A new light in Cassiopeia
In 1572, Tycho Brahe saw an unfamiliar bright star in Cassiopeia. Telescopes had not yet been invented. He knew the field by eye and asked others to confirm what he saw. At its brightest the object rivalled Venus, then faded over the following months until it disappeared from unaided view.
His first observation was dated November 11 in the Julian calendar, equivalent to November 21 in the proleptic Gregorian calendar. He published his observations and arguments in De nova stella in 1573.
Using position to judge distance
Tycho measured the angular distances between the object and neighbouring stars, looking for shifts during the night. Nearby objects show parallax as the observer’s position changes; the Moon has measurable daily parallax. The new star showed neither that shift nor the day-to-day motion of a planet.
He therefore placed it beyond the Moon, among the fixed stars. His book recorded the positions and comparison stars, allowing other observers to examine the same reasoning.
Change beyond the Moon
The Aristotelian tradition treated the heavens beyond the Moon as unchanging, often placing transient phenomena such as comets nearer Earth. Tycho located an object that appeared, brightened and vanished in the distant heavens, confronting that view with measurements.
He followed its brightness and colour, comparing it with familiar stars and planets. A chance sighting became a sequence of observations extending over many months.
The supernova remnant
The explosion is now called SN 1572, or Tycho’s supernova. Astronomers have reconstructed its light curve from the historical brightness records and used it in supernova classification. Observations of the same region also identified an expanding remnant.
The recorded position helped locate that remnant, while the brightness sequence connected sixteenth-century visual observations with research on stellar explosions. Tycho recorded the changing light of the explosion; the remnant was detected much later.