day moon
Tonight the Moon is essentially full... or at least, as full as it will get this month. It will be visible throughout night, and will transit around midnight, when it will appear much smaller than when it is rising or setting - a phenomenon known as the Moon Illusion. An observation tonight might reasonably span midnight, starting on one day and finishing on the next (hence the two-day heading for this page). Apart from the features described here, a separate Tour of Bright Objects covers features which become prominent across the whole lunar disc around the time of the full Moon.

Unless there is due to be an umbral lunar eclipse, the Moon will pass slightly above or below the Earth's cone of shadow, so there will be a slight terminator at one of the lunar poles. Also, you may be observing some hours before or after full. In other words, there may be a terminator on any edge of the Moon's disc. Coupled with the effects of libration, this may affect the apparent position and even the visibility of some features. Details of eclipse times and locations can be found in the Inconstant Moon Diary.

Tycho Once again, the dominant feature tonight is class 1 ring mountain Tycho, its rays reaching out across much of the southern highlands. Two powerful rays extend towards the southern limb: one to the south-south-east, the other running south-south-west from the western edge of the crater itself. At the point where the line of this latter ray intersects the limb, some small lumps may be observed in profile. These are the Doerfel Mountains. At 26000' they come within 3000' of the maximum height of the Himalayas.

Close to the limb, with its eastern edge tangent to the same ray, is the class 5 walled plain Bailly. It is 200 miles across, making it the largest crater visible on the Moon.

Grimaldi & Riccioli Continuing westward along the limb, the bright southern highland area tapers to a point, or a thin ribbon, depending on current libration. One or two Crisium lengths to the south of here, the dark oval of class 5 Grimaldi, first seen last night, stands out conspicuously dark. Tonight it is joined by Riccioli, just to the north west. This class 3 crater is named after Giovanni Riccioli, who in 1651 assigned the majority of the lunar feature names in current use. It is 2/3 the size of Grimaldi, but appears considerably smaller as only a small part of its floor is dark.

About one Crisium length north of Grimaldi lies bright Olbers A, a small, young crater with a ray system extending up to 650 miles.

Tycho & Maginus Trace a line from Olbers through the centre of Tycho. On this line, beyond the centre of Tycho by twice its diameter, is the centre of ancient Maginus. Despite being a large, strong feature one week ago, tonight it is completely invisible.

Near the eastern limb, north of the Mare Crisium by about its width, is the large walled plain Gauss. With extremely favourable libration, a small crater may be glimpsed to its east at an apparent separation of just over twice its width. This is Giordano Bruno, an unassuming feature with the distinction of being possibly the youngest significant crater on the Moon, and perhaps the result of the only impact ever to have been witnessed. This extraordinary event is documented in the Chronicle of Gervaise, a monk at Canterbury Cathedral in 1178.