Farther south and just inside the lunar limb, look for a third, lens-shaped gray patch. Interior to this stripe you'll spot the longer, continuous thread of Lacus Veris (“Lake of Spring”), located between the inner and outer Rook ranges. Lacus Autumni separates the Montes Cordillera from the outer Rooks.
The outer stripe, named Lacus Autumni (“Lake of Autumn”), is closest to Grimaldi and composed of several separate segments that together form a broken arc. He used a C9.25 Edge HD telescope for the observation and photo. “I was fortunate to encounter skies that were just favorable enough to capture these views of Mare Orientale (November 28, 2021) that represent the maximum extent of the region visible from Earth, with both libration to the west and the south being at extreme values,” writes Tom Glenn of San Diego. Note that libration doesn’t depend on the Moon's phase - it does its work whether or not your target is in daylight or darkness. In 1961, the International Astronomical Union adopted a more universal definition, defining east and west from the viewpoint of someone standing on the Moon, the same way we determine directions on Earth.īecause Mare Orientale lies in the Moon’s southwestern quadrant, two conditions must be met for a good look at it: The southern and western libration extremes need to coincide, and the southwestern limb must be in sunlight. Shouldn't it be called Mare Occidentale? More than a century ago, when astronomers first studied the mare in detail, convention defined the eastern part of the Moon as the side facing east as viewed from Earth. Mare Orientale translates to the “Eastern Sea,” which seems a bit odd since it sits on the Moon’s western edge. This wide view of the waning gibbous Moon shows Mare Orientale in context during the favorable libration of Jan. This would seem to imply that only 50% of its surface is visible, but thanks to librations - the cyclic north-south nodding and east-west rocking of the Moon - the actual amount is 59%. It turns at the same rate that it revolves, with one side facing Earth. We only see the lunar nearside because the Moon’s rotation period is exactly synchronized with its period of revolution. This little-sailed sea only comes into its own when a favorable libration periodically swivels it into view. Despite being 930 kilometers (580 miles) across, the mare straddles the Moon's southwestern limb, where foreshortening often “compresses” it out of sight. If you’ve never observed the Mare Orientale bull's-eye, the Moon's youngest and best-preserved impact basin, don't kick yourself. Color-coding for gravity reveals where mass is concentrated (red) and where there’s a deficit (blue).Įrnest Wright / NASA-GSFC Scientific Visualization Studio Spanning 930 kilometers or about the distance between New York City and Charlotte, North Carolina, the Moon’s Mare Orientale exhibits three distinct, mountainous rings surrounding a central, magma-flooded basin.
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