Visually or in small binoculars, the lunar maria [ˈmaːriə'] (singular: mare [ˈmaːre']) are large, dark plains on the surface of Moon. They were called maria, Latin for "seas", by early astronomers who were thinking they really contain the water. Today we know that they are of volcanic origin and contain various types of basalt. There is also oceanus (ocean) on the Moon, as well as smaller, dark patches with the names lacus (lake), palus (marsh), and sinus (bay). The names of maria can be found on any good Moon atlas. But one mare will be important for our next discussion of Moon's libration. It is placed in the upper right corner of the Moon (i.e. near its north-eastern boundary - remember reversing East and West on the Moon!), and it is called Mare Crisium (the "Sea of Crises"). This mare is easily observable from about 3-4 days after New Moon to about 3-4 days after Full Moon. Please, try to observe it in December by naked eye and/or by small binoculars. Does it preserve its circular form or not? More about this easy and interesting observation in next article.