L11 Aristarchus Crater

When to Observe Aristarchus (L11)

Aristarchus sits deep in the western reaches of Oceanus Procellarum, at 47.5°W — far enough from the central meridian that the terminator reaches it noticeably later than most targets. This western position gives it two well-separated terminator windows and makes it one of the most phase-versatile targets on the entire Lunar 100.

• Best Viewing — Waxing Gibbous (Day 12–13) — the sunrise terminator sweeps across the crater, throwing the central peak and layered inner walls into dramatic relief against the dark floor.
• Second Window — Waning Crescent — the sunset terminator returns from the opposite direction, revealing the western wall terraces and casting the central peak shadow in reverse.
• Full Moon Bonus: Unlike most craters, Aristarchus remains spectacular at Full Moon. The blazing ray system fans south and southeast across the dark mare — a completely different target under high Sun.
• The Earthshine Window: During a Waxing Crescent, Aristarchus is bright enough to be spotted on the dark, Earthlit portion of the lunar disk — the only crater routinely visible without the terminator’s help.

What to Look For

1. The Brightest Point on the Moon

Even at the lowest magnification, Aristarchus is unmistakable — an intense white point blazing against the dark basalt of Oceanus Procellarum. Its albedo is extraordinarily high for a lunar crater, a consequence of its youth (~450 million years) and the relatively unweathered anorthosite and impact glass exposed in its ejecta blanket.

2. The Layered Inner Walls

At 150x–200x near the terminator, the inner walls of Aristarchus offer one of the most geologically complex cross-sections visible in an amateur telescope. The impact excavated through multiple distinct layers of the lunar crust, exposing alternating bands of dark mare basalt, bright crustal anorthosite, impact melt glass, and pyroclastic ash — each at a different depth. No two bands are the same width, and the contrast between the dark and bright material is striking even in modest apertures.

3. The Ray System

Near Full Moon, Aristarchus transforms. The central peak and wall detail wash out, but the asymmetric ray system takes over — bright ejecta streaks spreading primarily south and southeast across the dark mare floor. The asymmetry reflects the oblique angle of the original impact, with the impactor arriving from the southeast and spraying ejecta preferentially downrange to the northwest, with the longest rays extending several hundred kilometres across the mare.

4. The Wider Volcanic Context

At 50x–100x, pull back to appreciate the extraordinary volcanic setting. Immediately to the west sits Herodotus — a crater of similar size but dark, lava-flooded, and featureless by comparison, a striking geological contrast. To the northwest, the irregular upland of the Aristarchus Plateau (L22) rises above the surrounding mare, its dark pyroclastic mantle visible as a subtle tonal difference. From the plateau’s northern edge, the widening mouth of Schröter’s Valley (L17) — the largest sinuous rille on the Moon at ~140 km long — curves away to the northwest, its “Cobra Head” vent just detectable under low Sun.

The Science: The Brightest Crater and What It Tells Us

Aristarchus is not just the brightest crater on the Moon — it is one of the most scientifically significant, sitting atop one of the Moon’s most volcanically active regions.

1. The Impact: Roughly 450 million years ago, an asteroid struck the western edge of Oceanus Procellarum from the southeast, excavating 2.7 km deep through multiple layers of mare basalt and ancient highland crust. The freshness of the ejecta — still rich in unweathered anorthosite and impact glass — is what gives the crater its extraordinary brightness today.
2. The Exposure: The impact cut through a pre-existing stack of lava flows of varying composition and age, creating the layered wall stratigraphy visible today. This makes Aristarchus a natural drill core through the western lunar crust — each band a different volcanic episode.
3. The Outgassing: The Aristarchus Plateau beneath the crater is one of the most volcanically restless regions on the Moon. Apollo 15 detected elevated alpha particle emissions passing overhead, suggesting radon gas seeping from the interior. Over 122 Transient Lunar Phenomena — temporary obscurations and colorations — have been reported here, the highest count for any single feature on the Moon.

What you are looking at is not just a young, bright crater — it is a window into an actively outgassing, volcanically complex region of the lunar interior that may not be entirely dormant even today.