Set Your Sights on This Lunar Bull’s-Eye

If you’ve never observed the Mare Orientale bull's-eye, the Moon's youngest and best-preserved impact basin, don't kick yourself. Despite being 930 kilometers (580 miles) across, the mare straddles the Moon's southwestern limb, where foreshortening often “compresses” it out of sight. This little-sailed sea only comes into its own when a favorable libration periodically swivels it into view. Fortunately, that happens this month.

We only see the lunar nearside because the Moon’s rotation period is exactly synchronized with its period of revolution. It turns at the same rate that it revolves, with one side facing Earth. 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%.

Libration Liberation

“Nodding” or libration in latitude occurs because the Moon’s orbit is inclined 5.1° to the plane of Earth’s orbit, and its axis is tilted 1.5°. Every lunation, the Moon ducks below and then above the Earth-Sun line, letting us peer under the south pole and over the north pole an additional ±6.5° (5° + 1.5°).

“Rocking” or libration in longitude is a consequence of the Moon’s elliptical orbit. The Moon speeds up when it’s closer to the Earth and slows down when it's farther away. Since its rotation speed remains constant throughout, the two get out of sync. After perigee, the Moon’s swift orbital motion outpaces its rotation, exposing a slice of the far side beyond the trailing (eastern) limb. After apogee, the Moon’s orbital motion lags behind its rotation, and we see farther around the leading (western) limb.

Maximum librations occur about a week after perigee and a week after apogee, when observers can see up to 8° of longitude beyond either limb. Both types vary in amount from one lunar cycle to the next. Diurnal libration, caused by the difference in perspective when we view the Moon at rising vs. setting, exposes an additional ±1° of (mostly) longitude.

East Meets West

Mare Orientale translates to the “Eastern Sea,” which seems a bit odd since it sits on the Moon’s western edge. 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. 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.

Because 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. 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.

Astronomers estimate that an asteroid about 40 miles (64 km) across crashed into the Moon around 3.8 billion years ago and excavated the huge Orientale basin. The tremendous energy generated in the impact partially liquefied the crust. Concentric waves of molten rock spread outward from the epicenter similar to the expanding ripples of water when you toss a pebble into a pond.

The rock solidified to form three nested, raised rings — the inner and outer Montes Rook ranges and the Montes Cordillera — separated by smoother lava plains that together form an enormous bull’s-eye. Were it located closer to the center of the lunar disk, Mare Orientale would compete with Tycho as one of the Moon’s most arresting sights. While the odds didn’t shake out that way, a favorable libration and a little imagination will go far in appreciating the mare’s impactful magnificence despite its out-of-the-way location.

Viewing Guide

From January 19th to 23rd, longitudinal and latitudinal libration extremes join forces for one of the best views of Mare Orientale this year. Within that interval the peak nights are January 20th and 21st, when the Moon will be in waning gibbous phase and well placed for viewing after 10 p.m. local time. A 3-inch or larger telescope should be all the glass you need.

To get there, start at the prominent, dark-floored crater Grimaldi and look two Grimaldi-lengths southwest for a pair of narrow, dark stripes not far from the lunar limb. These are sections of the ring-shaped lava plains that occupy the lowlands between Orientale’s rugged peaks. They look stringy because you’re seeing them virtually edge-on, like Saturn’s rings.

Unsqueezing a Sea

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. Lacus Autumni separates the Montes Cordillera from the outer Rooks. 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. Farther south and just inside the lunar limb, look for a third, lens-shaped gray patch. That’s Mare Orientale — ground zero! Keep going — a white “thread” exactly at the limb is the ultra-foreshortened western half of the inner Rook range!

If you run into a cloudy spell, two additional windows of opportunity are open this year: February 17–18 and December 14–18. During next month’s apparition, sunrise at the mare occurs on the night of February 16th, highlighting the 3D topography of the outer mountain belts similar to the view in the photo below.

You can simulate the Moon’s every nod and waver (and identify hundreds of craters and other features) with the Virtual Moon Atlas. It’s my fave. Once you’ve downloaded and installed the software, be sure to click on Show Moon’s Phase under Configuration / Display so you know whether your chosen feature is in shadow or not.

The monthly issues of Sky & Telescope list select librations in the Lunar Almanac column (page 42), and NASA’s Dial-A-Moon and Moon Phase and Libration 2022 are also useful for checking libration extremes. Finally, the RASC Observer’s Handbook lists libration values in its monthly calendar. By convention, north and south librations are labeled with a plus (+) and minus (-), respectively. Longitudinal librations are positive (+) for east and negative (-) for west.

Rare Sights

On average, a complete cycle of lunar librations takes about six years. But that doesn’t guarantee you’ll necessarily see any lunar feature under the same lighting conditions tonight and six years hence, since librations aren't tied to phase. It’s possible your target could be in shadow on that future date.

In fact, one full Saros period (18 years, 11 days, 8 hours) must elapse before you see a crater, peak, crack, or mare under nearly identical libratory conditions and illumination. Practically speaking, views are extremely similar several times a decade. But essentially, every time you look at the Moon, you're seeing it like you never have before. What a great reason to get out and observe!

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