The Mare Orientale tilts into view
WHEN: 10-16 February
FROM EARTH WE can always see 50 per cent of the Moon’s globe, but it isn’t always the same 50 per cent every night. Variations in the Moon’s orbital speed caused by the fact that its orbit is an ellipse rather than a circle, combined with a tilt relative to the plane of Earth’s orbit around the Sun, means that over time we get to see up to an extra nine per cent of lunar real estate due to ‘wobbling’.
The effect is called libration and the regions that rock and roll in and out of view are known as the libration zones. Unlike the features that are more central to the Moon’s Earth-facing disc, those that exist in the libration zones are at times well presented and at others completely hidden from view.
A tantalising feature that lies in the southwest libration zone is the Mare Orientale, or the Eastern Sea. This lunar sea is surrounded by two distinct rings of mountains, the outer one measuring a whopping 900km across.
Seen head-on this feature would look like a bullseye, but from Earth our view is so heavily foreshortened that it’s hard to make out the mare’s true form. From 10-16 February, however, libration will present a favourable view of the Mare Orientale, with the best libration occurring on the 13th and 14th.
The mare itself is roughly circular, measuring 300km across. It’s surrounded by Montes Rook, the inner mountain range. Within this formidable 6km-high, 600km-diameter ring lies the dark lava ‘lake’ known as Lacus Veris, or the Lake of Spring. With the region under bright sunlight, the dark lava and contrastingly bright mountain peaks make good features to look out for and these can help make sense of the region.
The outer 900km mountain ring is formed by the Montes Cordillera and, like the Montes Rook, it also contains a dark lava lake. Lacus Autumni, the Lake of Autumn, is visible inside its northeast boundary.