L14 Sinus Iridum
A ~249 km flooded impact crater on the northwestern shore of Mare Imbrium — the Moon’s most famous “missing rim,” where the Montes Jura curve in an unbroken arc around a bay whose southern wall was drowned by lava long ago.
L14 Sinus Iridum
Mare Imbrium NW📉 Vital Statistics
🔭 Field Notes
Sinus Iridum is the spectacular remains of a giant impact crater whose southern wall was completely drowned by the same basaltic lava floods that formed Mare Imbrium. What remains is a graceful arc of mountains — the Montes Jura — curving from Promontorium Heraclides in the southwest to Promontorium Laplace in the northeast, cupping a flat, lava-smoothed floor laced with wrinkle ridges.
- ▶ The Golden Handle: Around lunar days 10–11 (roughly 24 hours after sunrise over Copernicus), the Montes Jura peaks glow in sunlight while the bay floor remains in shadow — the arc of mountain summits appears to float free of the Moon’s disk. Visible in binoculars and one of the most celebrated clair-obscur effects in all of lunar observing. The window lasts only a few hours per lunation.
- ▶ Moon Maiden: Promontorium Heraclides takes on the silhouette of a woman’s head with flowing hair gazing across the bay — first depicted on Giovanni Domenico Cassini’s 1679 lunar map. Whether Cassini himself or his engraver Claude Mellan is responsible for the detail is disputed among historians, but the pareidolia has been beloved by observers ever since. Best appreciated with slightly soft focus and low magnification around day 10, as the terminator just clears the western Jura. Origin of the depiction is debated; the feature’s existence and long observing history are well documented.
- ▶ Wrinkle Ridges: The flat floor is cross-hatched by dorsa — low sinuous ridges formed by compressional buckling as the lava plain cooled and the basin slowly subsided. They emerge best under low oblique illumination around days 10–11 and again on the waning gibbous, appearing as gentle waves lapping toward the vanished southern shore.
📍 Nearby L100 Targets
- L23 Mons Pico: A solitary 25 × 15 km massif rising 2.4 km above Mare Imbrium, roughly 200 km to the southeast. An orphaned fragment of the Imbrium basin’s inner ring, almost entirely swallowed by lava — only its crest protrudes. Under low-angle illumination it casts a dramatic shadow many times its own width; at high Sun it fades to a bright speck. A textbook lesson in how basin rings are buried by mare volcanism.
- L49 Gruithuisen Delta & Gamma: A pair of steep-sided volcanic domes roughly 36°N, 40°W — to the south-southwest of Sinus Iridum, near the Mare Imbrium / Oceanus Procellarum boundary. Delta (~27 km wide, ~1.8 km tall) and Gamma (~20 km wide, ~1.5 km tall) were built from highly viscous, silica-rich magma confirmed by LRO Diviner data — making them the Moon’s best analogue to terrestrial rhyolite or dacite volcanism. How such silicic magmas formed on a water-free, tectonically dead body remains an open scientific question.
- L83 Plato Craterlets: The dark, lava-flooded floor of 101 km Plato — just northeast of Sinus Iridum — hosts a field of tiny impact pits at the limit of Earth-based resolution. The “Big Four” (the largest ~2.4–2.7 km across) are reported visible in 6–8 inch apertures under excellent seeing; claiming all four is an achievement, and reporting more than that deserves scrutiny. Centuries of contradictory observation have made Plato’s floor one of the most debated patches of ground on the Moon.
🚀 Mission Log
Target Acquisition
Anchor on Plato
Start at Plato — the dark, lava-flooded 101 km crater that stands out instantly against the bright Montes Alpes and Mare Frigoris highlands to its north. Its flat, near-black floor makes it one of the easiest features on the Moon to spot at a glance, even at low power.
Sweep south-southwest to the open bay
From Plato, sweep roughly 500 km to the south-southwest toward Mare Imbrium’s northwestern shore. You are looking for something unlike any closed crater nearby — an open, gently curved arc of mountains cupping a flat floor, with one whole side simply missing. That open “bite” out of the mare’s edge is Sinus Iridum.
Distance is an approximate great-circle figure from published selenographic coordinates (Plato ~51.6°N, 9.4°W; Sinus Iridum center ~45.0°N, 31.7°W) — useful as a rough sweep distance, not a precise eyepiece measurement.Trace the Jura at any magnification
Even at low power, the Montes Jura read clearly as an unbroken arc running from Promontorium Heraclides in the southwest to Promontorium Laplace in the northeast. Increase magnification to look for the network of low wrinkle ridges crossing the otherwise flat, lava-smoothed floor — most visible under a low Sun, when they cast faint shadows like waves rolling toward a shoreline that’s no longer there.
Confirm timing off Copernicus (L5)
If you’re unsure whether the terminator is approaching Sinus Iridum yet, check Copernicus first. Lunar observers have long used the rule of thumb that the sunrise terminator reaches Sinus Iridum about 24 hours after it crosses Copernicus — a handy rough cross-check while you wait for the Golden Handle window, though exact timing shifts slightly from one lunation to the next.
