The Supermassive Podcast · The Royal Astronomical Society

The Major Lunar Standstill

·48 min·4 clips

Hear how Stonehenge's builders may have tracked the moon for burials, linking astronomy to ancient rituals.

Host Izzy Clark and astrophysicist Dr. Becky Smethurst are joined by Royal Astronomical Society deputy director Dr. Robert Massey for a deep dive into the major lunar standstill. Massey explains the phenomenon: the moon's orbital plane precesses over an 18.6-year cycle, meaning that at the major standstill, the moon reaches its most extreme northerly and southerly rising and setting points on the horizon, appearing unusually high or low in the sky on alternate fortnights. Stonehenge curator Heather Sabir walks through the site on audio, describing the known solar alignment of Stonehenge's trilithon horseshoe (midsummer sunrise / midwinter sunset) and the current research hypothesis that the four station stones — forming a rectangle around the circle — may also encode a lunar alignment, specifically the most southerly moonrise and most northerly moonset during a major standstill. Dr. Amanda Chadburn of Bournemouth University describes the sight-line methodology: teams photograph and measure whether the moon rises or sets within the alignment defined by pairs of station stones. She notes that tree-ring dating of beams in the Chimney Rock great house shows it was built and renovated during lunar standstill years. Dr. Erica Ellingson from the University of Colorado describes Chimney Rock — a narrow knife-edge ridge in southwest Colorado where the Pueblo people of the Chaco civilization built a multi-story great house around 900-1100 AD. The two pinnacles at the ridge end frame the moonrise during major standstill years. Ellingson's team has rigged satellite uplinks to live-stream moonrise from the summit, which is too narrow and dangerous for large groups, to accessible amphitheaters below. Dr. Becky explains lunar phases as a result of the sun-Earth-moon angle, and dispels moon illusion and supermoon hype: the moon is physically the same size at all times; its apparent enlargement near the horizon is an optical illusion. She explains tidal locking via the giant impact hypothesis and the analogy of two friends spinning while holding hands. On the question of the moon's recession, Massey describes the laser ranging experiments still using Apollo-era reflectors that measure the moon's drift of 3.8 centimetres per year. Over geological time, this means total solar eclipses — currently possible because the moon and sun appear nearly the same angular size from Earth — will eventually be replaced by only annular eclipses.

As heard by us

A familiar Moon illusion opens into a larger sky question.

The piece follows the moon illusion from a simple observation: the Moon can look larger near the horizon, yet photo checks, arm's-length comparisons, and small objects like a pebble or finger keep showing the same size.

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Why you'd press play

If you want the Moon explained without losing the wonder, start here.

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