Chang'e 1 and Chang'e 2 were launched 2007 October 24 and 2010 October 1 respectively. The Chang'e 3 project is on course for late-2013. Taking the three dates together, it seems China set in train a planning process, probably driven by cash availability, to make a launch every three years. We'll have to await another cycle to see if Chang'e 4 is set for 2016.

The timing also points to October as the favoured month for launch, suggesting that something about the alignment of the Earth, Moon and Sun is best in those months. It may be coincidence as lunar launches are theoretically possible at any time of year.

Although a mission can be launched any day, there are other considerations that limit when launches may happen. Examples are surface lighting conditions on arrival at the Moon for a lander or a photographic orbiter, direction of the Sun for illumination of solar panels on the way out, and the local trajectory as the spacecraft approaches the Moon. Together, they tend to create a set of conditions for launch (Launch Windows) that are satisfied for a few days at approximately monthly intervals.

The actual position of the Moon round its orbit during flight may favour some months more than others.

For Chang'e 1 and Chang'e 2 the dates and times show a pattern. Both launches were in October. Times of day were slightly different in a way that suggests the Sidereal Time of launch is a consideration but the relationship is not based on an exact number of Sidereal Days.

RAAN is the Right Ascension of the Ascending Node of an orbit. It marks the point where a satellite crosses the Earth's Equator in a northbound direction (ie "Ascending"). Two values are shown: one for Chang'e and one for the Moon.

For Chang'e 1, the RAAN comes from the Twoline Orbital Elements sets issued for it by SpaceTrack. For Chang'e 2 there were no Elements Sets because it entered a barycentric orbit immediately, rather than going via a geocentric orbit like its predecessor. The Chang'e 2 value is an estimate based on assuming that it followed a similar Azimuth to Chang'e 1 as it left the Xichang Launch Centre.

RAAN of the Moon's orbit changes with time. Over a period of about 18 years, it wanders cyclically back and forth between about 348° and 368° (or 8° in simpler terms). The value was different between the 2007 and 2010 launches and will be different again for a 2013 mission.

The Declination at Chang'e lunar arrival is the celestial latitude of the Moon at the time, measured from the centre of the Earth. A value of 0 would be on the Equator and negative values indicate south of the Equator.

There are some significant points to note. First - the RAAN values for Chang'e and the Moon differ by about 180° for both launches. Second, the Declination on arrival is near zero in both cases.

Rather than approach the Moon using a trajectory in the same plane as the Moon's orbit, the two Chang'e vehicles approached completely 'out of phase' which is why arrival had to occur near the Equator - the only place the paths of Moon and spacecraft crossed. It was probably done to allow entry into polar rather than the equatorial orbit around the Moon that a co-planar trajectory would have produced.

For both, the Moon was moving southwards at its orbital Descending Node) and Chang'e was heading north at the Ascending Node). The time of day (UTC) of arrival was almost the same in both cases. It occurred while the Moon was high in the sky as seen from China. It was set-up in order to monitor lunar approach, and entry into orbit, from the Chinese mainland tracking stations.

The Greenwich Sidereal Time of launch is calculated only out of interest.

In the table, the interesting values are the differences between the RAAN of the Chang'e and Lunar orbits. For the two missions, they are a little more than one degree different from each other. It is unlikely to be coincidence and provides a good starting point to determine likely launch dates and times for Chang'e 3.

Finally, the rightmost columns show the Azimuth and Altitude of the Moon as seen from Beijing at the time of arrival.

In the final quarter of 2013, four combinations of date and time fit the conditions derived from the Chang'e 1 and Chang'e 2 missions. As long as the Chang'e 3 launch obeys the same rules then the dates and times are good estimates.

In hindsight, Chinese news outlets, late 2012, were starting to describe the Chang'e 3 mission as due to occur "in the second half of 2013". It somewhat widens the period and brings in the possibility of launch as early as June, about the same time as Shenzhou 10 is set to be in orbit.

To cover for this, the table below covers all possible dates from June onwards that use the Chang'e 1/2 style launch windows.

Within the error margins of the calculations, each date has a day either side on which launch might take place. Opportunities occur at intervals of one sidereal day (23h 56m) so will come later or earlier by four minutes for each calendar day earlier or later.

All arrivals happen when the Moon is high in the sky as viewed from tracking stations within China, and the landing will be timed similarly.

Following the Shenzhou 10 piloted mission, not all of the Chinese tracking fleet went directly home. Yuan Wang 5 stayed at sea and a snippet of information about its future plans suggested that the aim may be to launch Chang'e 3 during the opportunity that occurs around November 22.

The advantage of flying the mission late in the year is that Lunar arrival occurs near the time of New Moon.

Chang'e 3 is planned to go into orbit then, after several days, land on the surface in Sinus Iridum (surveyed by Chang'e 2 during a simulated landing approach). The Sun will rise over Sinus Iridum several days after arrival so Chang'e 3 has time to refine its orbit and set up an approach to the landing site that allows touch down soon after dawn local time. The roving vehicle it carries can then start work with nearly two weeks of operation before the first sunset.

Chang'e 3 is unpiloted and will land under computer control so touch down can theoretically occur in darkness. However, controllers will want to see images of the landing area as soon as it arrives and some of the landing sensors may depend on being able to 'see' the terrain. Daylight will also allow the first operations to be executed immediately and the solar panels will start to supply power at once - a sensible precaution against failure of a storage battery.