Co-Longitude is a number that defines when sunrise is occurring on the Moon; i.e. the position of the terminator, the line that separates the sunlit portion of the Moon from the part that's in shadow.

If you're new to Moon observing, you might think that the percentage of the Moon that's illuminated (it's phase) or its age are the important factors in observing. In fact, these only give an indication as to what's visible.

So, with that in mind, the Find Similar Illumination screen has been introduced into Version 2. What this screen does is give future dates and times for when a specified co-longitude will re-occur; i.e. when the terminator will pass through a given position on future occasions.

When the screen is called, it uses the co-longitude value for the current position of the terminator (the CoLong figure on the main LunarPhase Pro screen). Once the screen is open, you can overwrite this value with whatever value you choose.

The screen lists future times and dates for the next 25 months although you can change how many months the screen does calculations for, and the start month and year for the calculations (so you can look for past times and dates as well).

The screen will automatically update whenever you change the co-longitude figures, the start month or year or the number of months over which to perform the calculations.

In addition to the times and dates, the screen also lists the Moon's libration in latitude (N-S movement; negative is South); its libration in longitude (E-W movement; negative is West) and its Azimuth and Altitude.

The screen will also tell you whether or not the Moon is visible from your location when the specified co-longitude occurs and, if the Moon is visible, whether the co-longitude occurs during daylight or night-time.

The data on the screen is rounded off with the inclusion of Moon rise/set and Sun rise/set times for the found dates.