Lengths represent the length (duration) of a cycle or sequence in Mystcraft Ages. These are used for two main purposes: determining the speed of some celestial body (Suns, Moons, or Stars), or the relative duration of a color step in a Gradient. There are four symbols in the group: "Zero Length," "Half Length," "Full Length," and "Double Length."
Similar to Colors, these symbols combine pairwise when reading a Descriptive Book. The Grammar maintains a single "current length" value: If there is no current length, the first Length symbol will set it, otherwise each new symbol will be averaged with the current length. This allows length values to be specified almost anywhere between 0.0 and 2.0.
"Zero Length" is a little special, in that it combines with the others in the usual fashion, but if used alone, it will be interpreted as "no cycle", for example a sun that does not move in the sky.
For most uses, the values are relative to a "Minecraft day" of 20 minutes (24,000 game ticks), so a "half length, normal sun" will rise and set twice while the overworld sun completes a single day. Likewise for gradients used by such symbols as "Sky Color"; the default length of the color sequence is a Minecraft day. However, the "Sunset Colors" symbol applies its gradient lengths to the length of the sunset/sunrise effect, and addons may use Lengths for other purposes.