lunarcal Lite) when that colongitude occurs. The box also gives you the latitude of the Sun on the Moon (which tells you if the shadows will be coming slightly more from the north or south), and the Moon’s azimuth and altitude above your local horizon. Knowing the Moon’s altitude is of course important, because the next time the sunlight/shadows are just right, the Moon might be below your horizon.
You can use the mouse to select sections of the text, as well as use the normal window commands Ctrl-A to select all, and Ctrl-C to copy to the clipboard, then paste into other documents. Programs such as Word and Wordpad will maintain the formatting.
To have Lunar Calculator go to the date, single left-click anywhere on the line, then click GoTo Date on the menu bar. The future colongitude window will minimize (in case you want to try another date), and the new view will begin generating.
Remember that in a colongitude search, the most important parameter is the lighting of the object, and that libration comes second. If libration (tilt away from the limb) is more important, then use the libration search.
If you prefer looking at a table of days and values, you can do this through the utility menu.
Lite users can only go back and forth 1 month.