The moon will reach full on 18 May. This is a good opportunity to take the time to really look at its bright disc. The dark markings are called lunar maria, named after the Latin word for seas because early astronomers thought they were seeing bodies of water. We now know that the moon is largely dry and the maria are actually solidified lava plains, deposited in volcanic eruptions on the moon some 3–3.5 billion years ago. The maria are not uniformly distributed across the lunar surface; they cover more of the west than the east, and that’s not the only difference. Mare Imbrium (sea of rain) and Oceanus Procellarum (ocean of storms) are slightly lighter in colour than the eastern maria because of differences in their chemical composition. This month’s full moon is a seasonal blue moon, meaning that it is the third out of four full moons to happen in an astronomical season, counted between equinox and solstice. There are usually only three full moons per season.
