The Fire Star that Chinese astronomers could not pin down
A silk manuscript buried at Mawangdui gives working cycles for Jupiter, Saturn and Venus and none at all for Mars — the one planet that refused to keep a schedule.
In 1973, archaeologists opened tomb 3 at Mawangdui, near Changsha, sealed in 168 BCE. Among the grave goods was a silk manuscript of roughly 8,000 characters, the Wuxingzhan — "Prognostics of the Five Planets." Its tables run from 246 BCE to 177 BCE, seventy years of planetary positions, and for most of the planets they are impressive: a twelve-year cycle for Jupiter, a thirty-year cycle for Saturn with a 377-day synodic period attached, an eight-year phase table for Venus running to the month and the lodge. For Mars there is no period at all. The section survives, and it simply does not give one.
The name says why. Chinese astronomers called Mars 熒惑, Yinghuo — Christopher Cullen renders it "Dazzling Deluder"; later usage settled on 火星, the Fire Star. The Huainanzi says flatly that its comings and goings are irregular; a later commentator added that no standard can be made from them. The Shiji's treatise on the celestial offices gives phase lengths in months that work out to a synodic period near 790 days, against a true mean of 779.94. That is not carelessness. It is the Huainanzi's complaint restated as a number: Mars is the worst-behaved object a naked eye can track, no two of its circuits take the same length of time, and a mean period is a description of nothing that actually happens.
A planet that cannot be predicted becomes a warning. The most dreaded configuration in Chinese astrology was 熒惑守心, Yinghuo shou Xin — Mars lingering in the lodge Xin, the "Heart", which contains Antares, a red star of almost the same colour and brightness as Mars itself. Court astrologers read it as a threat aimed at the ruler personally, and the histories treat it as one of the gravest things the sky can do.
In 7 BCE the omen was reported at the court of Emperor Cheng of Han. His counselor-in-chief, Zhai Fangjin, was handed poisoned wine and pushed into taking his own life, so that the disaster would land on him instead of the throne. The emperor died anyway, about a month later.
The observation was false. Huang Yi-Long recomputed the recorded instances and found that of 23 entries for "Mars guards the Heart" in the dynastic histories, 17 describe something that never happened — and 7 BCE is one of the 17. The real configuration is rare, on the order of once in eighty years. What produced the false entries is still argued: Huang reads them as deliberate fabrication, while other recounts put most of them down to misdated records, "guarding" confused with a lesser approach, and Jupiter mistaken for Mars. Either way, Chinese astronomy produced some of the most careful sky records on Earth — eclipse and comet observations still used by modern researchers — and also killed a chancellor over where Mars was not.