📝 Observation Log — L14 Sinus Iridum
0/4 CompleteIs Sinus Iridum visible tonight?
Sinus Iridum’s signature “Golden Handle” appears when the terminator falls across the Montes Jura — check if the Moon is approaching Waxing Gibbous (Day 10–11).
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When to Observe Sinus Iridum
Sinus Iridum sits at colongitude ~32°W in the northwestern part of Mare Imbrium — well clear of the limb, with none of the foreshortening that distorts features like Schiller. Its single most celebrated moment comes 10–11 days after New Moon, when the floor of the bay is still in darkness but the peaks of the Montes Jura, rising up to roughly 2.6–4.7 km depending on which part of the range, are already catching sunlight. The arc of summits appears to hang detached above the dark limb — the “Golden Handle,” one of the Moon’s best-known clair-obscur effects.
- Best Viewing: 10–11 days after New Moon (Waxing Gibbous), roughly two to three days after First Quarter — and again at sunset, days 22–24, as the terminator returns from the other direction. The Golden Handle effect itself is a waxing-side phenomenon and does not have a true waning mirror image, but the wrinkle ridges and Jura shadows are well placed for study on the waning gibbous too.
- No Limb Distortion: Unlike far-southwestern features such as Schiller, Sinus Iridum’s position well inside the disk means its true outline — roughly 249 km across by IAU convention, or about 260 km by older Cambridge Atlas measurement — is seen close to actual proportions, not stretched by perspective. Sources differ on exact diameter depending on measurement convention; figures are not directly interchangeable.
What to Look For
At low power Sinus Iridum reads as an open-mouthed bay rather than a closed crater — the defining trait that earns it the Lunar 100 designation “very large crater with missing rim.” The Montes Jura curve unbroken from Promontorium Heraclides in the southwest to Promontorium Laplace in the northeast, but the southern third of the original crater wall has simply vanished beneath the Imbrium lava. Even without a telescope, sharp-eyed observers can detect the distinctive “bite” Iridum takes out of Mare Imbrium’s edge.
The Jura terminate in two distinct capes. Laplace, on the northeast, is the taller and sharper of the two at roughly 2.6 km, throwing a crisp triangular shadow at sunrise; Heraclides, on the southwest, is lower and less sharply defined — reference elevations for it vary by source and measurement method, so no single number is given here — but it carries the more famous nickname. Under soft seeing and low magnification, with the terminator just past the western Jura, Heraclides resolves into the silhouette of a woman’s head with windblown hair gazing across the bay: the “Moon Maiden,” a classic telescopic illusion noted by generations of lunar observers, depicted as early as Giovanni Cassini’s 1679 lunar map.
The flat, lava-flooded floor is otherwise almost featureless — its exceptionally smooth surface led it to be considered as a potential landing region for several lunar missions, including pre-Apollo studies and China’s Chang’e 3. Under low oblique light, though, a network of dorsa (wrinkle ridges) emerges, the result of compressional buckling as the basin slowly subsided after the lava cooled. One ridge, running just east of the small crater Laplace A, is large enough that lunar observers have given it an informal name — Dorsum Laplace A — though this is not an official IAU designation.
Despite its size, the bay’s interior contains no large craters — a point long noted by lunar observers, including Patrick Moore, who noted that telescopically the floor appeared to be broken by only one reasonably conspicuous craterlet (catalogued as Heraclides E). Modern orbital imagery reveals many more small craters and pits than are visible from Earth, but for a backyard telescope, Laplace A near the eastern edge remains the other commonly cited floor feature and a useful reference point for tracking the nearby wrinkle ridge.
The Science: A Two-Act Story, Not a Mystery
Unlike some lunar features whose origin is genuinely contested, Sinus Iridum’s formation has a well-supported, broadly agreed sequence backed by crater counts, geologic mapping, and orbital imagery. There isn’t a live scientific debate here so much as a satisfying two-act story of impact followed by burial.
Act One — A Crater on the Rim of a Crater
Around 3.85 billion years ago, the giant Imbrium Basin was excavated. Sometime after — current estimates put Sinus Iridum’s own formation in the range of roughly 3.84 to 3.7 billion years ago — a separate, smaller impactor struck near Imbrium’s northern rim, throwing up the ejecta that became the Montes Jura.
Act Two — Subsidence and the Lava Floods
As Mare Imbrium’s center sank under the weight of repeated basaltic lava flows, the southern portion of the newer Iridum crater became buried beneath those later mare basalts. Multiple distinct lava flows — not a single event — are thought to have been involved, based on mapped differences in surface age across the bay’s floor. The northern rim, propped on older and more rigid terrain, survived intact as the Jura we see today.
What makes Sinus Iridum remarkable isn’t a debate over how it formed, but how cleanly that ordinary, well-documented sequence — basin, then crater, then flood — produced something this visually singular. There’s nothing else quite like it anywhere else on the Moon.